Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR1: Tribology
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
9:15 AM - RR1.1
Water Droplet Freezing: Humidity, Shearing Gas Flow and Surface Energy Effects.
Stefan Jung 1 , Manish Tiwari 1 , N. Vuong Doan 1 , Dimos Poulikakos 1
1 Mechanical and Process Engineering, ETH Zurich, Zurich Switzerland
Show AbstractFreezing of water is a ubiquitous phenomenon and therefore crucial to understand. Ice formation on supercooled surfaces exposed to different environmental conditions affects applications such as power transmission lines and aerodynamic surfaces from the aviation to wind turbine industries. Substrate heating and coatings with anti-icing chemicals have been used with partial success in tackling the frosting problem. Recently, use of superhydrophobic surfaces with hierarchical micro-to-nanoscale roughness has been proposed since their outstanding water repellency imparts them with an additional advantage of being able to minimize ice accretion and adhesion. Therefore, the role of surface roughness of superhydrophobic surfaces, which are inherent in their synthesis, on their icephobic behavior needs critical examination. For example, we show here that a droplet on a polished silicon wafer (a hydrophilic surfaces) with nanometer (nm)-scale roughness (smaller than the size of the first stable ice nuclei) and higher wettability freezes with an order of magnitude longer freezing delays compared to typical superhydrophobic surfaces. However, the low ice adhesion on superhydrophobic surfaces is an advantage. Therefore, herein we report the role of environmental conditions such as shearing gas flow, humidity and presence of frost on freezing of supercooled sessile water droplets on supercooled surfaces (both at -15oC). The experiments were performed in a custom built double layered, low temperature chamber (cooled using nitrogen vapor) fitted with a ventilator fan (to impose shearing gas) and with optical access for visualization using a high speed camera. We show herein that environmental humidity and gas flow have strong influence on freezing of water droplets on supercooled superhydrophobic surfaces, thereby also drastically affecting their icephobic behavior. The gas flow leads to a strong evaporation of the supercooled droplets, which alters the droplet temperature due to evaporative cooling. Through a scaling analysis, the effect of this cooling on nucleation of ice is examined using nucleation theories factoring in both surface roughness and contact angle. The role of shear gas flow on droplet roll-off vis-à-vis freezing dynamics is also analyzed by image processing of the high speed videos recording. Beyond a critical value of the gas velocity, the droplet begins to roll-off on the supercooled drolets. During roll-off, we show that freezing can ensue if the droplet comes in contact of an ice nucleus. It is observed that in dry humidity condition, the frozen droplets can roll-off at critical shear velocity, however, under saturated humidity condition the frozen droplet remains stuck to the surface. These results are interpreted by in terms of droplet state on the superhydrophobic state switching from Cassie state (in dry humidity) to Wenzel state (in saturated humidity).
9:30 AM - **RR1.2
Quantum Chemical Molecular Dynamics Simulation on Tribochemical Reaction Dynamics for Super-Low Friction System.
Momoji Kubo 1 , Nobuki Ozawa 1
1 Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, Japan
Show Abstract Diamond-like carbon (DLC) has been expected as a promising material which indicates super-low frictions for reducing carbon dioxide emissions and saving energies. It is experimentally pointed out that the detailed understanding of the tribochemical reactions of the DLC is necessary for fully explaining its super-low friction. Here, classical molecular dynamics simulation is frequently employed to investigate the tribological phenomena on atomic scale. However, the classical molecular dynamics method cannot simulate the chemical reaction dynamics. Therefore, we developed a quantum chemical molecular dynamics simulator for the elucidation of the tribochemical reaction dynamics. In the present study, we applied our quantum chemical molecular dynamics simulator to the investigation on the tribochemical reaction dynamics of the DLC. This simulator is based on our original tight-binding theory and realized over 5,000 times acceleration compared to the first-principles molecular dynamics method. The tribochemical reaction dynamics of the hydrogen-terminated DLC under 1 GPa pressure condition was simulated. Our calculation results indicate that the hydrogen-hydrogen repulsion at the interface leads to the super-low friction of the DLC. Moreover, we observed the formation of H2 molecule at the friction interface by the tribochemical reaction of hydrogen atoms which terminate carbon atoms. The time profile of the friction coefficient of the hydrogen terminated DLC is analyzed. The super-low friction coefficient of 0.07 is realized after the formation of H2 molecule. Then we suggested that the formation of vapor phase at the solid-solid interface is the reason for the super-low friction of the DLC. Moreover, it is very interesting that the distance between the DLC substrates increased just after the formation of H2 molecule. The increment of the DLC substrates distance was found to be another important reason for the super-low friction of the DLC. We also calculated that the tribochemical reaction dynamics of the hydrogen terminated DLC under the higher pressure condition of 10 GPa. At that time, the creation of new C-C bonds at the friction interface was observed. Moreover, the formation of new C-C bonds increased the friction coefficient of the DLC. Therefore, we suggested that the avoidance of the C-C bond formation is key factor to realize the super-low friction of the DLC. Moreover, we also suggested that the experimental fluctuation of the friction coefficient of the DLC is due to the dynamic tribochemical reactions of the hydrogen and carbon atoms in the hydrogen-terminated DLC.
10:00 AM - RR1.3
Surface Forces in Confined Electrolyte Solution up to Saturation.
Rosa Espinosa-Marzal 1 , Manfred Heuberger 2
1 Materials Department, ETH Zurich, Zurich Switzerland, 2 Advanced Fibers, Empa, St Gallen, 9014, Switzerland
Show AbstractIn this work we measure the surface forces across a single-slit pore consisting of two atomically flat mica surfaces submerged in a reservoir of potassium nitrate (KNO3) solution. We use solution concentrations ranging from 0.08 mM to 2.6 M in the extended surface force apparatus (eSFA). Our experimental results are in general agreement with previously reported results, yet the higher resolution of our SFA reveals some new structural insights. We discuss our data in terms of three different concentration regimes that are defined by shifts of the energetics of structures and interactions at the molecular scale, as probed by the eSFA. Below a concentration of 0.3 mM, the surface force fits the classical DLVO theory at all surface separations. A mechanical instability leads to a jump into a mica-mica contact. Above 0.3 mM hydrated ions populate the interface to produce a collective layering transition; a layer of hydrated ions can be expelled in a 5.8±1 Å film thickness transition, which is unprecedented evidence of the existence of water cluster ions, i.e. ions with strongly associated hydration water, in salt solutions. In a concentration range between 1 and 10 mM multiple layers of cluster ions are detected. The ion hydration shell has a soft character, which gives rise to layering transitions of irregular sizes (4±1 Å). There is a tendency to smaller cluster sizes at higher concentrations. The water coordination number of ions at the interface, as inferred from layering transitions, is 5-10x smaller than in the surrounding bulk solution. Ions adjacent to the surface can reversibly be semi-dehydrated and immobilized in a 2.9±0.3 Å film thickness transition by external pressure; once immobilized they prevent intimate surface contact and lead to a reduced mica-mica surface adhesion. Above 20 mM the attractive ion correlation force comes into play as an additional attractive force. It affects both surface force and cluster ion structure. A highly dehydrated ionic condensate at the interface finally undergoes solidification around 100 mM. A 1-3 nm thick ionic solid finally forms at the surface while the bulk salt solution is still unsaturated. The surface force measured in the presence of this solid is weakly attractive and the measured adhesion increases again towards bulk solution saturation. Our results are of particular relevance to the fundamental understanding of saline hydration of charged hydrophilic surfaces, colloidal stabilization, biological systems, electrochemistry, and of the destructive salt weathering of concrete and stone by pore-cracking mechanisms, and, the pressure induced-dissolution at mineral-water interfaces.
10:15 AM - RR1.4
Comparison of the Kinetic Friction of Planar Neutral and Polyelectrolyte Polymer Brushes Using Molecular Dynamics Simulations.
Yangpeng Ou 2 , Jeffrey Sokoloff 1 , Mark Stevens 3
2 Physics Department, Northeastern University, Boston, Massachusetts, United States, 1 Physics, Northeastern University, Boston, Massachusetts, United States, 3 MS 1411, Sandia Nathional Laboratory, Albuquerque, New Mexico, United States
Show AbstractWe have simulated the relative shear motion of both neutral and polyelectrolyte end-grafted polymer brushes using molecular dynamics. The flexible neutral polymer brush is treated as a bead-spring model, and polyelectrolyte brush is treated same way except that each bead is charged and there are counterions present to neutralize the charge. We investigated the friction coefficient, polymer monomer density, and brush penetration for the two kinds of brushes with both the same grafting density and the same normal force under good solvent condition. For the case of equal grafting density, we find that polyelectrolyte brushes had a smaller friction coefficient and monomer penetration than neutral polymer brushes, although the polyelectrolyte brushes supported a much higher normal load than the neutral brushes for the same degree of compression. Charged and neutral brushes with their grafting densities chosen so that they support the same load exhibited approximately the same degree of interpenetration, but the polyelectrolyte brush exhibited a significantly lower friction coefficient. We present evidence that the reason for this is that the extra normal force contribution provided by the counterion osmotic pressure that exists for polyelectrolyte brushes permits polyelectrolyte brushes to support the same load as an identical neutral polymer brush of higher grafting density. Because of the resulting lower monomer density for the charged brushes, fewer monomer collisions take place per unit time, resulting in a lower friction coefficient.
10:30 AM - **RR1.5
Atomic Friction Experiments under Electrochemical Control.
Roland Bennewitz 1
1 , INM Leibniz Institute for New Materials, Saabrucken Germany
Show AbstractElectrochemical methods allow for fast and reversible modification of metal surfaces through deposition and dissolution of metal films, adsorption and desorption of anions, as well as oxidation and reduction. The surface composition and structure undergo dramatic changes in these processes, which should cause significant changes in the friction on the surface. We will report on our friction force microscopy experiments, in which we study atomic friction processes in order to explore the prospects of friction control through electrochemical methods.The resolution of atomic stick-slip events in an electrochemical cell is improved by the development of a dedicated instrument [1]. The lateral force contrast reveals the atomic structure of the Au(111) surface and its herringbone reconstruction. After deposition of one monolayer of Cu by underpotential deposition, the atomic stick-slip changes into a periodicity which indicates frictional response of a CuCl rather than a Cu layer.Wear-less friction on Au(111) surfaces is extremely weak and exhibits almost no load dependence. Upon electrochemical oxidation of the surface, significant friction with linear load dependence is observed. The process is reversible and allows switching repeatedly between high and low friction [2]. More subtle effects are found in the regime of anion adsorption, namely a frictional response with threshold behaviour. The threshold depends on both applied normal load and the electrochemical potential, indicating confinement effects [3].[1] A. Labuda et al., Rev. Sci. Instruments 81, 083701 (2010)[2] A. Labuda et al., Langmuir 27 (2011) 2561[3] F. Hausen et al., Electrochimica Acta (2011) doi:10.1016/j.electacta.2011.03.013
11:30 AM - **RR1.6
Adhesion and Friction on Patterned Surfaces.
Robert McMeeking 1 2 3
1 Department of Mechanical Engineering, University of California, Santa Barbara, California, United States, 2 School of Engineering, University of Aberdeen, Aberdeen United Kingdom, 3 , INM-Leibniz Institute for New Materials, Saarbruecken Germany
Show AbstractPatterned surfaces having small fibrils of various shapes and depressions when fabricated from soft materials such as PDMS are known to have superior adhesion properties compared to unstructured surfaces. Further features of note are that fibrils having flanges or spatula at their ends are more adhesive than those that have square or rounded perimeters at the adhering end. Flat on flat adhesion involving one patterned surface against an unpatterned one is very sensitive to alignment, with the pull-off force falling off dramatically with even slight misalignment of a few degrees. These insights and other evidence indicate that peeling is the dominant mechanism for detachment of such surfaces, whether at the scale of individual fibrils or dimples, or at the macroscopic scale of the entire patterned surface. It is also apparent that the mechanisms of detachment are significantly influenced by the presence of friction on the surface, both when it eliminates sliding completely and when sliding occurs. These phenomena are also relevant to the adhesion utilized by the gecko and exploited by mussels through their byssal attachments. Another feature that influences adhesion is the buckling of fibrils under compression that can eliminate adhesion through rate effects that prohibit reattachment after unbuckling. These phenomena are described through models of the various mechanisms and interactions among them, with the important influences of large strain allowed for. Insights into the resulting adhesion, pull-off forces and the interaction of adhesion and friction are provided.
12:00 PM - RR1.7
Contact of Adhesive Randomly Rough Surfaces.
Lars Pastewka 1 , Mark Robbins 1
1 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractAdhesive contact of rigid, randomly rough surfaces and elastic substrates is studied using molecular statics and continuum simulations. The surfaces are self-affine with Hurst exponent 0.3 to 0.8 and different short λs and long λl wavelength cutoffs. The range and strength of the adhesive potential is also varied. In all cases the effect of adhesion decreases as the ratio λl/λs increases. In particular, the pull-off force decreases and the area of contact becomes linear in the applied load. The results will be discussed in the context of the Maugis-Dugdale [1] theories for individual asperities and the more recent scaling theory of Persson [2].[1] D. Maugis, J. Colloid Interface Sci. 150, 243 (1992)[2] B.N.J. Persson, Phys. Rev. Lett. 74, 75420 (2006)
12:15 PM - RR1.8
Synthesis, Characterization and Tribological Implications of Surface-Tethered Hydrogel Thin Films with Various Vertical Architectures.
Ang Li 1 , Edmondo Benetti 1 , Rosa Espinosa-Marzal 1 , Shivaprakash Ramakrishna 1 , Davide Tranchida 2 , Holger Schoenherr 2 , Nicholas Spencer 1
1 Department of Materials, Laboratory for Surface Science and Technology, Zurich, Zurich, Switzerland, 2 Department of Physical Chemistry I, University of Siegen, Siegen Germany
Show AbstractIn this study, thin films of poly(acrylamide) (PAAm) brushes and hydrogel-brushes with various degrees of crosslinking were grown from iniferter-functionalized silicon substrates by UVLED-initiated photopolymerization and their properties studied by means of a variety of analytical methods. In the case of homogeneous PAAm hydrogel-brushes, both bulk and interfacial properties of the polymer films were found to be strongly influenced by the lateral crosslinking of the grafted polymer chains. In agreement with theoretical expectations, the decrease of polymer-brush mobility with increasing crosslinking density results in a substantial increase of film wettability. The swelling ratio of polymer hydrogel-brushes, as measured by ellipsometry and atomic force microscopy, also confirms the formation of grafted networks and is directly related to the amount of crosslinker in the monomer feed. Young's moduli and friction coefficients of PAAm hydrogels are higher than those of the grafted polymer-brush analogues and can be tuned by varying the crosslinking densities.In the second part of the study, stratified brushes/gel films with various vertical architectures were successfully fabricated by repeatedly reinitiating the dormant species at the polymer chain ends in a monomer solution containing a predefined amount of crosslinker. The fabricated stratified hydrogel thin films differ in both lubricating performance and tribological durability. A three-layer film structure (brush-gel-brush) is proposed, in order to minimize the stress at the iniferter binding sites, to average the load, to strengthen the film and to achieve low friction coefficients. The optimized brush-gel-brush polymer film showed outstanding tribological stability and lubricating performance compared to unstratified polymer brushes or gel analogues. Thus, these stratified water-born polymeric thin films may find promising applications in the fast-developing aqueous-based coatings industry and provide a platform to design films with excellent mechanical and aqueous lubricating properties.
12:30 PM - RR1.9
Comparison of Friction Forces Measured in Adhesive and Non-Adhesive Systems with SFA and AFM.
Jagdeep Singh 1 , Yutao Yang 1 , Marina Ruths 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractThe boundary friction of different polyaromatic self-assembled monolayers was measured in single asperity contacts with the surface force apparatus (SFA) and atomic force microscopy (AFM). Measurements were done in ethanol and in dry N2 gas. In ethanol, the adhesion was significantly reduced. This allowed a direct, quantitative comparison of load-dependent friction between surfaces of very different radii. A linear dependence of the friction force on load was obtained, and good agreement was found between friction coefficients measured with the two techniques despite the large differences in contact areas, applied loads, and pressures. The investigations have been extended to measurements in dry N2 gas, where the surfaces adhere due to van der Waals interactions. In the adhesive systems, the friction force shows a non-linear dependence on load and is dependent on the radius of curvature of the interacting surfaces.
12:45 PM - RR1.10
Ultra Low Friction of Multilayer Graphene Studied by Coarse-Grained Molecular Simulation.
Hitoshi Washizu 1 , Seiji Kajita 1 , Mamoru Tohyama 1 , Toshihide Ohmori 1 , Hiroshi Teranishi 2 , Noriaki Nishino 2 , Atsushi Suzuki 2
1 , Toyota Central R&D Labs. Inc., Aichi, Aichi, Japan, 2 , Toyota Motor Corporation, Toyota, Aichi, Japan
Show AbstractCoarse-grained Metropolis Monte Carlo Brownian dynamics (MCBD) simulations are employed to investigate the friction dynamics of a transfer film of multilayered graphene sheets. The mechanism of ultra low friction of some type of layered materials such as graphite is not well explained yet. This is because the transfer film is consisted by a huge amount of atoms which is hard to treat by atom based simulation such as molecular dynamics. MCBD is the numerical solver of the Fokker-Plank diffusion equation and physically equal to the Langevin dynamics. Each circular graphene sheet consist of 400 to 1,000,000 atoms are allowed to move in 3 translational and 1 rotational directions due to thermal motion in 300K. Sheet-sheet interaction energy is calculated by the sum of the pair potential of sp2 carbon. The time scale is related to the diffusion coefficient. The mono- and the multilayer up to 20 layer graphene sheets are confined by two sliders. The upper slider is fixed to the upper solid body and the lower slider is modeled as infinitely wide graphite sheet. The end effects are included to all calculation of interactions of neighboring sheets except the interaction of the lowest sheet and the lower slider, by decreasing the potential energy when two sheets are separated in translational direction. The effect of incommensurate surface interaction is included by adopting smooth energy surface when two sheets are in twisted yaw angle. Then the sliding simulations are done by moving the upper slider in constant velocity. In monolayer case, the friction force showed stick-slip like curve and the average of the force was large. In multilayer case, the friction force did not show the oscillation and the average of the force was very low even in commesurate condition. This is because the whole transfer film obtain the internal degree of freedom in multilayer case and the lowest sheet of the layer are able to follow the equipotential surface of the lower slider. The dependence of sheet size, number of sheets are explained from the point of view of thermostatistical stability.
RR2: Interfaces in Mechanics
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
2:30 PM - **RR2.1
Designing Interfaces for Nano Crystalline Diamond Coatings.
Yue Qi 1
1 Chemical Sciences and Materials Systems Lab, General Motors R&D, Warren, Michigan, United States
Show AbstractTo enable nano-crystalline diamond (NCD) as tool coatings for aluminum machining, first principles calculations were integrated with cohesive zone and growth chemistry models to investigate all three interfaces: the Al/NCD interface, the grain boundaries (GB) inside NCD, and the coating/substrate interface. First, the tribochemical reactions on diamond surfaces was captured by first principles thermodynamics, we predicted the environmental conditions (gas species, pressure and temperature) to achieve fully –H or –OH terminated surface with minimized adhesion with Al. Second, our integrated model revealed that lower substrate temperatures increases the hydrogen content at the surface, which reduces tensile stress during film growth. Third, to enhance the adhesion of the coating, we discovered the anisotropic fracture behavior of interlayer can be utilized to form oriented microcracks, thus providing a new interface toughening mechanism. These predictions were validated with experiments at each step and guided NCD coating design.
3:00 PM - RR2.2
Analytical Model for Nanoscale Ploughing and Wear.
Maneesh Mishra 1 , Izabela Szlufarska 2 1
1 Materials Science Program, University of Wisconsin Madison, Madison, Wisconsin, United States, 2 Materials Science and Engineering, University of Wisconsin Madison, Madison, Wisconsin, United States
Show AbstractAlthough nanoscale friction has been widely studied in the elastic regime, very few studies have focused on friction and wear in the elasto-plastic regime where material removal can have significant contributions to the coefficient of friction. Existing analytical models, which have been developed for macroscale elasto-plastic friction, fail to describe a ploughing contribution to the coefficient of friction in nanometer sized contacts. Two reasons for this breakdown are a large contribution to nanoscale friction from pile up and a reduction of friction due to elastic recovery in the wake of the cutting tip. We have developed a new analytical model of ploughing friction that includes the above two contributions. The model was validated using large scale molecular dynamics simulations of machining on silicon carbide (SiC) and copper (Cu) single crystals using a spherical tip with radius of curvature R=10 nm. We find that the elastic recovery in the wake of the cutting tip is a function of depth of cut and significantly reduces coefficient of friction at small depths of cut (<0.1 R) where deformation is predominantly elastic. At larger depths of cut (0.2-0.5 R), deformation is elasto-plastic and both SiC and Cu deform via dislocation plasticity. Dislocation plasticity leads to formation of a pile up and the coefficient of friction increases as the cutting tip pushes the piled up material in front of it. In addition, we show that once the total coefficient of friction is known, a transition from ploughing to cutting during nanoscale scratching of singe crystals can be accurately predicted by macroscale geometric models.
3:15 PM - RR2.3
Mechanics and Friction of Individual Nanoparticles.
Eric Bucholz 1 , Susan Sinnott 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractIn situ transmission electron microscopy (TEM) experiments can be used to visualize and manipulate nanoparticles, including subjecting them to compression and shearing, which facilitate the study of their responses to compression and shearing. Understanding these responses is critical for optimizing their use as solid lubricants or lubricating additives to base oils in tribological applications. Despite the new insights provided by these experimental studies, it is nonetheless difficult to determine the source of changes in mechanical behavior and specific lubrication mechanisms from the resulting data; therefore, a complementary computational analysis of these systems is needed. Here, classical molecular dynamics simulations are performed to characterize the behavior of individual amorphous carbon (aC) and inorganic fullerene-like (IF) MoS2 nanoparticles at an interface when subjected to externally applied forces. In particular, the response of aC nanoparticles as a function of diameter and normal load are quantified based, in part, on the ratio of sp2:sp3 carbon atoms. The simulations predict that the transition from elastic to plastic deformation is triggered by an increase in the percentage of sp3 carbon atoms with the mechanical response also being independent of nanoparticle size over the range of diameters considered (2 – 5 nm). In the case of IF MoS2 nanoparticles, the simulations reveal how their mechanical and tribological properties depend on the atomic-scale details of the nanoparticle’s structure. Two specific configurations are considered with three nested MoS2 layers each: 1) a curved, ellipsoidal structure and 2) an octahedron. These different structures allow for the characterization of the role of curved and faceted morphologies as well as grain boundaries on the rolling and sliding behavior as well as any lamellar exfoliation of the individual MoS2 nanoparticles. Through the combination of experimental and simulation analysis, a comprehensive picture of the mechanical and tribological properties of nanomaterials is provided which should ultimately aid in the optimization of their use in tribological applications. This work is supported by the Office of Naval Research.
3:30 PM - **RR2.4
Hybrid Modeling Approaches for Characterizing Interfacial Structure and Dynamics.
Donald Brenner 1 , Lipeng Sun 1 , Shijing Lu 1 , Hongli Dang 1 , Mohammed Zikry 2
1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractWe have been developing hybrid approaches that combine analytic models, first principles calculations, atomic and continuum modeling to characterize the structure and properties of solid interfaces. This talk will focus on two examples of this work. In the first example a meso-scale defect model for tilt grain boundary energies that is parameterized to first principles calculations is used to predict and understand solute-induced grain boundary stabilization in AlPb and CuZn alloys. In contrast to prior suggestions in the literature, no correlation was apparent between the magnitude of the stabilization energy and the grain boundary energy of the pure systems. This result is attributed to an elastic term in the model that is unchanged by doping. Instead the stabilization energy is a well behaved function of tilt angle with a maximum at 53.13o. This relation occurs because the common elastic term for the pure and doped system, which produces cusps in the energy versus tilt angle relations at low sigma structures, cancels when calculating the stabilization energy. In the second example molecular simulations were used to model the dynamics of dislocations moving at very high strain rates in an Al 2139 alloy as they interact with nanometer-thick platelet precipitates. A unique deformation mechanism was observed that is a combination of precipitate cutting and Orowan looping. The resulting “step-ladder” structure of the sheared precipitates correlates with experimental high resolution transmission electron micrographs of shock-loaded alloys. A new analytic expression for the critical shear stress needed for platelet deformation was developed that is based on an activated reaction kinetics model. The strain rate in this expression appears in a log term so that the very high strain rates inherent to the simulations can be scaled to experimental conditions. This expression reproduces the dependence of the critical shear stress on temperature and platelet size given by the molecular simulations; it also reproduces the large dependence of flow stress on strain rate observed experimentally but not apparent in the high strain rate simulations.
RR3: Defect and Oxygen Dynamics near Interfaces I
Session Chairs
Monday PM, November 28, 2011
Commonwealth (Sheraton)
4:30 PM - RR3.1
Structural Transition of SrTiO3 Asymmetric Tilt Grain Boundaries.
Akihisa Fukumoto 1 , Haksung Lee 1 , Teruyasu Mizoguchi 1 , Yuichi Ikuhara 1 2 , Takahisa Yamamoto 1 2
1 , The University of Tokyo, Tokyo Japan, 2 , Japan Fine Ceramics Center, Nagoya Japan
Show Abstract Grain boundary (GB) in ceramics often plays an important role on obtaining unique properties. For example, nonlinear I-V characteristics of SrTiO3[1] and positive temperature coefficient of resistivity (PTCR) of BaTiO3[2] are originated from GBs and strongly depend on GB atomic structures. Therefore, to predict and control GB structures are essential for the materials design. To understand GB atomic structures, the combination of transmission electron microscopy (TEM) observation and theoretical calculation is a powerful way. So far, a number of related studies have been reported. However, most of them are concerned to symmetric tilt boundaries because such type of GB is easy to be analyzed from a view point of theoretical calculation. In contrast, the understanding of asymmetric GB structures is limited although GBs in practical polycrystalline materials are mostly asymmetric. In this study, structures of the SrTiO3 asymmetric tilt GB annealed in oxidizing or reducing atmospheres were investigated.Bicrystal samples of SrTiO3 [1-10](114)//(110)Σ3 asymmetric tilt boundary which were heat-treated in air or H2 were observed by conventional TEM (CTEM), high-resolution TEM (HRTEM), and Cs-corrected scanning transmission electron microscope (STEM) from the edge-on direction, and further, compositional analysis at the GB was also performed by STEM equipped with energy dispersive X-ray spectrometer (STEM-EDS). In order to understand the structural transition theoretically, GB structures and GB energies were calculated by the first principles projector augmented wave (PAW) method.It is revealed that GB structural transition is induced by changing oxygen partial pressure. The CTEM observation shows that the asymmetric tilt GB is periodically faceted with two kinds of GB planes of which plane indices are (11-2)//(552) and (115)//(11-1) in air-annealed sample. On the other hand, GB planes parallel to the original plane of (114)//(330) appear in H2-annealed sample. The STEM observation reveals that all GB atomic structures consist of 3 types of structural units, of which plane indices correspond to (111)//(11-1) ((111) twin), (112)//(11-2) ((112)twin), and (221)//(001). Using the structural unit model theory, GB energies and their dependence on annealing atmosphere are calculated. With the results, we discuss the structural transition of GBs by considering the stability of GBs that depends on the oxygen partial pressure.[1]T. Yamamoto et al. Journal of Materials Science, 40, 881(2005)[2]B. Hyubrechts et al. Journal of Materials Science, 30, 2463(1995)
4:45 PM - RR3.2
Surfactant Stabilization of Polar Oxide Surfaces: Enabling the Creation of Novel Functional Interfaces.
Benjamin Gaddy 1 , Elizabeth Paisley 1 , Mark Losego 1 , James Tweedie 1 , Ramón Collazo 1 , Zlatko Sitar 1 , Jon-Paul Maria 1 , Douglas Irving 1
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractHeteroepitaxy has the potential to enable the coupling of properties at interfaces between active materials, such as polar semiconductors and complex oxides, and will facilitate rich functionalities including non-linear dielectrics, non-linear magnets, and 2-D conductors. Many of the most interesting interfaces require the integration of dissimilar materials, with different structures, symmetries, and types of bonding. One such interface is the one between CaO, which has a rocksalt structure, and wurtzite GaN. The hexagonal symmetry of (0001) GaN leads to an epitaxial preference for seeding <111> oriented CaO growth, but the high surface free energy of this rocksalt orientation quickly leads to the formation of {100} facets and prevents smooth layer by layer growth. In this talk we will present results from ab-initio thermodynamics calculations. These results demonstrate that a hydrogen surfactant can overcome the preference for {100} faceting common to rocksalt oxides by altering the relative surface free energies, thereby enabling smooth layer by layer growth of <111> CaO on (0001) GaN. We compare our results to experimental growth of rocksalt oxides where water vapor is used instead of molecular oxygen in Molecular Beam Epitaxy. The effect of the hydrogen surfactant on the stability of the surface is examined through analysis of electrostatic effects, local density of states, interplanar relaxations, atomic reconstructions at the surface, electronic relaxations, and surfactant configurations. This work was supported by a U.S. Dept. of Defense NDSEG Fellowship, U.S. Dept. of Education GAANN Fellowship, an NSF Graduate Fellowship, and NSF DMR grant 0547134.
5:00 PM - RR3.3
Investigation of Nucleation and Growth of Hafnium Oxide Deposited by Thermal and Plasma-Enhanced Atomic Layer Deposition Using In Situ Spectroscopic Ellipsometry.
Amir Afshar 1 , Kenneth Cadien 1
1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada
Show AbstractThe nucleation and growth of hafnium oxide on silicon during thermal and plasma-enhanced atomic layer deposition (ALD) has been studied. Changes in the thickness of the HfO2 layer after reactant doses and during the purge times were studied using in-situ spectroscopic ellipsometry (SE). Tetrakis(dimethylamino)hafnium (TDMAH) was used as the hafnium precursor, and oxygen plasma and water were used as oxidizing reagents for plasma-enhanced and thermal ALD, respectively. Argon was used as the carrier gas. The substrate temperature was in the range of 100 - 300 °C. The results showed that the apparent thickness of the HfO2 layer (Δt1) increased sharply after the TDMAH dose and then decreased gradually during the purge time. This was attributed to the desorption of TDMAH from the surface. After introducing the oxidizing reagents, the apparent thickness (Δt2) decreased sharply due to replacement of large dimethylamino groups by small OH groups. The apparent thickness remained constant during the second purge time indicating that reaction was completed and the HfO2 layer was stabilized. Δt1 and Δt2 decreased with increasing temperature for both thermal and plasma-enhanced ALD. This is explained by the fact that at higher temperatures the rate of desorption would be higher than the rate of chemisorption resulting in a decreased Δt1 and Δt2. The results also showed that Δt1 was higher for the first cycle when the TDMAH reacts with the native silicon oxide on the substrate. After the first step Δt1 and Δt2 remained constant.
5:15 PM - RR3.4
First Principles Analysis of the Initial Oxidation of Treated Si(001) and Si(111) Surfaces and Its Implications for Protecting Quantum Confined Silicon Nanostructures.
Huashan Li 1 , Zhigang Wu 1 , Mark Lusk 1
1 Physics, Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe photovoltaic properties of quantum confined silicon (Si) nanostructures are extremely sensitive to oxidation because the high surface-to-volume ratio amplifies the influence of defects on electron-hole recombination, and because even small oxide films can change the optical gap, a sensitive function of the size of the underlying pristine structure. Even more extreme, sufficiently small Si nanostructures can completely oxidize before the encroaching oxide layer becomes a barrier to further oxygen attack. Understanding the mechanisms and the detailed processes of the earliest first steps oxygen interaction with functionalized Si surfaces is therefore critical to designing surface treatments that result in robust Si nanostructures. Interestingly, no systematic computational investigations have been carried out to elucidate the role of surface passivants for the extreme curtailing oxidation required to protect quantum confined Si particles and wires. In this study, we use density functional theory combined with transition state analysis to identify the most likely paths for dissociative adsorption of the first O2 molecules on hydrogen, methyl, and siloxane terminated Si (111), Si(001) surfaces as well as Si quantum dots. Specifically, barrier saddle point searching via an iterative application of linear synchronous transition and quadratic synchronous transition searching is supplemented by an eigenvector following method to determine the activation barriers for initial stage oxidation. Our results indicate that oxidation occurs without any energy barrier on H-terminated surface via direct dissociative adsorption, and that the passivant provides a degree of protection only because of its influence on the subsequent hopping of O atoms. On the other hand, O2 molecules are forced to react first with surface ligands on methyl and siloxane terminated surfaces, and the associated intermediate barriers offer more substantial protection that is possible with the H-treatment. In addition, the coverage dependence of adsorption barriers as well as the influence of ligands on the lateral and inward hopping mobilities of O atoms shed light on the role of ligands in preventing the initial stage of oxidation within these more complex systems. The important role of steric shielding leaves nanostructures vulnerable to initial oxygen attack, but it may be possible to close off these avenues by designing systems with very low O mobility from these sites.
5:30 PM - RR3.5
Optimization of the Properties of the Interface of SiNx:H/Si in Crystalline Silicon Solar Cells and Its Effect on Cell Efficiency.
Machteld Lamers 1 , Keith Butler 2 , Ingrid Romijn 1 , John Harding 2 , Arthur Weeber 1
1 , ECN Solar Energy, Petten Netherlands, 2 , University of Sheffield, Sheffield United Kingdom
Show AbstractThe efficiency of a crystalline silicon solar cell depends to a large extent on the passivating qualities of the layers placed on either side of the cell. Passivating layers should reduce the recombination losses at the surfaces of the cells as this will increase the cell output. In standard silicon cell production an Aluminium Back Surface Field (BSF, which is a layer with a higher dopant concentration than the base of the cell) is created on the rear side as a surface passivating layer, while on the front side SiNx:H (hydrogenated silicon nitride) is used. The latter layer acts not merely as a passivating layer, but also as an anti-reflection coating. The properties required for a highly passivating SiNx:H layer do not necessarily correspond to the properties needed to ensure maximum light transmission. Usually, highly passivating SiNx:H layers have a high absorption, which reduces the light transmission and subsequently the short circuit current Jsc of the solar cell. In optimizing the SiNx:H layer used on the front side of solar cells a careful balance is therefore required between the light management on one hand and the passivating properties on the other hand. Passivation itself can subsequently be divided in bulk- and surface defect passivation, and in this case also a careful balance needs to be maintained. In this paper we show experimental data from various SiNx:H layers and its SiNx:H/Si interfaces fabricated with an industrial remote-PECVD system (PECVD is Plasma Enhanced Chemical Vapour Deposition). We show that the gradient and the concentration of Si-N bonds at the interface determine the surface passivating properties. We show that a steep gradient or a very gradual gradient gives low passivating qualities and low open circuit voltage Voc of a solar cell. The optimal gradient can be found in between the extremes. We present molecular dynamics simulations in which the geometries of layers with various gradients are analyzed and the locations of important defective geometries are mapped. Surface passivation is determined by the fixed charge and defect density at the interface. A higher fixed charge and a lower defect density improves the surface passivation. We show that a steep gradient gives less defects, while a more gradual gradient will give a higher fixed charge. The optimal surface passivation is a balance between both the defect density and the fixed charge. This finding of the optimal gradient is supported by the molecular dynamics simulations. We show how this interface can be optimized by varying the deposition properties using the PECVD tool and how this understanding will lead to further increase solar cell efficiency.
5:45 PM - RR3.6
Studying the Effect of Interface Structure on the Electronic and Optical Properties of Silicon/Silicon Nitride Interfaces in Solar Cells Using Molecular Dynamics Simulations.
Keith Butler 1 , Machteld Lamers 2 , John Harding 1 , Arthur Weeber 2
1 Materials Science and Engineering, University of Sheffield, Sheffield United Kingdom, 2 Solar Energy, ECN, Petten Netherlands
Show AbstractOne of the major research areas in high performance solar cell production is the optimization of the so-called passivation interface, between crystalline silicon (c-Si) and silicon nitride (SiNx). This interface is important for two reasons. Firstly it acts an anti-reflective coating (ARC) to trap light within the c-Si wafer. Secondly it serves to passivate defects, which act as charge carrier recombination centres, present at the surface of the c-Si. Unfortunately it is often found that the optimization of one of these properties is achieved at the detriment of the other. Much of the work done on the development of the passivation interface to date has focused on finding the optimal ratio of Si to N (i.e. the value of x in SiNx). Plasma-enhanced chemical vapour deposition (PECVD) has become a popular technique for deposition of the SiNx layer, due to its ability to produce interfaces with the c-Si wafer containing fewer defect states. Under different conditions, either an abrupt interface, or one with a nitrogen gradient can be produced. Recent research has shown that a gradual varying of the nitrogen content across the interface rather than an abrupt interface between SiNx and c-Si can give solar cells with improved optical and electronic properties. We show how molecular dynamics simulations can explain how a gradual structure of the interface can result in a reduction in the density of the defective atom environments which have been implicated as charge traps which hamper solar cell performance. In addition we highlight the effects of varying the stoichiometry of the SiNx layer.We then investigate how the locations of certain defects are related to the width of the layer over which the N gradient spans. Mapping of the defects reveals how the majority of defects are located away from the c-Si surface in the presence of a N gradient. We also find that where the N gradient is too wide the number of defects at the interface increases. These results point clearly towards an optimal interface deign for improved efficiency in silicon based solar cells.
RR4: Poster Session: Interfacial Physics, Chemistry, and Mechanics
Session Chairs
Tuesday AM, November 29, 2011
Exhibition Hall C (Hynes)
9:00 PM - RR4.1
Characterization of Ceramic Powders during Compaction Using Electrical Measurements.
Timothy Pruyn 1 , Rosario Gerhardt 1
1 Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractIn this study we evaluated the electrical response of ceramic compacts during dry pressing as a function of applied pressure. Semiconductive SiC and insulating Al2O3 powders were used for the experiments. In order to determine the influence of porosity in the ceramic powder compacts, a custom made die with an insulating outer sleeve was used to carry out dc and ac measurements. Measurements were performed as a function of loading and unloading compaction pressure. Dc measurements can only detect the combined response from the powders and the porosity. However, from the SiC impedance spectroscopy data, at least two semicircles were observed in the complex impedance plot that allows separation of the two processes. One of these semicircles represents the bulk material property, while the other is likely due to the void space and interfaces between particles. The admittance, modulus and permittivity were also examined and showed behavior highly dependent on these two processes. The impedance behavior of the insulating Al2O3 was more sensitive to the compacted microstructure and humidity and often displayed trends different from the semiconducting SiC.
9:00 PM - RR4.10
Superlattice Vibrations and Debye Temperature of Nanoparticle Supercrystals.
Kaifu Bian 1 , Ananth Kaushik 1 , Paulette Clancy 1 , Detlef Smilgies 2 , Tobias Hanrath 1
1 Chemical Engineering, Cornell University, Ithaca, New York, United States, 2 Cornell High Energy Synchrotron Source (CHESS), Cornell University, Ithaca, New York, United States
Show AbstractThe Debye theory of lattice vibrations and specific heat in atomic crystals marks a cornerstone in the development of solid-state physics. Supercrystals comprised of colloidal nanoparticles present an interesting analogy to atomic crystals with the mass of the inorganic core corresponding to the atomic mass, while the complex interactions of the organic ligand shells correspond to the interatomic force. An important, yet unresolved question is whether highly ordered nanoparticle assemblies exhibit superlattice vibrations analogous to phonon modes in atomic crystals. Phonon modes in atomic crystals are commonly approximated by Debye model. Can Debye theory also describe vibrations in nanoparticle superlattices?To answer this question, we investigated supercrystals (SC) comprised of lead sulfide (PbS) nanoparticles using small-angle X-ray scattering (SAXS). We determined the Debye temperature of PbS SCs via two complementary experimental approaches, which probed the SC ordering in response to changes in pressure and temperature. We measured the bulk modulus of PbS SCs from high-pressure SAXS measurements with a diamond anvil cell. In complementary experiments, we probed temperature-dependent scattering intensities and determined the Debye-Waller factor. We will compare our two experimental determinations of the superlattice Debye temperature with results from molecular dynamics simulations of nanocrystal superlattices. Together, these results provide interesting new insights into the vibrational dynamics of nanoparticle superlattices.
9:00 PM - RR4.12
Temperature Evolution of the Cooperative Characteristic Length in Drawn Polymers.
Florian Hamonic 1 , Allisson Saiter 1 , Daniele Prevosto 2 , Eric Dargent 1 , Jean-Marc Saiter 1
1 , LECAP, Institute for Materials Research. – Université de Rouen, Faculté des Sciences, Saint Etienne du Rouvray France, 2 , Italian National Research Council - Institute for Chemical and Physical Processes, Pisa Italy
Show AbstractThe existence of a characteristic length scale of structural relaxation in glass formers has been introduced by Adams and Gibbs [1] with the concept of cooperatively rearranging regions and more recently gained new interest in the framework of the dynamic heterogeneity concept [2]. The characteristic length scale should be connected with the evolution of the structural relaxation time and the glass transition phenomenon. For this reason its investigation is of great relevance for glass formers. In particular, when a polymer is mechanically drawn a certain macromolecular order is induced, which eventually leads to the formation of crystal regions coexisting with amorphous ones if the material has some tendency to crystallize. In such situation chains in the amorphous regions can suffer for constrains due to the presence of the crystalline parts, similar to the case of confined amorphous materials. Such constrains modify the dynamic properties of the polymer, altering the distribution of relaxation time and the glass transition itself.The goal of this work is to follow the evolution of cooperativity along the Arrhenius plot for drawn PET and PETg samples, the former developing crystalline regions the latter remaining almost completely amorphous during drawing. For this goal dielectric spectroscopy and temperature modulated differential scanning calorimetry measurements will be used, which allow studying the CRR size temperature dependence over a wide temperature interval [3]. The comparison of the results for the two materials allows evidencing the effect on structural dynamics of pure drawing (no crystals, PETg) and that of the confinement of amorphous regions within the crystalline ones (PET). Moreover, the interpretation of the results in term of modification of the size of cooperatively rearranging regions will be proposed.[1] G. Adam, J. H. Gibbs, J. Chem. Phys., 43, 139, 1965.[2] L. Berthier, G. Biroli, J.P. Bouchaud, L. Cipelletti, D. El Masri, D. L’Hôte, F. Ladieu, M. PiernoScience, 310, 1797, 2005.[3] A. Saiter, L. Delbreilh, H. Couderc, K. Arabeche, A. Schönhals, J.-M. Saiter Phys.Rev. E 81, 041805, 2010.
9:00 PM - RR4.13
First-Principles Study on the Self-Induced Oscillation for Charged-Particle Pairs Confined in Square Quantum Dots.
Atsushi Tsubaki 1 , Tomoki Tagawa 1 , Kyozaburo Takeda 1
1 Electrical Enginnering and Bioscience, Waseda University, Tokyo Japan
Show AbstractDynamics in charged particles confined in the quantum dot (QD) system is attractive not only for the scientific understanding on the kinetics in Coulomb interaction but also for the technical application in the quantum devices. Here, we theoretically study the time-dependent (TD) features of the two kinds of charged-particle pairs (electron-electron and electron-hole) confined in the square QD system. We computationally solve TD Schroedinger equation under the typical two mean-field approaches of UHF and DFT calculations. In order to deepen our understanding on TD behaviors, we compare results of the electron-hole pair with those of the electron-electron pair and also the DFT results with the UHF ones. The TD UHF calculation demonstrates that the charged-particle pairs vacillate spatially when the individual particles initially have the same irreducible symmetry. The projection analysis reveals that the vacillation changes to be resonative when the confinement is strengthened. The FFT investigation further elucidates that this characteristic vacillation is caused by the mutual transition of charged particles by the self-induced Coulomb interaction. The TD DFT calculation, however, decreases the vacillation frequency Ω of the electron-electron pair whereas it increases the Ω value of the electron-hole pair. This opposite tendency in Ω is caused by the electron correlation (EC) sophisticatedly included in the DFT calculation. The EC for the electron-electron pair works to delocalize the two-electron wave function. Consequently, the confinement is effectively weakened to reduce the resulting Coulomb. Contrary, the wave function for the electron-hole pair originally localizes at the SQD center. The enegetical stabilization due to the EC now enhances this central localization of the electron-hole pair. Accordingly, the confinement is effectively strengthened and the vacillation frequency increases. Thus, the present UHF and DFT calculations demonstrate that electron correlation works oppositely for the charge-particle pairs, in accordance with the Coulomb attractive or repulsive interaction.
9:00 PM - RR4.14
Adsorption and Interaction Forces of Serum Proteins as a Measure of the Biocompatibility of Novel Polymeric Biomaterials.
David Cozzens 1 , Umaprasana Ojha 1 , Marina Ruths 1 , Rudolf Faust 1
1 Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts, United States
Show AbstractWe have studied the surface characteristics and biocompatibility of thermoplastic polyurethanes (TPUs) with applications in blood-contacting medical devices. Commercial TPUs as well as novel polyisobutylene-based TPUs with superior oxidative stability were characterized through contact angle measurements, XPS, and AFM imaging. Since thrombogenicity is a key concern with long-term blood-contacting biomaterials, the first step in this process, the adsorption of serum proteins, was quantified by quartz crystal microbalance measurements with dissipation monitoring (QCM-D). The adsorption of fouling fibrinogen or passivating albumin onto thin films of the polymers spin-coated onto the quartz crystal electrode was studied, as well as competitive adsorption from a mixture of these proteins. The interaction forces and adsorption strength of the proteins with the polymer surfaces were measured in buffer solution with the colloidal probe technique. The results indicate comparable adsorbed amounts and similar adhesion strengths of the proteins on the novel materials and on the commercial ones. This suggests that the urethane nature of the surface is of high importance for the protein interactions, and that the novel TPUs may have excellent biocompatibility.
9:00 PM - RR4.2
Hierarchical Textured Architectures for Continuous Dropwise Condensation and Collection of Water Droplets.
Xuemei Chen 1 , Jun Wu 2 , Shuhuai Yao 2 , Zuankai Wang 1
1 Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon Tong Hong Kong, 2 Department of Mechanical Engineering , The Hong Kong University of Science and Technology, Hong Kong Hong Kong
Show AbstractThe miniaturization of electronic devices demands novel thermal management techniques to efficiently dissipate heat over a small area. Two-phase heat transfer devices such as condensers allow for high heat transfer coefficients. In particular, dropwise condensation is able to produce heat transfer coefficients an order of magnitude higher than film condensation. Therefore engineering surfaces that enable dropwise condensation is of paramount importance. Surfaces that enable both efficient droplet nucleation and droplet self-removal (i.e. droplet departure) are essential to accomplish successful dropwise condensation. However it is extremely challenging to design such surfaces. This is because droplet nucleation requires a hydrophilic surface while droplet departure necessitates a super-hydrophobic surface. Here we report that these conflicting requirements can be satisfied using a hierarchical (multiscale) nanograssed micropyramid architecture that yields global superhydrophobicity as well as locally wettable nucleation sites, allowing for ~65% increase in the drop number density and ~450% increase in the drop self-removal volume as compared to a superhydrophobic surface with nanostructures alone. Further we find that synergistic co-operation between the hierarchical structures contributes directly to a continuous process of nucleation, coalescence, departure, and re-nucleation enabling sustained dropwise condensation over prolonged periods. Exploiting such multiscale coupling effects can open up novel and exciting vistas in surface engineering leading to among other applications, optimal condensation surfaces for high performance electronics cooling, heat transfer and water harvesting applications.
9:00 PM - RR4.3
The Almost Universal Occurrence of Dry Friction is Due to Tomlinson Instabilities on Small Length Scales.
Jeffrey Sokoloff 1
1 Physics, Northeastern University, Boston, Massachusetts, United States
Show AbstractThe virtual universal existence of “dry” (i.e. velocity independent in the slow sliding speed limit) friction when two solids slide relative to each other is responsible for a large percentage of the energy loss that occurs in machines and other mechanical devices. It is well known that Tomlinson model instabilities, that have long been considered as the explanation for occurrence of “dry” friction, cannot occur for the micron scale asperities that occur at most interfaces because these asperities are generally too short and fat to exhibit such instabilities. When one considers the fact that smaller length scale asperities exist on the surface of each of the micron scale asperities and still smaller length scale asperities occur on the surface of each of these new asperities, and this continues until one reaches atomic length scales, it is possible to show that Tomlinson model instabilities will always occur for sufficiently small length scale asperities. This provides a likely explanation for the almost universal occurrence of “dry” friction. This hypothesis is substantiated by calculations based on a multiscale contact mechanics model due to Persson, which includes effects of elastic and plastic deformation as well as adhesion on multiple length scales.
9:00 PM - RR4.4
Mass and Point Defect Transport at Solid-Solid Semicoherent Interfaces.
Kedarnath Kolluri 1 , Michael Demkowicz 1
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractUsing atomic-scale modeling of several fcc-bcc interfaces, we show that mass and point-defect transport at semicoherent interfaces depends strongly on interface structure. Point defects trap at specific interface sites and migrate from one trap to another when the traps are close enough. What's more, interface structure strongly influences migration of point defects that are not trapped as well. Our findings suggest that constitutive models that currently describe diffusion at solid-state interfaces need revision.This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
9:00 PM - RR4.5
Relaxation of Polymer Melts in Nanofilms at the Polymer-Solid Interface.
Yongjin Wang 1 , Jianing Sun 2 , Lei Li 1
1 Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , J.A. Woollam Co, Lincoln, Nebraska, United States
Show AbstractThe relaxation of polymer chains in bulk melt is fast, e.g., on the order of milliseconds to seconds. However, the relaxation in nanometer-thick films on solid substrates could be orders of magnitude slower. The very slow relaxation brings up the concerns on the long-term performance and the lifetime of the thin films. In order to design more robust thin-film materials, it is critical to understand the thermodynamics and the kinetics governing the relaxation. Here we report the experimental results indicating that the polymer-solid interfacial interaction, which does not have a bulk counterpart, plays a key role in both thermodynamic driving force and kinetics of the relaxation. Two Perfluoropolyethers (PFPEs) with the same backbone and different endgroups, one polar and the other non-polar, were coated on a hydrophilic substrate and the relaxation of the polymer films were monitored by contact angle testing. The result shows that the PFPE with polar endgroups relaxes with time and the relaxation time constant, obtained from Kohlrausch-Williams-Watts (KWW) model, is ten orders of magnitude higher than that of bulk polymer. However, the PFPE with non-polar endgroups does not show relaxation behavior. Based on the experimental results, we proposed that the relaxation is driven by the attractive interaction between the polar endgroups of the polymers and the polar sites on the substrate. The very slow kinetics of the relaxation has been attributed to the heterogeneity of the polymer-solid interaction at the interface and the cooperative nature of the molecular motions in the relaxation.
9:00 PM - RR4.6
Chemical Changes Underlying Aging of Silica in Nanoscale Frictional Contacts.
Yun Liu 1 , Izabela Szlufarska 1
1 Materials Science & Engineering Department, University of Wisconsin - Madison, Madison, Wisconsin, United States
Show AbstractFriction and adhesion of silica are important both in naturally occurring phenomena, such as shallow tectonic earthquakes, as well as in engineering applications, including wafer bonding for device applications. Since water is often ubiquitously present in the environment and because of the high reactivity of silica, water can dominate the tribological response of silica-silica interface. Here we focus specifically on aging of silica, which is a fascinating phenomenon that is dramatically accentuated by the presence of humidity and is relevant for understanding of earthquakes. Aging refers to the process by which static friction changes (generally increases) with time when the two surfaces are held still. One of the two main hypotheses for physical mechanisms of aging is that it results from chemical changes at the silica interface, which may include dissolution of silica, reorganization of H-bond network in confined water, and incorporation of water into silica. In this work we will discuss chemical reactions that can lead to aging of silica in aqueous environments. Using ab initio calculations based on the Density Functional Theory and molecular dynamics simulations based on ReaxFF reactive force field we determined rates of relevant reactions and identified processes that underlie the experimentally observed logarithmic dependence of aging on hold time. The effect of the interfacial chemistry on adhesion will also be discussed.
9:00 PM - RR4.7
Defects in a Block Copolymer Photonic Gel.
Yin Fan 2 , Jae-Hwang Lee 1 , Steven Kooi 3 , Alfredo Alexander-Katz 1 , Edwin Thomas 1 3
2 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Institute for Soldier Nanotechnology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractBy selectively swelling one block in a lamellar forming diblock copolymer, we successfully made a tunable and responsive photonic gel. Applications include sensing, active display etc. It is of interest to study the swelling process for better control over the gel's properties, e.g. rate of response as well as robust behavior. Fast-optical reflectivity spectra during the swelling process show variations among samples made with the same material and the same processing conditions. The real-time swelling behaviors can be categorized into two types: fast response with low final swelling ratio, or slow response and high swelling ratio. We propose two swelling profiles to explain the difference: type A is of a uniform swelling profile and type B a gradient swelling profile. The two types of swelling profile can be further explained with regards to the types of defects and the defect density in the layered structure. Physical models linking the defect structure and the swelling properties are developed.
9:00 PM - RR4.8
Polymer Globule Dynamics in the Presence of Surfaces and Shear Flows.
Charles Sing 1 , Alfredo Alexander-Katz 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHomopolymer globules have long been considered important as model systems for the study of proteins, with the understanding of globular polymers represents the origins of many theoretical treatments of protein behavior. In particular, recent work has shown that the quaternary structure of the blood protein von Willebrand Factor (vWF) is well represented by these simple models, especially in understanding its response to external fluid flow stimuli. vWF is activated during blood clotting, where it adsorbs readily to surfaces under high shear conditions. Our research seeks to understand this counterintuitive adsorption behavior by investigating through theory and Brownian Dynamics simulation the interaction between a collapsed polymer globule and an attractive surface in the presence of shear flow. These investigations reveal a rich interplay between hydrodynamic, cohesive, and entropic effects that ultimately determine the dynamics of a collapsed homopolymer in shear flow. This allows for a more nuanced understanding of the structural and chemical features governing vWF behavior, and more generally will provide insight into methods to manipulate homopolymer adsorption using fluid flows.
9:00 PM - RR4.9
Resonating UHF Study on Electron Correlation in a Ground State of Two Electrons Confined in 2D Quantum Dot.
Takuma Okunishi 1 , Kyozaburo Takeda 1
1 , Waseda university, Tokyo Japan
Show AbstractSemiconductor quantum dots (QDs) are meaningful system to deepen our inherent understandings on the electronic characteristics. When electrons are confined in such nano-meter sized QDs, the competition with a Coulomb force and a confinement potential is crucial, and the inclusion of the electron correlation is requested in the theoretical consideration. In this study, we reconsider two electrons confined in a 2D square QD, because this system is simple but is critical for fundamental considerations of electron correlation. In order to include the electron correlation, we employ the resonating UHF (res-UHF) method, where the configuration interaction (CI) is taken into account through the multi-reference description of many-electron wave functions. This method is straightforward to approximate a many-body wave function by the superposition of the nonorthogonal Slater solutions. An UHF approach might give an inconsistent ground state due to neglect of an electron correlation. However, res-UHF approach consistently corrects with reverting the symmetry nature in Hamiltonian. This res-UHF approach is also powerful because an inclusion of only 9 UHF scf-solutions is sufficient for describing the singlet ground state within an accuracy of 2% in the total energy. This is because these UHF scf-solutions are nonorthogonal mutually but represent the conceivable electron-spin configurations rationally. We further combine this res-UHF approach with solving the time-dependent (TD) equation, and study how an electron correlation causes a quantum fluctuation in determining the ground system. The TD approach demonstrates that an electron correlation induces a resonative vacillation between two energetically degenerated UHF ground states. The FFT analysis further reveals that the "bonding and anti-bonding" energetically separation between these two UHF ground states determines the vacillation frequency of the quantum fluctuation. The transitions into other UHF scf-states cause a characteristic beating in the electron-correlated quantum fluctuation.
Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR5: Defect and Oxygen Dynamics near Interfaces II
Session Chairs
Elizabeth Dickey
Blas Uberuaga
Tuesday AM, November 29, 2011
Commonwealth (Sheraton)
9:30 AM - **RR5.1
Point Defect Dynamics at BaTiO3 – Electrode Interfaces.
Elizabeth Dickey 1 , Matthew Burch 1 , Clive Randall 2 , Malay Samantaray 2
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractThe migration of charged point defects under applied voltage bias is a phenomenon that can lead to long-term degradation of oxide devices or which can be utilized to create unique device functionalities. This talk discusses point defects and interfacial reactions in BaTiO3 capacitors as a function of processing history and voltage biasing. The relationship between local microchemistry and electrical transport is studied through a combination of transmission electron microscopy and electrical transport measurements, and it is found that the local microchemistry at the electrode interfaces is particularly important for the device leakage current. Low oxygen activities at this interface can lead to interfacial reactions and/or high concentrations of oxygen vacancies, both of which lower the interfacial resistance.The BaTiO3 - electrode interface becomes increasingly influential on the overall electronic and ionic transport as the dielectric layers are scaled down in thickness. In particular, porosity and interfacial roughness lead to field concentrations, which increase electrode-interface leakage and can accelerate the point defect redistribution kinetics. Scaling laws are studied by finite element modeling in which three-dimensional microstructures obtained by FIB/SEM imaging are used as input models. The scaling behavior and relative contributions of electrode porosity versus electrode roughness will be discussed. Finally, processing strategies for controlling interfacial properties at small layer thicknesses will be presented. This work has been supported by the National Science Foundation Ceramics Program through grant No. 0606352.
10:00 AM - RR5.2
Tuning the Limiting-Thickness of a Thin Oxide Layer on Al (111) with Oxygen Gas Pressure.
Na Cai 1 , Guangwen Zhou 1 , Kathrin Mueller 2 , David Starr 2
1 Department of Mechanical Engineering & Multidisciplinary Program in Materials Science and Engineering , SUNY-Binghamton, Binghamton, New York, United States, 2 Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractThe oxidation of metal surfaces is of great importance for a wide range of technological applications including heterogeneous catalysis, electrochemistry, corrosion, gate oxides, and lubrication. Specifically, aluminum oxide films exhibits a variety of unique properties including a large dielectric constant (~10), a large barrier height for electron tunneling (~2eV), high corrosion resistance, good thermal and mechanical stability, and good adhesion to the underlying metal. Amorphous aluminum oxide films, formed by low-temperature oxidation of aluminum, fulfill the requirements for these applications because of their better structural perfections and better interface at the oxide-metal interface. A generic model describing low-temperature oxide film growth is the Cabrera-Mott model. According to this model, an electric field is formed across the oxide film due to the potential difference (called the Mott Potential) between the metal/oxide work function and the oxygen/oxide work function as a result of electron tunneling between the Fermi level of the parent metal substrate and acceptor levels of chemisorbed oxygen at the surface. The self-generated electric field reduces the energy barrier for the migration of ions through the oxide film (the limiting step for mass transport in oxidation), leading to rapid initial oxidation rates at low temperature. As the tunneling current diminishes with increasing oxide film thickness, the oxidation virtually stops at a limiting-thickness. We report an x-ray photoelectron spectroscopy (XPS) study of the oxidation of Al(111) surfaces at room temperature. Our results reveal that the actual value of the self-generated electrostatic potential (designated as the kinetic potential) can deviate significantly from the Mott potential and is tunable by varying the oxygen pressure during oxidation providing control of the limiting thickness of the oxide film. We found that a significantly large oxygen gas pressure (> 1 Torr) is required to form sufficient adsorbed oxygen at the oxide surface to accept the tunneling electrons in order to develop the saturated kinetic potential and therefore the maximum limiting thickness of the oxide film. At lower oxygen pressures, the lower coverage of oxygen anions leads to a kinetic potential of lower magnitude and therefore a thinner limiting-thickness of the oxide film.
10:15 AM - RR5.3
Precursor to the Onset of the Bulk Oxidation of Cu(100).
Liang Li 1 , Xi Mi 2 , Yunfeng Shi 2 , Guangwen Zhou 1
1 Mechanical Engineering, SUNY-Binghamton, Binghamton, New York, United States, 2 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractThe oxidation of metals plays a critical role in many important technological processes, such as corrosion, chemical catalysis, fuel reactions, and thin film processing. The general reaction sequence of the oxidation on a clean metal surface is thought to proceed as oxygen surface chemisorption, oxide nucleation and growth, and then bulk oxide growth. Much is known about the oxygen surface chemisorption, and particularly, the atomic structure of the chemisorbed phases on metal surfaces. However, the atomistic processes leading to the crossover between oxygen chemisorption and the nucleation of bulk oxide phase are still quite unexplored, although this information is critical for developing a complete understanding of the mechanism of metal oxidation. Using density-functional theory, we investigate the energetics of oxygen sub-surface adsorption governing the onset of bulk oxidation of Cu(100) terrace. Our results reveal that the presence of domain boundaries formed from merged (2√2×√2)R45°-O oxygen chemisorbed nanodomains mismatched by a half unit-cell leads to the preferred oxygen adsorption at the sub-surface tetrahedral sites. The resulting Cu-O tetrahedron resembles strikingly that of the bulk oxide phase of Cu2O and facilitates the subsequent oxide growth along the domain boundary. The insights and approach obtained from this study can be extended to understand the transient oxidation of many other metals, where oxygen-chemisorption induced surface restructuring generally occurs, but its effect on initiating the onset of the bulk oxide formation has been hitherto rarely been addressed.
10:30 AM - RR5.4
Passivity Breakdown and Pitting Nucleation of Copper Oxide Films under Aqueous Conditions: Simulations by Reactive Molecular Dynamics.
Byoungseon Jeon 1 , Subramanian Sankaranarayanan 2 , Adri van Duin 3 , Shriram Ramanathan 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, Pennsylvania, United States
Show AbstractPassive oxide films formed on metal or alloy surfaces play deterministic role on the surface chemistry and properties such as corrosion, wear resistance and so forth. The chemical and electronic nature of these oxide films are therefore of significance and how they are perturbed in realistic environments containing reactive elements such as halide ions in aqueous media need to be understood. For example, under aqueous conditions, like marine environment or water-based electrolyte, oxide films are degraded by corrosion, yielding passivity breakdown. Once pitting is nucleated, bare metals are contacted by corrosive elements and corrosion is accelerated. Protection and corrosion control is of great importance but understanding the atomistic scale mechanism is not done yet completely.Employing ReaxFF reactive force-fields, we conduct simulations on interaction of technologically relevant metal such as copper with water containing chlorine ions. Under aqueous conditions with chlorine ions, the behavior of copper and its dissolution is analyzed. Chemical thinningof oxide films by chlorine is observed while surface condition plays a key role on initial adsorption of chlorine. Pitting is nucleated when chemical thinning reaches the inner bulk substrate, yielding high stresson metal substrates.
10:45 AM - RR5.5
The Mechanical and Chemical Behavior of the Thin Oxide Skin on a Liquid Metal.
Ju-Hee So 1 , Mohammad Khan 1 , Michael Dickey 1
1 Chemical and Biomolecular Engineering, NC State University, Raleigh, North Carolina, United States
Show AbstractWe describe the mechanical and electrochemical behavior of the thin oxide skin that forms on a liquid metal alloy and discuss ways to modify these properties. The alloy, eutectic gallium indium, is useful for moldable microelectrodes, stretchable antennas, soft diodes and flexible solar cells. The ability to micromold the metal for these applications is enabled by the mechanical properties of a thin oxide skin that forms spontaneously on its surface. The oxide skin is elastic and yields under a critical stress, at which point the metal flows. We studied the mechanical modulus of the oxide skin under different environments using a rheometer with a parallel-plate geometry. The modulus of the skin can be tuned based on the chemical environment surrounding the metal and the physical hysteresis of processing. We demonstrate that water and acid can lower the modulus of the skin relative to that in air. Depositing materials on the skin offers a way to increase the modulus of the oxide skin. We found that certain polymers deposit spontaneously from solution on to the oxide skin. The modulus increases with time as it is exposed to the polymer solution (on the time scale of hours) and then levels off at a value that is two orders of magnitude higher than the native oxide skin. The kinetics of the deposition process can be correlated with the change of modulus. This technique offers a new, in situ method of studying film deposition kinetics and measuring thin film mechanics. We use electrochemistry to detect the physical breakage of the oxide skin since the exposed metal spontaneously oxidizes and generates current. Electrochemical oxidation or reduction of the skin that forms on the metal can also be used to change the thickness of the native oxide skin. These studies provide new tools to tune the modulus of the moldable liquid metal and engineer the forces under which the metal will flow.
11:30 AM - **RR5.6
The Competing Effects of Grain Boundaries on the Radiation Damage Response of Cu.
Blas Uberuaga 1
1 , Los Alamos Natl Lab, Los Alamos, New Mexico, United States
Show AbstractIt is well accepted that grain boundaries serve as effective sinks for radiation-induced defects (interstitials and vacancies). However, fundamental insight into the atomic-scale origin of this behavior is still lacking. The promise of engineering materials with interfaces to meet the radiation tolerance demands of future nuclear applications requires that the origin of this enhanced radiation tolerance be understood. We use molecular dynamics, temperature accelerated dynamics, and molecular statics to study radiation damage phenomena near a variety of grain boundaries in Cu over three different temporal regimes: the short-time damage production phase of a collision cascade, the longer-time scales over which defect annihilation and aggregation occur, and the thermodynamic-limiting behavior of the system. We find that both the production and the subsequent annealing of the radiation-induced defects are modified significantly by the presence of the grain boundary. In particular, we identify a new mechanism by which interstitials efficiently annihilate vacancies, promoting enhanced defect recombination. We compare to previous experimental results and identify three regimes over which different thermally activated processes are active, resulting in different responses, both better and worse than large-grained counterparts, of the material to irradiation. Our results show that nanostructured materials have a very sensitive response to irradiation and offer new insights into the design of radiation tolerant materials.
12:00 PM - RR5.7
Dynamic Propensity at Interfaces in a Radiation Environment.
Walid Mohamed 1 , Xiaojun Mei 1 , Jacob Eapen 1
1 Nuclear Engineering, NC State University, Raleigh, North Carolina, United States
Show AbstractRadiation tolerance is generally associated with the ability of a material to withstand the deleterious effects of radiation-induced phenomena such as amorphization, point-defect clustering, vacancy induced cavitation, swelling and precipitation of new phases. Following a collision cascade, the freely migrating point defects escape the cascade zone and contribute to the long-time, ‘extended’ defect structures in conventional materials. The accumulation of defects is regarded as a competition between defect formation by cascades, thermal fluctuations, interaction between the defects, and the transport or migration of defects.Several experimental studies and simulations show that nanocrystalline materials and interfaces, in general, can exhibit radiation tolerant properties. Transport of point defects to grain boundaries and interfaces (followed by absorption) is considered to be a central mechanism for radiation tolerance in such materials. The essential role of the interfaces is to act as an impediment to the growth of ‘extended’ defects. Using molecular dynamics simulations, we analyze the dynamical heterogeneity in nanocrystalline Cu following radiation. We quantify the radiation dynamics through directional diffusivities and dynamical propensity which is defined as the displacement of an atom averaged over an isoconfigurational ensemble. These measures allow a quantitative measure of migration of atoms following radiation and the dynamics at grain boundary interfaces that can potentially influence radiation tolerance in materials.
12:15 PM - RR5.8
Simultaneous Reduction of Radiation-Induced Defect Concentrations and Fluxes Using Interfaces with Precisely Controlled Sink Strengths.
Michael Demkowicz 1 , Richard Hoagland 2 , Blas Uberuaga 2 , Amit Misra 3
1 Department of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 MST-8: Structure-Property Relations Group, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Center for Integrated Nanotechnologies , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractWe use a reaction-diffusion model to demonstrate that solid-state interfaces in polycrystalline composites may simultaneously reduce the concentration and flux of radiation-induced defects, given a sufficiently high interface area per unit volume. The effect of interfaces on defect concentration is shown to be extremely sensitive to interface sink strength, η. These findings open an opportunity for mitigating both radiation-induced segregation, which depends on defect flux to interfaces, and radiation-induced swelling and hardening, which depend on defect concentration.This material is based upon work supported as part of the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number 2008LANL1026.
12:30 PM - RR5.9
The Effect of the Water-Silica Interface on Enhanced Hydronium Ion Formation and Proton Transport in Mesoporous Silica.
Glenn Lockwood 1 , Stephen Garofalini 1
1 Materials Science & Engineering, Rutgers, the State University of New Jersey, Piscataway, New Jersey, United States
Show AbstractElectrochemical studies have shown enhanced proton transport in mesoporous silica containing water, but modeling the kinetic processes underpinning this phenomenon requires molecular simulations which can capture the structural complexity of mesoporous silica as well as the chemical fidelity required to reproduce the reactive and dissociative properties of water molecules at interfaces. To this end, we have employed a dissociative water potential that matches many properties of bulk and nanoconfined water as well as the chemisorptive behavior of water at the silica surface, and we have simulated the enhanced formation of hydronium ions at these surfaces and shown behavior consistent with ab initio molecular dynamics simulations.In addition to showing the formation of surface silanol (SiOH) sites where protons are strongly bound, our simulations have revealed additional weakly binding proton adsorption sites on the silica surface. These sites contribute to enhanced proton transport beyond that observed in the nanoconfined water phase alone. The silica surface also affects the transport properties of molecular water, and the magnitude of this effect in nanoconfined geometries has been found to be dependent on both distance from the interface and the curvature of the confining silica surface. Thus, the enhanced proton conduction in hydrated mesoporous silica is affected by surface chemistry, topology, and curvature, and the details of our findings will be discussed.
12:45 PM - RR5.10
Thermodynamics, Mechanics, and Dynamics Properties of Water and Ions Confined in Nanopores: Application to Cement Hydrates.
Patrick Bonnaud 1 , Krystyn Van Vliet 1 , Benoît Coasne 4 , Roland Pellenq 2 3
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 4 Institut Charles Gerhardt Montpellier, CNRS and Université Montpellier, Montpellier France, 2 Civil and Environmental Engineering, MIT, Cambridge, Massachusetts, United States, 3 CINaM, CNRS and Aix-Marseille Université, Marseille France
Show AbstractHydrated counter-ions in a suspension or paste of charges colloids play a central role in the overall system’s stability and thermodynamics. For instance Ca2+ ions in a cement paste that can be seen at early ages as an aqueous colloidal system are inherent part of the structure. They compensate the substrate charge (due to local chemical/substitution defects in cement hydrate ribbon-shaped particles also named as C-S-H with C = CaO; S = SiO2; H = H2O). C-S-H particles combine to each other to form brick-shaped particles. Cement paste is a disordered assembly of these particles which are maintained together thanks to electrostatic forces. Because of the different scales involved in the formation of cement pastes, this material is a multiscale porous material with a wide range of pore sizes. Depending on the ambient conditions, the cement paste can encounter mechanical damage due to the presence of the confined fluid (e.g. cryo-suction effects in freezing conditions which are the consequence of thermodynamic equilibrium between solid-like and liquid-like water inside the pore). In this work, the thermodynamics, mechanics and dynamics properties of the electrolyte (Ca2+ ions and water) in the pore are studied to understand such effects and the influence of ions using classical simulation techniques (Grand Canonical Monte Carlo and Molecular Dynamics). Several models of pores are considered to study the role of chemistry and surface roughness: silica (hydroxylated and with Ca charge-compensating ions) and cement. The substrates were atomistically described and three pore sizes were considered (1, 2 and 4 nm) at various temperatures ranging from 175 K to 300 K.The adsorption/desorption isotherms of water at room temperature in the different pore sizes were first estimated. A very hydrophilic behaviour is observed in all studied systems where water can adsorb and for the larger pores, condensate into liquid at pressure lower than the bulk saturating pressure while it is in the gas phase in the outside Grand Canonical reservoir. Solvation pressures inside these pores are computed via the virial expression on a series of GCMC configurations. A drop of the solvation pressure at the capillary condensation is observed for silica system with hydroxylated surfaces when a different behaviour arises for systems with a charged surface compensated by calcium counterions and C-S-H. In all these hydrophilic and super hydrophilic mesopore models, confined water is strongly perturbed in the vicinity of the pore surface even at low temperatures. Water behaves like in the bulk in the core region of the pores. Finally, the MD trajectories show that the stronger the confinement, the slower the dynamics of water molecules and ionic species. Surface chemistry and surface roughness play an important role in the slowing down of the electrolytic film. Characteristic residence times were derived for water molecules in the adsorbed layer located close to the pore.
RR6: Dynamics in Porous Media
Session Chairs
Martin Mueser
Izabela Szlufarska
Tuesday PM, November 29, 2011
Commonwealth (Sheraton)
2:30 PM - RR6.1
Electrophoretic and Electroosmotic Flow through Carbon Nanotube Membranes as Chemical Pumps.
Ji Wu 1 , Bruce Hinds 1
1 Chemical and Materials Engineering, Univ. of Kentucky, Lexington, Kentucky, United States
Show AbstractCarbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. In general, the transport mechanisms through CNT membrane are a) ionic diffusion is near bulk expectation with no enhancement from CNT b) gas flow is enhanced by ~1-2 order of magnitude due to specular reflection off of flat graphitic surface c) and pressure driven flux of a variety of solvents (H2O, hexane, decane ethanol, methanol) are 4-5 orders of magnitude higher than conventional Newtonian flow [Nature 2005, 438, 44] due to atomically flat graphite planes inducing nearly ideal slip conditions. Nearly all applications require chemical selectivity in what is allowed to pass across the membrane. However the act of placing selective functional chemistry at pore entrance or along the core of CNTs, dramatically/completely eliminates the enhanced flow effects by eliminating the near perfect slip boundary condition[ACS Nano 2011 5 3867]. Needed is a mechanism to pump chemicals through the pore where selective chemistry is. This is routinely achieved in protein channels where permeates are accelerated through regions of precise functionality. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow. Through diazonium electrochemical modification we have successfully bound anionic surface charge to CNT tips and along CNT cores. High electro-osmotic flows of 0.16 cm/s-V at are seen by the pumping of neutral caffeine molecules. Improvements in electroosmotic power efficiency of 25-112 fold are seen in CNTs compared to conventional nanoporous materials with atomically rough interfaces [RCS Nanoscale 2011 DOI: 10.1039/c1nr10303b]. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices is discussed with the voltage gated delivery of clonidine and nicotine across CNT membrane at therapeutically useful fluxes [Proc. Nat. Acad. Sci. 2010 107(26) 11698-11702]. In small diameter SWCNTs ion mobilities are seen to be ~6 fold enhanced due to induced electroosmotic flow.
2:45 PM - RR6.2
Charge Transport in Confined Ionic Liquids.
Joshua Sangoro 2 1 , Ciprian Iacob 2 1 , Friedrich Kremer 2 1
2 Institute of Experimental Physics I, University of Leipzig, Leipzig Germany, 1 Molecular Physics, University of Leipzig, Leipzig Germany
Show AbstractCharge transport and glassy dynamics in several classes of ionic liquids confined in nanoporous silica membranes with mean pore diameters ranging between 4 nm and 15 nm are investigated in a wide frequency and temperature range by a combination of Broadband Dielectric Spectroscopy (BDS), Fourier Transform Infrared spectroscopy (FTIR), and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). Remarkable enhancement of the ionic mobility by more than two orders of magnitude is traced down to changes in the conformation of the ions as a result of the density reduction accompanying two-dimensional confinement. In addition, it becomes possible to determine the resulting change in density of ionic liquids in nanopores from dielectric spectra in quantitative agreement with recent molecular dynamics simulations. For some ionic liquids, slower ionic mobility is observed. This trend is fully reversed upon silanization as proved by FTIR spectra, proving the significant role of guest molecule-host membrane interaction. Technological implications of the observed confinement effects on transport properties of ionic liquids will be discussed.
3:00 PM - RR6.3
Controlling Wettability of Nanoporous Materials - A Novel Technique of Liquid Actuation.
Yu Qiao 2 1 , Weiyi Lu 1 , Taewan Kim 2
2 Program of Mater. Sci. Eng., UCSD, La Jolla, California, United States, 1 Dept of Struct. Eng., UCSD, La Jolla, California, United States
Show AbstractWith its ultra-large specific surface area, a nanoporous material is an ideal, yet relatively unexplored, platform for liquid actuation, with potential performance gains typical of disruptive technologies. Our preliminary results indicate dramatically improved performance not attainable by conventional intelligent materials. In a nanostructured solid material, due to the lack of control of solid-solid interaction, how to fully utilize the large interface area usually imposes tough technical challenges. Our recent exploratory study provides a novel way to circumvent this problem: When a liquid phase, with the motion being controlled by voltage or heat, is confined in nanopores, the ultralarge nanopore surface exposed to the liquid can greatly amplify the beneficial surface processes, such as the thermocapillary and the electrocapillary effects. As the solid-liquid interface energy is controlled by temperature or potential difference, the effective wettability at nanopore inner surfaces can vary considerably, which triggers the liquid molecules and ions to "flow" into or out of the nanopores. Since this process can be reversible, the system exhibits a volume memory characteristic. Due to the ultralarge value of surface area, the output energy density is order-of-magnitude higher than that of conventional intelligent materials. The deformability of the system is dominated by the porosity, leading to an effective maximum deformation of 20-40%.
3:15 PM - RR6.4
Comparison between the Phase Transition of Physically Confined Cyclohexane and 1-Decanol Inside Nano Porous Silica.
Samuel Amanuel 1 , Hillary Bauer 1 , Alexandrea Safiq 1 , Jargalsaikhan Dulmaa 1 , Amer Khraisat 1
1 Physics and Astronomy, Union College, Schenectady, New York, United States
Show AbstractThe linear relationship between melting (and freezing) temperatures and inverse physical size is explained qualitatively by the Gibbs-Thompson equation. It is not clear, however, how the other thermodynamic parameters, such as delta H and surface energy, change (or remain invariant) with physical size. We studied the phase transition of cyclohexane when it is confined in nano porous silica using a differential scanning calorimeter (DSC). Our experimental data revealed the same trend between the melting temperature and physical size as predicted by the Gibbs-Thompson equation and previous studies. The apparent delta H of the transition, however, seemed to vary with physical size, where cyclohexane confined in smaller pores has smaller apparent delta H. While this is in agreement with previously reported experimental studies, the change in delta H with physical size is in contradiction with the assumptions in the Gibbs-Thompson equation. Furthermore, it cannot be explained due to the functionality of the specific heat capacity with respect to temperature. In order to reconcile this seeming contradiction, we have made the assumption that there are layers of cyclohexane molecules at the interface that do not partake in the phase transition and, therefore, do not contribute to the measured heat of fusion. A correction in the mass of the participating sample to account for these molecules yields physical size invariant delta H. From this analysis, the thickness of the layers can also be approximated (ca. 2.14 ± 0.2 nm). Our findings further revealed that these molecules have higher density than the core molecules, which undergo phase transition. This suggests that the molecules at or near the interface tend to pack more closely. Similar analysis of physically confined 1-decanol revealed that its melting temperature and apparent delta H show the same trend as what was observed in physically confined cyclohexane. The estimated thickness of the nonparticipating 1-decanol molecules, however, is smaller (ca. 0.9 ± 0.2 nm). In addition, the density of the 1-decanol molecules at or near the interface is smaller than the density of the molecules in the core. These two results are surprising considering that 1-decanol is a larger and more polar molecule than cyclohexane. A plausible explanation is the buckling of the alkane in 1-decanol. While it is very likely that the OH groups in 1-decanol form hydrogen bonds with the OH groups in the silica, it is also conceivable that the molecules do not stand erect, given the limited rigidity of the alkane and the curvature in the silica pores. This possible buckling could preclude other decanol molecules from occupying the adjacent space and thereby creating pseudo voids. This, in turn, could reduce the density of the interfacial molecules.
3:30 PM - RR6.5
Observation of Surface-Charge-Induced Overlimiting Current in Porous Materials.
Daosheng Deng 1 , Vicki Dydek 1 , Ali Mani 1 , Sven Schlumpberger 1 , Martin Bazant 1
1 Chemical engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractSalt transport in bulk electrolytes occurs by diffusion and convection, but in microfluidic devices and porous media, the presence of charged side walls leads to additional surface transport mechanisms, surface conduction and electro-osmotic flows, which become more important as the bulk salt concentration decreases. As a result, it is possible to exceed the diffusion-limited current to a membrane or electrode. In this work, we present experimental observations of over-limiting current to an ion-exchange membrane through a porous glass frit with submicron pores. The current-voltage relation and approximately constant over-limiting conductance agree well with theoretical predictions, and high current behavior is consistent with the propagation of “desalination shocks”. We also demonstrate the continuous extraction of depleted solution from the frit near the membrane under conditions of over-limiting current. The advantage of working with porous media rather than microfluidic devices is that we can cheaply and easily “scale up” this phenomenon to macroscopic volumes for practical applications to water desalination and purification.
4:15 PM - RR6.6
Conversion Reactions in Surface-Functionalized Mesoporous Materials with Various Catalytic Site Distributions: Influence of Restricted Passing of Reactants and Products within Pores.
Jing Wang 1 , David Ackerman 1 2 , Kapil Kandel 1 2 , Igor Slowing 1 , Marek Pruski 1 2 , Jim Evans 1 3
1 Ames Laboratory USDOE, Iowa State University, Ames, Iowa, United States, 2 Chemistry, Iowa State University, Ames, Iowa, United States, 3 Physics & Astronomy, Iowa State University, Ames, Iowa, United States
Show AbstractFunctionalized mesoporous materials can integrate the selectivity of homogeneous catalysts with the stability/separability of heterogeneous catalysts. Control of surface properties is achieved by multi-functionalization, some anchored groups serving as catalysts, others modifying selectivity or activity. The distribution of functional groups inside the pores depends on the synthesis approach. While nominal pore diameters, e.g., for mesoporous silica are 2 nm or above, significantly functionalization can reduce effective diameters below 2 nm. In this case, transport of reactants and products within pores (and, critically, their ability to pass each other) can be severely inhibited. We apply statistical mechanical modeling to explore the dependence of reactivity and related properties on the extent of inhibited passing in this quasi-single-file transport regime [1,2]. This analysis is performed for various distributions of catalytic sites. The key requirement is an accurate description of chemical diffusion in these quasi-single-file reaction-diffusion systems. This is achieved by adapting a “hydrodynamic” transport theory to incorporate an effective tracer diffusion coefficient which is enhanced near pore openings. [1] D.M. Ackerman, J. Wang, J.H. Wendel, et al., J. Chem. Phys. 134 (2011) 114107.[2] D.-J. Liu, J. Wang, D.M. Ackerman, I.I. Slowing, et al., ACS Catalysis 1 (2011) 751.
4:30 PM - RR6.7
Saltwater Desalination across a Nanoporous Graphene Membrane.
David Cohen-Tanugi 1 , Jeffrey Grossman 1
1 Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe describe an innovative approach for water desalination based on nanoporous graphene. This approach represents a promising step towards addressing global water scarcity in an energy-efficient manner. By introducing nanoscale pores in the structure of a graphene layer, it becomes possible to use graphene as a filtration membrane, in order to effectively separate out salt ions and impurities and produce a clean water supply. Potential advantages of graphene over existing nanofiltration materials include negligible thickness (one atomic layer) and high mechanical strength, which may enable faster water transport and higher operating pressures than previously possible.In order for such a concept to become a practical reality, a number of fundamental questions must be answered first. Using Molecular Dynamics simulations, we have investigated the theoretical behavior of a saltwater-graphene system under applied pressure. We find that an effective size exclusion mechanism prevents Na+ and Cl- ions and their hydration shells from passing through the confining pores at sufficiently small pore diameters. The pore diameter and the chemical interactions at the water-membrane interface thus become the most important criteria for effective desalination performance.Our key results regarding the effect of system properties (pore morphology, surface chemistry, applied pressure, etc.) on desired performance characteristics such as ion selectivity, maximal water flux and energy requirements will be presented.
4:45 PM - RR6.8
Regulating Solute Transport Using Nano-Structured Surfaces.
Hassan Masoud 1 , Alexander Alexeev 1
1 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractWhen a colloidal suspension flows in a microfluidic channel, the competition between diffusion and hydrodynamic effects sets the spatial distribution of the colloids. While the intensity of diffusion is governed by the physical properties of the mixture which is fixed for a given process, the forces due to hydrodynamic interactions with channel walls can be modified by altering the local surface topography. Using dissipative particle dynamics, we demonstrate that nanoscopic posts decorating channel surfaces can be used to regulate the density distribution of colloidal suspensions. We consider a mixture of nanoparticles and polymer chains and show that depending on the flow velocity patterned surfaces can either hydrodynamically attract or repel nanoscale objects suspended in the flowing fluid. Furthermore, we demonstrate that such patterned surfaces can effectively separate colloid-polymer mixtures.
5:00 PM - RR6.9
On the Influence of Interlayer Water Intercalation on the Global Mesoporous Water Transport in a Clay: X-Ray Studies.
Henrik Hemmen 1 , Lars Alme 1 , Jon Otto Fossum 1 , Yves Meheust 2
1 Physics, NTNU, Trondheim Norway, 2 Geosciences Rennes, Université de Rennes 1, Rennes France
Show AbstractThe swelling of layered smectite clay particles causes changes in the interlayer repetition distance (d-spacing) as a function of temperature and humidity. For the synthetic clay sodium fluorohectorite, hydrodynamically stable hydration states with zero, one and two intercalated monolayers of water have previously been reported, with discrete jumps in d-spacing at the transitions between the hydration states. Keeping the temperature fixed and varying the ambient relative humidity, we find small reproducible d-spacing changes also within the pure hydration states. These changes are monotonous as a function of relative humidity, and one order of magnitude smaller than the shift in d-spacing that is typical of the transition between two hydration states. The reproducibility and reliability of this relative humidity controlled d-shift enables us to use the interlayer repetition distance d as a measure of the local humidity surrounding the clay particles in a powder.We provide an example of an application of this observation: By imposing a humidity gradient along the length of a 1 mm thick glass capillary filled with clay powder, and using x-ray diffraction to record the average d-spacing within scattering volumes along the capillary, we are able to extract profiles of the relative humidity vs. time along the sample length. The observed time evolution describes the transport of water through the mesoporous space inside the clay powder. The density of water inside the interlayer space is liquid-like, implying that the clay particle swelling acts as a very effective and strong sink for the diffusing humidity in the mesopores once a change in hydration state has started occurring. As the humidity only enters our capillary from one side (i.e. quasi one-dimensional diffusion), water molecules are found in limited amount in the region where intercalation occurs, and thus leads to a quantifiable slowing down of the global diffuse transport in the mesoporous powder.
5:15 PM - RR6.10
Modeling the Transport of Nanoparticle-Filled Binary Fluids through the Porous Medium: A Lattice Boltzmann Approach.
Yongting Ma 1 , Amitabh Bhattacharya 1 , Olga Kuksenok 1 , Dennis Perchak 2 , Anna Balazs 1
1 Chemistry engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , Kodak Research Laboratories, Rochester, New York, United States
Show AbstractUnderstanding the transport of multi-component fluids through the porous medium is of great importance for a number of technological applications, ranging from ink jet printing, the production of textiles and enhanced oil recovery. The process of capillary filling is well understood for a single-component fluid, however much less attention has been devoted to the behavior multi-phase fluids, and especially to nanoparticle-filled fluids, in porous media. Here, we examine the behavior of binary fluids containing nanoparticles that are driven by capillary forces to fill well-defined pores of porous material. To carry out these studies, we use a hybrid computational approach that combines the lattice Boltzmann model for binary fluids and a Brownian dynamics model for the nanoparticles. The hybrid model allows us to capture the interactions between the binary fluids and the nanoparticles, as well as model the interactions among the fluid, the nanoparticles and the pore walls. We show that the nanoparticles dynamically alter both the interfacial tension between the two fluids and the contact angle on the pore walls; this, in turn, strongly affects the dynamics of the capillary filling. We demonstrate that by tailoring the properties of the nanoparticles, such as their affinity to the fluid components and their interaction with the pore walls, we can effectively control not only the filling velocities, but also the deposition of nanoparticles on the pore walls. Our findings provide fundamental insights into the dynamics of this complex multi-component system, as well as potential guidelines for a number of technological processes that focus on capillary filling with nanoparticles in porous media and in microchannels with differing geometries.
5:30 PM - RR6.11
Transparent, Thermally Stable and Mechanically Robust Superhydrophobic Surfaces Made from Porous Silica Capsules.
Xu Deng 1 , Lena Mammen 1 , Yan Zhao 1 , Philipp Lellig 1 , Periklis Papadopoulos 1 , Klaus Muellen 1 , Chen Li 1 , Hans-Juergen Butt 1 , Doris Vollmer 1
1 , Max Planck Institute for Polymer Research, Mainz Germany
Show AbstractSuperhydrophobic surfaces are advantageous for a cost-effective main¬tenance of a variety of surfaces. The combination of micro and nano-sized roughness increases the contact angle of water such that water droplets cannot adhere but roll off.1,2 Therefore, superhydrophobic coatings are self-cleaning and anticorrosive. If the superhydrophobic surface were even transparent, the range of possible applications could be expanded to glass-based substrates such as goggles or windshields and, equally important, prevent an efficiency degradation of solar cells by pollution accumulation. Moreover mechanical robustness is also particularly critical because the dual scale roughness can easily be destroyed irreversibly leading to a rapid decrease of the contact angle and an increase of contact angle hysteresis. Herein,3 we present a simple method to fabricate a superhydrophobic coating based on porous silica capsules. The superhydrophobic coating shows a static contact angle of 160° and a sliding angle of less than 5°. Moreover it is thermally stable up to 350°C and shows excellent transparency. Its transparency is verified by applying the coating on organic solar cell devices. The coating does not diminish the light conversion efficiency. On the other hand, it will prevent fouling which decreases the efficiency of solar cells by more than 10% in course of time. Furthermore, the coating retains it’s superhydrophobicity under adhesion tape peeling testing and sand abrasion.3 [1]A. Lafuma, D. Quere, Nat. Mater. 2003, 2, 457.[2]M. D'Acunzi, L. Mammen, M. Singh, X. Deng, M. Roth, G. K. Auernhammer, H.-J. Butt, D. Vollmer, Faraday Discussions 2010, 146, 35.[3]X. Deng, L. Mammen, Y. Zhao, P. Lellig, K. Müllen, C. Li, H.-J. Butt, D. Vollmer, Advanced Materials 2011, DOI: 10.1002/adma.201100410.
5:45 PM - RR6.12
Imaging and Tracking Single-Walled Carbon Nanotube Dynamics in Rock-like Porous Media.
Shannon Eichmann 1 , Matteo Pasquali 1
1 Chemical & Biomolecular Engineering, Rice University, Houston, Texas, United States
Show AbstractSingle-Walled Carbon Nanotubes (SWNTs) have unique electronic and mechanical properties that have led to an increased interest for their use in a wide range of applications (i.e., composite materials, drug delivery, and electronics). Because of their extreme aspect ratio and slenderness (~1 nm diameter), SWNTs can increase the electrical conductivity of fluids more effectively than other particles. Moreover, under appropriate conditions, SWNTs precipitate into highly conducting chains that can span macroscopic length scales.1 Finally, because of their slenderness, SWNTs are expected to penetrate much tighter pores than isotropic particles of equivalent volume. Therefore, SWNTs can be useful in oil reservoirs as tools for sensing, targeting, and as contrast agents. Recently, we have used direct imaging of individual SWNTs by tagging with fluorescent dyes2 and individual semiconducting SWNTs by near-infrared (NIR) fluorescence3,4 to directly measure diffusion and bending dynamics and stiffness in aqueous media. We have also studied SWNT dynamics in agarose gels, a special case of porous media mimicking biological tissues and cells, and showed that pore size does not affect the rotational and translational diffusion, whereas stiffness slows down rotational diffusion.5 We now study the dynamics of individual SWNTs within a rock-like porous media as a model for oil reservoirs. The porous media was produced by packing concentrated colloidal silica particles between clean glass surfaces, where the packed layer is a few particles thick, allowing for optical imaging without index matching. We image directly SWNT motion within this porous media both by dye-induced visible and intrinsic near-infrared fluorescence. Results are presented for SWNTs undergoing Brownian motion and under externally imposed flow. In the absence of flow, reptation dynamics within the packed silica are measured and compared to that previously observed in agarose gels.5 We find that the lower porosity, large and fixed obstacles present in the silica pack versus that of agarose gels lead to slower and highly path dependent dynamics for SWNT motion. In both cases measured SWNT mobility is then compared to models for anomalous diffusion when objects encounter large obstacles based on bending energy and mean passage time.6,7 Ongoing work is focused on investigating the effects of SWNT surface functionalization toward controllable SWNT localization and formation of conductive SWNT networks for signaling, as well as, specific targeting of oil-water interfaces within porous media. 1.Kamat, P. V., et. al., (2004) JACS 126, 107572.Duggal, R. & Pasquali, M. (2006) PRL 96, 2461013.Weisman, R. B., et. al., (2004) App Phys A 78, 11114.Fakhri, N., et. al., (2009) PNAS 106, 142195.Fakhri, N., et. al., (2010) Science 330, 18046.Odijk, T. (1993) Macromolecules 26, 68977.Saxton, M. J. (1994) Biophys J 66, 394
Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR7: Soft/Bio Interfaces I
Session Chairs
Thomas Speck
Francesco Stellacci
Wednesday AM, November 30, 2011
Commonwealth (Sheraton)
9:30 AM - **RR7.1
Why Organic Surfaces with Nanostructured Composition Present Novel and Unexpected Properties?
Francesco Stellacci 1
1 Institute of Materials, EPFL, Lausanne Switzerland
Show AbstractIn this talk I will discuss results obtained in the last eight years in my research group that highlight the role of structure on interfacial properties of soft surfaces. In particular, I will describe how it is possible via self-assembly to generate surfaces that present an alternation of hydrophobic and hydrophilic group with a sub-nanometer critical wavelength. The presence of structure at a length scale that is molecular in nature provides these surfaces a series of unexpected properties. Namely, the interfacial energy of these surfaces shows a dependence -up to 20%- on surface structure, the solubility of the nanoparticles that carry these structured surfaces is consequently modulated. A plausible reason for these complex behaviors will be presented. Additionally we will show that these nanoparticles have a novel physical chemical driving force towards spontaneous fusion with lipid bilayers.
10:00 AM - RR7.2
Molecular Dynamics Study of the Effects of Nanoscale Surface Structure on the Interfacial Properties and Functionalities of Mixed-Monolayer Protected Nanoparticles.
Hao Jiang 1 , Francesco Stellacci 3 4 , Sharon Glotzer 1 2
1 Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Institute of Materials, Ecole Polytechnique Fédérale de Lausanne, Lausanne Switzerland, 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractRecent experiments have shown that surface structure has significant influence on the interfacial properties of substrates protected by mixed surfactant monolayers. Interestingly, the total work of adhesion of a monolayer-protected nanoparticle (NP) varies non-monotonically with the size of the striped surface domains of the hydrophilic and hydrophobic coating molecules. We investigated the physical mechanism of this non-monotonic behavior using molecular dynamics (MD) simulations. Our simulations revealed that the molecular organization at the interface is substantially affected by the sizes of the hydrophobic and hydrophilic domains on the NP surface. When the size of the hydrophobic domains is small ( < 1 nm), bridge-like hydrogen-bonded water structures are formed across these domains, stabilizing the water interface and increasing the water density in the regions immediately above the domains. At the same time, due to the entropic effect, water molecules are driven away from the hydrophilic domains as the domain size decreases, decreasing the interfacial water density. These two competing effects, both of which depend on domain size, combine in a complex way to result in the non-monotonic dependence of interfacial energy and work of adhesion on the size of surface domains. We also show via MD simulations the application of these patchy particles to sensors and show how interfacial mechanisms give rise to sensitive selectivity.*This material is based upon work supported by the U.S. Defense Threat Reduction Agency under Grant No. HDTRAI-09-1-0012, under subaward agreement No. 5710002583-01.
10:15 AM - RR7.3
Investigation of Adsorption Process of Thiol Self- Assembled Monolayers on Gold by Monitoring Energy Dissipation Shifts as a Function of Chain Length and Functionality by Means of Quartz Crystal Microbalance.
Agata Pomorska 1 , Ozlem Ozcan 1 , Guido Grundmeier 1
1 Technical and Macromolecular Chemistry, University of Paderborn, Paderborn, NWF, Germany
Show AbstractSelf- assembled monolayers (SAMs) have attracted great attention in the last years due to their superior properties, like thermal and chemical stability. SAMs show moreover strong adhesion to a substrate, giving an opportunity to functionalize metal surfaces accordingly to the desired application (e.g. adhesion of organic coatings).
SAM modified gold substrates were employed in our former adsorption studies to tailor an accurate system for deposition of multi- functional particles as well as their homogenous distribution within a self- healing coating [1,2]. It was shown recently that the surface energy and the surface stress variations occur during self- assembly. This phenomenon takes place due to the three contributions, namely gold- thiol group, chain to chain and functional group- solvent bulk interactions and controls the self assembly process [3].
In this paper we are presenting the in-situ study of adsorption of bifunctional thiol molecules on gold substrates with the aim of gaining a better understanding of the self-assembly process at the metal/liquid interface. Investigation of the individual contributions of various interactions to the self assembly mechanism by means of Quartz Crystal Microbalance (QCM) will give a new insight into the phenomenon. Here we are focussing on the energy dissipation shift which is the parameter that provides information about interface stress variation along with the frequency shift that displays mainly mass changes. The selection of thiols with different functionalities and chain lengths made the analysis of different contributions possible and presents a new approach to the design of functional surfaces by means of interface engineering.
[1] A. Pomorska, K. Yliniemi, B. P. Wilson, D. Shchukin, D. Johannsmann, G. Grundmeier; manuscript accepted to be published in Journal of Colloids and Interface Science (2011).
[2] A. Pomorska, D. Shchukin, R. Hammond, M. Cooper, G. Grundmeier, D. Johannsmann, Anal. Chem., 82 (2010), 2237-2242.
[3] G. Zuo, X. Li ; Nanotechnology 22(2011)045501.
10:30 AM - RR7.4
Adhesion of Poly(Dimethylsiloxane) and Sub-Monolayers of Alkylsiloxane on Solid Substrates.
Yoshiko Harada 1 , Hiroshi Jinnai 1 , Atsushi Takahara 1
1 ERATO, JST, Fukuoka Japan
Show AbstractThe surface properties of a solid substrate can be altered easily by the application of coatings such as self-assembled monolayers (SAMs) and polymers. Because adhesion of a given material is related to the surface properties of the material, surface modification with SAMs provides a convenient route to fine tuning the adhesion behavior of the solid.In this report, we will present the effect of alkylsiloxane monolayer coatings on adhesion between monolayer-coated glass (or Si wafer) and poly(dimethylsiloxane) or PDMS. While many of similar studies of the effect of SAMs on adhesion focus on the functional groups at the surface, we will discuss the impact of having partial, incomplete coverage of the solid substrate by SAMs. Incomplete films can serve as models of defective SAMs. Defects in SAMs are intrinsic; they originate from various sources, such as grain boundaries, geometrical arrangement of constituent molecules, or simply from incomplete reactions. Regardless of their causes, defects in coatings can lead to undesired failure of the system, for example, in MEMS.We have developed an adhesion tester on the basis of the JKR (Johnson-Kendall-Roberts) theory to investigate adhesion of soft materials. The instrument consists of a load cell to measure the force exerted between samples, a motored stage to control the distance between samples, and a microscope to visualize the contact area. In our setup, a PDMS hemisphere or lens was pressed against a flat substrate coated with SAMs, and from the acquired data, we obtained the work of adhesion, and pull-off force for further analysis. We found that substrates coated with partial SAMs show more negative pull-off forces compared with those at full coverage. The main interactions between PDMS and SAMs are assumed to be from the alkyl groups of the polymer and monolayer, but for partial films, interactions between PDMS and the substrate, and an increased number of contacts from disordered alkyl chains complicate the situation. We will consider those additional forces in our analysis, and moreover, examine how adhesion of sub-monolayer coatings is affected by the change in surrounding environment.
11:15 AM - **RR7.5
Self Healing Materials: A New Approach to More Durable Materials and Structures.
Sybrand van der Zwaag 1 , Santiago Garcia Espallargas 1
1 Aerospace Engineering, Delft University of Technology, Delft Netherlands
Show AbstractSelf healing materials are materials which are capable to repair cracks and to restore functionality more or less autonomously. Two key requirements to generate self healing behaviour are i) the presence of mobile agents (either as a separate additive or an extended state of the matrix itself) which can move to the damaged site and ii) the healing agent turning into a solid load bearing product after reaction at the damage site. The surfaces of the crack play a crucial role in this process, being a trigger for the healing reaction and a the interface between the original matrix and the reaction product. In this presentation we will show examples of autonomous or triggered self healing behaviour for polymers, composites, metals, ceramics and concrete, involing liquid flow, reversible chemical and physical networks, oxidation, precipitation or bacterial activity as the healing process.
11:45 AM - RR7.6
A Synthetic Omniphobic Slippery Surface Inspired by the Nepenthes Pitcher Plants.
Tak Sing Wong 1 2 , Sung Hoon Kang 1 2 , Sindy K. Y. Tang 1 2 , Joanna Aizenberg 1 2
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, Massachusetts, United States
Show AbstractCutting-edge liquid-repellent surfaces are inspired by the lotus effect, which makes use of physical surface textures to support water droplets and maintain a stable air-liquid interface [1]. While these bio-inspired surfaces show exceptional water-repellency, they have limited resistance to the impalement of low surface tension liquids, and are vulnerable to physical damage [2 - 5]. To address these issues, we recently developed a conceptually different approach that is inspired by the Nepenthes pitcher plants [6]. Specifically, these plants make use of a stable, lubricated surface to repel the oily insect footing, so as to enhance their insect capturing efficiency. Inspired by the natural phenomenon, we developed a robust, self-healing, and optically transparent synthetic slippery surface that is capable to repel various polar and non-polar liquids with a broad range of surface tensions. Detailed discussions on the materials design and wetting characterizations of the synthetic surface will be presented.REFERENCES:[1] Barthlott, W. & Neinhuis, C. Planta 202, 1-8 (1997).[2] Quere, D. Ann. Rev. Mater. Res. 38, 71-99 (2008).[3] Bocquet, L. & Lauga, E. Nature Mater. 10, 334 – 337 (2011).[4] Tuteja, A., Choi, W., Mabry, J. M., McKinley, G. H. & Cohen, R. E. Proc. Natl. Acad. Sci. USA 105, 18200-18205 (2008).[5] Nguyen, T. P. N., Brunet, P., Coffinier, Y. & Boukherroub, R. Langmuir 26, 18369-18373 (2010).[6] Bohn, H. F. & Federle, W. Proc. Natl. Acad. Sci. USA 101, 14138-14143 (2004).
12:00 PM - RR7.7
Dynamics of Particle Transport by a Motile Adhesive Ciliated Surface.
Anna Balazs 1 , Amitabh Bhattacharya 1 , Gavin Buxton 2 , O. Berk Usta 3
1 Chemical Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 , Robert Morris University, Moon Township, Pennsylvania, United States, 3 , The Center for Engineering in Medicine, Boston, Massachusetts, United States
Show AbstractTransport of particle and fluids by ciliated surfaces plays an important role in several biological processes. Recently, there have also been several experimental realizations of externally actuated, biomimetic, artificial cilia, with the goal of directing the motion of microscopic particles. In this study, we use theory and simulations to model the transport of a neutrally buoyant, microscopic particle, in a fluid-filled slit, via a regular array of beating elastic filaments. These synthetic cilia, anchored to the substrate, are actuated by an external periodic force and undergo a cyclic motion. The tips of the beating cilia experience an attractive interaction with the particle’s surface and thus, can transfer momentum to the suspended particle. In our simulations, the fluid is modeled via the Lattice-Boltzmann method, while each cilium is modeled as a chain of point particles, which are hydrodynamically coupled to the fluid via a frictional force. By running simulations over a range of cilia stiffness and cilia-particle adhesion strength, we explore the parameter space over which the particle can be “released”, “propelled” or “trapped” by the cilia. We find that the particle attains maximum net velocity in the “propelled” state. We also use an independent semi-analytical theory to predict the parameters for which the cilia are able to attach themselves to the particle and “propel” the particle. We find a good agreement between our semi-analytical theory and simulations. This is the first study that shows how the capture of particles by cilia depends primarily on transient cilia-particle interactions.
12:15 PM - RR7.8
Self-Orientation of Nanoparticles at the Oil/Water Interface.
Chiwon Lee 1 , Eunhye Jeong 1 , Kihoon Kim 1 , Hyesoo Han 1 , Sunil Jeong 1 , Wonbo Lee 1 , Taewook Kang 1
1 Chemical and Biomolecular engineering, Sogang Univ., Seoul Korea (the Republic of)
Show Abstract Transport of nanoparticles across cell membrane is a key step toward in-vivo application in that it affects to both the cellular uptake and intracellular fate of these materials. However, although various effects such as size, composition, and surface property of nanoparticles on crossing cell membrane have been well studied in so far, the detailed mechanism of how these particles interact with the surface of the cell membrane remains poorly understood. Here, to overcome this challenge, we demonstrate computational simulation results which are focused on the orientation and the position of the nanoparticles at the oil/water interface mimicking the endocytosed nanoparticles through the cell membrane. In case of nanorod, it showed the lowest interface energy at the 90 degree which is vertically-oriented with respect to the oil/water interface. In addition, our results revealed that the nanorod is the most stable up onto the oil/water interface at the fixed angle of 90 degree indicating that intracellular uptake of nanoparticles into the cell membrane is energetically favorable. We also studied the orientation and position of various shapes of nanoparticles in different combinations of media with long fatty acid hydrocarbon chains which usually consist of lipid bilayer of cell membrane. The results presented here may assist in design of optimized nanoscale drug delivery and therapeutic systems for biomedical application.
12:30 PM - **RR7.9
Hierarchical Structuring in Plants and Animals as Basis for Novel Bio-Inspired Materials and Surfaces.
Thomas Speck 1
1 Faculty of Biology, Botanic Garden, Freiburg Germany
Show AbstractBiological ‘constructions’ often possess outstanding mechanical properties that are mainly based on a complex hierarchical structuring including a multitude of interfaces on the various structural levels. They do not show the huge variety of constitutive materials as typically used in traditional engineering but are characterized by a limited number of basic chemical components and a large variety of micro- and nanostructures. This extremely efficient biological ‘materials and surface design’ is brought about by the evolution of hierarchical structures covering more than ten orders of magnitude and being well adapted to the requirements at each level of hierarchy. Many biological materials and surfaces posses in addition to their fascinating mechanical functions ‘self-x-properties’ (self-organization, self-adaptability, self-cleaning, self-healing...). The entity of these properties allows them to interact very efficiently with their respective environment. These biological solutions are cost- and energy-efficient, multi-functional and environmentally friendly. And with several billion test runs, they have surely stood the test of time.Novel sophisticated methods for quantitatively analysing and simulating the form-structure-functions-relationship on various hierarchical levels allowed over the last decade new fascination insights in multi-scale mechanics and other functions of biological materials and surfaces. On the other hand, new production methods allow for the first time the transfer of many outstanding properties of the biological role models into innovative biomimetic products. However, the transfer of characteristic functions of life, as self-adaptation, self-organization, self-repair, self-cleaning etc. still represents a major challenge for materials-research.Using actual R&D-projects of the Plant Biomechanics Group Freiburg as examples, the interdisciplinary approach in the development of hierarchically structures biomimetic materials and structures is presented. Examples include (1) biomimetic light-weight design in hierarchically structured branched and un-branched fiber-reinforced composite materials with gradient structure inspired by bamboos, horsetails, dragon trees and columnar cacti; (2) self-repairing coatings for pneumatic structures and self-healing elastic polymers for sealings exposed to high cyclic loading inspired by self-healing processes in lianas and rubber plants, respectively; (3) biomimetic shock-absorbing fiber-reinforced gradient foams inspired by pumelo peels; (4) adaptive biomimetic attachment systems inspired by the attachment organs of plants; and (5) hierarchically structures anti-adhesive surfaces.
RR8: Soft/Bio Interfaces II
Session Chairs
Anthony Brennan
Stanislav Gorb
Wednesday PM, November 30, 2011
Commonwealth (Sheraton)
2:30 PM - **RR8.1
Biologically-Inspired Reversible Adhesives: Where Are We Now?
Stanislav Gorb 1
1 , Zoological Institute at the University of Kiel, Kiel Germany
Show AbstractBiological hairy attachment systems demonstrate their excellent adhesion and high reliability of contact. The structural background of various functional effects of such systems is discussed in the present paper. Additionally, it is demonstrated here, how comparative experimental biological approach can aid in development of novel adhesives. Based on the broad structural and experimental studies of biological attachment devices, the first industrial bioinspired reversible adhesive foil was developed, which adhesive properties were characterised using variety of measurement techniques and compared with the flat surface made of the same polymer. The microstructured foil demonstrates considerably higher pull off force per unit contact area. The foil is less sensitive to contamination by dust particles, and after washing with water, its adhesive properties can be completely recovered. This glue-free, reversible adhesive is applicable in dynamic pick-and-drop processes, climbing robots, and other systems even under water or vacuum conditions. The foil represents therefore a considerable step towards development of industrial dry adhesives based on the combination of several principles previously found in biological attachment devices.
3:00 PM - **RR8.2
Interfacial Spectroscopy: In Situ Approaches to Understand Barnacle Adhesion.
Kathryn Wahl 1 , Daniel Barlow 1 , Daniel Burden 1 , Richard Everett 1 , Christopher Spillmann 1
1 , Naval Research Laboratory, Washington , District of Columbia, United States
Show AbstractProteinaceous secretions are widely recognized to be significant contributors to marine biofouling. The resulting interfacial films can be physisorbed or chemisorbed, and have varying degrees of permanency – they may be highly polymerized and cross-linked, or simply sticky enough to allow surface exploration. Conventional approaches to examining interfacial films derived from bioadhesive junctions is forensic in nature – biofoulant removal (separating the surfaces) followed by ex situ examination of the adhesive composition and surface morphology. While “what” the adhesive is may be gleaned from ex situ approaches, “how” the adhesive is applied and cures cannot. These time-dependent changes can’t be examined “after the fact” and instead require real-time measures of interfacial interactions. At NRL, we have made significant progress developing in situ methods to demonstrate the chemical, mechanical, and rheological processes at interfaces. We are now applying and extending these approaches to examine underwater adhesion in marine organisms, specifically the little striped barnacle, Balanus amphitrite. We are developing in situ and in vivo spectroscopic approaches to determine how protein structure and chemistry influence marine foulant adhesion. We are particularly interested in determining the structure and chemistry of the cement, the biochemical processes influencing polymerization, cross-linking, and water displacement, as well as the physiochemical nature of the adhesion. Our in situ approaches include performing temporally- and spatially-resolved microscopy and spectroscopy through adhesive interfaces transparent at UV, visible, IR, and x-ray wavelengths. I will describe how we have used these and other materials science tools to extend our understanding of the properties and development of the adhesive interface of barnacles. Specifically, we demonstrate that the proteinaceous adhesive interface is formed in two steps, with the second process modifying the interfacial chemistry and increasing adhesion twofold.
3:30 PM - RR8.3
Conformal Contact Behaviour of Biomimetic Microstructured Surfaces.
Hamed Shahsavan 1 , Boxin Zhao 1
1 Department of Chemical Engineering and Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractInspired by the superior frictional adhesion properties of gecko toe pads, extensive research activities have been devoted to fabricating, investigating, and exploiting topographically patterned adhesive surfaces. Surface patterning at the micro and nano scales has been recognized as a versatile method to fine tune material properties at interface. In this work, we present a recent study on the conformal contact behaviour of biomimetic microstructured surfaces, which is important for the assembling of individual components into functional devices. In contrast to the studies of the non conformal spatular contact adhesion of microstructured surfaces, the conformal adhesion is far less explored and understood. To our knowledge, this work is one of the first few systematic studies of the conformal contact on biomimetic microstructured surfaces. Hexagonal arrays of both soft poly(dimethylsiloxane) (PDMS) and rigid SU8 micropillars with varied heights were fabricated via the UV lithography and soft molding techniques. The transfer processes of micropatterns were facilitated by a monolayer of surfactants self assembled on the surface. A series of JKR-type adhesion, friction and peeling experiment coupled with optical interference imaging were performed to investigate the dynamic contact behaviours as a function of the micropillar height and the associated delamination and wearing mechanisms. We found that the energy release rate of separating PDMS from SU8 micropillars can increase 550 folds as the aspect ratio of the pillars increases from 0 to 6. The contribution of different energetics involved in the separation process such as elastic energy dissipation, plastic deformations, and side wall friction were analyzed. We concluded that the side wall friction plays the utmost role in delamination mechanism and consequently adhesion enhancement when the micropillar height exceeds a certain predictable value. The combined use of the micro/nanostructured surfaces with the van der Waals interactions seem to be a potentially more universal solution than the conventional adhesive bonding technology, which depends on the chemical and viscoelastic properties of the materials.
4:15 PM - **RR8.4
Nature Inspired Topographies for Antifouling.
Tony Brennan 1 , Scott Cooper 1 , Chelsea Kirschner 2 , John Finlay 3 , Maureen Callow 3 , James Callow 3 , Linnea Ista 4 , Gabriel Lopez 5 4
1 Materials Sci & Eng, University of Florida, Gainesville, Florida, United States, 2 Biomedical Engineering, University of Colorado, Boulder, Colorado, United States, 3 Biosciences, University of Birmingham, Birmingham United Kingdom, 4 Chemical Engineering, University of New Mexico, Santa Fe, New Mexico, United States, 5 Chemical Engineering, Duke University, Durham, North Carolina, United States
Show AbstractBiofouling is a technically complex issue that directly impacts our economic stability and environmental health. Chemical antifouling compounds can have a long-term, toxic effect and such substances have recently been banned in marine applications. Therefore, there is a need for environmentally friendly, non-toxic surfaces that resist fouling. We have been investigating the impact of physical/chemical characteristics of surfaces on the biological attachment process. Our focus on topography led to the development of the Sharklet AF™ pattern, which is a microstructure inspired by the topography of shark denticles, which are embossed onto polymeric surfaces. The Sharklet AF™ pattern shows a strong (~80%) inhibition of the swimming spores of the green alga Ulva. A four-hour kinetic study with spores of Ulva showed that the Sharklet AF™ pattern inhibits the aggregation behavior of the organism on the surface, preventing the cells from establishing a biofilm community. The attachment kinetics of spores of Ulva was fit with high correlation (r2 > 0.9) to equations historically used to describe the attachment of bacteria. This same Sharklet AF™ pattern has also shown the ability to reduce the settlement of the marine bacteria Cobetia marina. A unifying model is presented for bacteria and algal zoospores which correlates attachment density with surface roughness. Our model demonstrates, for the first time, that organisms respond in a uniform manner that can be described in terms of wetting characteristics of the surface and a Reynolds number associated with the settling organism relative to the topography.
4:45 PM - RR8.5
Interface Effects on Polymer Dynamics Investigated with Nanometric Resolution.
Daniele Prevosto 1 , Kim Hung Nguyen 2 1 , Massimiliano Labardi 1 , Simone Capaccioli 2 1 , Mauro Lucchesi 2 1 , Pierangelo Rolla 2 1
1 , Italian National Research Council - Institute for Chemical and Physical Processes, Pisa Italy, 2 , Dept. of Physics, University of Pisa, Pisa Italy
Show AbstractThe development of new organic materials for functional as well as structural applications is quite often pursued by mixing two or more components at the nanoscale, such as polymer with carbon nanotube or silica clay. In such nanocomposites final macroscopic properties are related to polymer properties at the interface with the other component. Furthermore, the comprehension of fundamentals of physical properties of organic disordered materials is pointing even more towards phenomena occurring at the nanometric scale, to solve such problems as the identification and characterization of dynamic heterogeneity regions in single component amorphous materials, as well as dynamics heterogeneities in asymmetric polymer blend, as well as dynamics properties at interfaces. All of these applicative and fundamental problems demand the investigation of properties in nanometric regions of the material.Some experimental techniques have intrinsic in their nature the capability of investigating the physical properties with nanometric spatial resolution, such as the dynamics properties, However, most of them measure properties averaged over many large portions of the sample, in other words they lack the possibility of selecting single nanometric portion of the sample evidencing the difference in dynamic they have.To address these issues we employed an AFM based technique [1-3] to perform spatially resolved dielectric relaxation measurements and to investigate interfacial effects on relaxation dynamics of poly(vinyl acetate), PVAc [2]. In the first part I will show the investigation of relaxation dynamics of several ultrathin PVAc films, showing that changing interfacial interaction with the supporting substrate modifies the relaxation dynamics. In the second part I will focus on nanocomposite of PVAc with montmorillonite. The investigation with the spatially resolved techniques allowed directly investigating some important aspects of the polymer relaxation behaviour. First, Dielectric relaxation imaging on nanometric scale was performed at constant temperature demonstrating nanometer scale resolution of local DS on heterogeneous materials. Then dielectric spectra acquired across the boundary between pure poly(vinyl acetate) and montmorillonite allowed to evidence a broadening of the structural peak and a slowing down of structural dynamics due to the presence of the clay. The spatial transition between the two regions has been also investigated with a resolution down to few tens of nanometers. Such results directly verify, with local scale measurements, the effect on polymer mobility due to well dispersed and interacting montmorillonite layers, previously argued by macroscopic measurements on nanocomposite materials.[1] P.S. Crider, M.R. Majewski, et al., Appl. Phys. Lett. 91, 013102, 2007.[2] C. Riedel, R. Arinero, et al., J. Appl. Phys. 106, 024315, 2009.[3] M. Labardi, D. Prevosto, et al. J. Vacuum Sc.Techn. B 28, C4D11, 2010.
5:00 PM - RR8.6
Probing the Intrinsic Switching Kinetics of Thermoresponsive Polymer Brushes at the Water/Substrate Interface.
Crispin Amiri Naini 1 , Sven Frost 1 , Steffen Franzka 1 , Mathias Ulbricht 1 , Nils Hartmann 1
1 Chemistry Department, University of Duisburg-Essen, Essen Germany
Show AbstractStimuli-responsive polymers are widely used as actuators and sensors in a variety of applications including adaptive microoptics, microfluidic chips and smart membranes. A detailed knowledge of the intrinsic switching kinetics of such materials is of key importance. Here we demonstrate a stroboscopic photothermal laser manipulation technique, which allows for real-time observation of the switching behavior of poly-N-isopropylacrylamide brushes at the water/substrate interface [1]. A modulated beam of a microfocused laser is used to intermittently heat the substrate surface and locally trigger swelling and deswelling of the thermoresponsive polymer film. Spatial variations of the swelling ratio are monitored using reflectometric interference video microscopy. This facilitates direct parallel measurement of the temperature-dependent switching kinetics of brush layers with thicknesses below one hundred nanometers. Response times range from the millisecond down to the microsecond range demonstrating the prospects of surface-grafted polymer films in fabrication of nanosized polymeric actuators and sensors with fast responsiveness.[1] C. Amiri Naini, S. Franzka, S. Frost, M. Ulbricht, N. Hartmann, Angew. Chem. Int. Ed. 50 (2011) 4513.
5:15 PM - RR8.7
Structure and Properties of Interfaces between Dissimilar Materials by Atomistic Simulations.
Katherine Becker 1 , Chen Shao 1 , Maxim Makeev 1 , John Kieffer 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe interactions of amorphous polymers with an ordered surface chiefly affectthe thermo-mechanical properties of multi-layer constructs. Naturally, the underlying phenomena must be examined at the atomistic scale, and multi-scale molecular simulations provide a powerful complement to experimental investigations. The first aspect in a predictive simulation approach is to generate realistic structural models of the interfacial regions. We achieve this by reproducing transport and reaction processes that underlie the natural formation of these regions using reactive molecular dynamics simulations. A second aspect is to achieve an accurate description of atomic interactions, which we obtain via ab initio calculations. We illustrate our approach using the examples of alkane chains and cyclic aromatic molecules, e.g., copper phthalocyanene (CuPC), adsorbed on metal surfaces, as well as poly[(4,4’-diphenilene) pyromellitimide] (PMB) molecule and multi-molecule amorphous PMB bonded to Si(001). We achieve excellent agreement with experimentally known characteristics. Furthermore, in structures so created we observe phase separation between an adsorbed surface layer that strive for congruency with hard crystalline surfaces and the bulk polymer. The degree of order in this adsorbed layer depends on the chain length of the polymer. The structural characteristics are correlated with the thermal and mechanical properties at the interface.
5:30 PM - RR8.8
Dynamic Mechanical Behavior and Phase Identification in Polymer Nanolaminates.
Erik Dunkerley 1 , Hilmar Koerner 2 3 , Richard Vaia 2 , Daniel Schmidt 1
1 Plastics Engineering, University of Massachusetts Lowell, Lowell, Massachusetts, United States, 2 Nanostructured and Biological Materials Branch, United States Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, 3 , UES Inc., Dayton, Ohio, United States
Show AbstractHybrids of polymer and layered nanofiller have captured the attention of researchers in academia and industry for over two decades. As conventional wisdom holds that exfoliated nanostructures are the most desirable, most efforts have focused on the dispersion of low concentrations of nanolayers in large quantities of polymer. Recently, another class of materials at the other end of the composition spectrum has begun to emerge: polymer nanolaminates.These materials, with their structure often referred to as “bricks and mortar” and reminiscent of nacre, have attracted attention due to the unique properties this structure provides. It is often discussed that combining high nanofiller concentrations with highly confined polymer chains fundamentally alters the mechanics and dynamics of such materials, with the potential for interfacial slip and the realization of hidden length. With that said, measuring and quantifying the significance of such alterations can be challenging, and information on the nature of these changes is currently limited.Here we present an overview of our work to date with a range of model nanolaminates combining various polymers with organically modified layered silicate nanofillers. Automated spray deposition is introduced as a means of easily and consistently preparing nanolaminates of arbitrary composition, ranging from pure polymer to pure (modified) nanofiller. Nanolaminate structure is characterized via scanning electron microscopy and two-dimensional x-ray diffraction, allowing for quantification of both intercalation and orientation states.The dynamic mechanical response of a range of materials is investigated, showing combinations of stiffness and mechanical loss not well-described by current micromechanical treatments. In addition, the deconvolution of multiple dynamic mechanical transitions observed in this data allows for the identification of the phases present and highlights changes in dynamics as a function of composition and intercalation state. Taken together, these results provide a more complete picture of the consequences of confinement on the dynamical properties of these systems in particular as well as polymer / layered silicate nanocomposites more generally, and highlight the power of this approach in characterizing such materials.
Symposium Organizers
Douglas L. Irving North Carolina State University
Susan B. Sinnott University of Florida
Martin H. Mueser Saarland University
Izabela Szlufarska University of Wisconsin-Madison
RR9: Methods in Interfacial Science
Session Chairs
Thursday AM, December 01, 2011
Commonwealth (Sheraton)
9:45 AM - RR9.1
In Situ Spectroscopic Ellipsometry and Nanogravimetry for Preparation and Performance of Aptamer Biosensor.
Jennifer Gerasimov 1 , Keith Rodenhausen 2 , Hao Wang 3 , Rebecca Lai 1 , Mathias Schubert 3
1 Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Chemical and Biomolecular Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 3 Electrical Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractDNA aptamer molecules passivated by alkanethiols can be used for biological detection and screening applications. Through the use of spectroscopic ellipsometry (SE, optical) and quartz crystal microbalance with dissipation (QCM-D, mechanical) techniques, the selective binding of analytes to chemisorbed aptamer probes can be observed in-situ. The measured system consists of a gold-coated quartz substrate, a multiple-layer organic thin film (aptamer probe, alkanethiol, and single-stranded DNA analyte), and physiological buffer solution. The attachment of material, the hybridization efficiency of the aptamer probes, and changes in the porosity of the multiple-layer organic thin film are all determined by SE/QCM-D.In this contribution, we present the real-time SE/QCM-D characterization of (a) the formation of the aptamer probe layer, (b) the subsequent chemisorption of alkanethiol, and (c) the interrogation of single-stranded DNA that is non-complementary or complementary to the sequence found on the aptamer probe. The aptamer DNA sequence is for codon 12 of the K-ras gene; mutations of this gene are often found in pancreatic cancer lesions.We find the introduction of either complementary or non-complementary DNA causes increases of the multilayer organic thin film thickness. However, our SE/QCM-D analysis shows that the porosity of the multilayer organic thin film responds differently depending on the compatibility of the DNA analyte with the aptamer probe. The SE/QCM-D technique provides evidence for different surface attachment mechanisms and can be useful for characterizing biological interfaces.
10:00 AM - RR9.2
Two-Dimensional Membrane Profiling through Silver Plasmon Ruler Tracking Using Polarization Resolved Plasmon Coupling Microscopy.
Guoxin Rong 1 , Hongyun Wang 1 , Björn Reinhard 1
1 Chemistry& Photonics Ctr, Boston Univ, Boston, Massachusetts, United States
Show AbstractIndividual polymer-tethered silver nanoparticle dimers, so-called silver plasmon rulers, enable distance and orientation measurements on the nanoscale. The reduced linear dichroism and the spectrum of the light scattered off from individual plasmon rulers encode information about their orientation and average interparticle separation, respectively. We took advantage of the gain in information silver plasmon rulers offer as probes in optical tracking and analyzed the translational and rotational motions as well as the extension of individual silver plasmon rulers diffusing on the plasma membrane of lysed HeLa cells. Consistent with a compartmentalization of the cell surface on the length scales of the plasmon rulers, most rulers were either immobilized or performed a confined lateral diffusion. Structural details of a plasmon ruler’s confinement region became accessible utilizing the orientation and interparticle separation dependent optical response of the plasmon rulers. This approach, which we refer to as polarization resolved plasmon coupling microscopy (PRPCM), enabled a detailed structural characterization of individual membrane compartments and provided a quantitative metrics to characterize the cellular membrane morphology. In combination with adequate tracking methods, the “dance” performed by membrane confined dimers of flexibly linked noble metal nanoparticles revealed deep insight into dynamic changes of the cell membrane organization with high temporal resolution and spatial precision. The gain in information in polarization resolved ratiometric tracking when compared with conventional particle tracking makes PRPCM a reliable biophysical tool for analyzing the lateral heterogeneity of complex cellular surfaces.
10:15 AM - RR9.3
Functionalized Fluorous MOFS with a Gate-Control Interface for Gas Storage and Separation.
Xiaoping Wang 1 , Chi Yang 2 , Mohammad A. Omary 2
1 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Chemistry, University of North Texas, Denton, Texas, United States
Show AbstractFluorous metal-organic frameworks (FMOFs) are a new class of advanced porous material with fluorine-lined pore surface and hydrogen-free. Reaction of silver(I) and 3,5-bis(trifluoromethyl)-1,2,4-triazolate in toluene/acetonitrile yields FMOF-1, the first example in the FMOF family, that shows high-density gas uptake and unique hysteretic sorption of H2[1]. Variable single crystal X-ray diffraction measurements reveals that FMOF-1undergoes remarkable breathing and negative thermal expansion when the crystal is exposed to N2 at ambient pressure[2]. Upon cooling a apparent negative thermal expansion takes place with very large changes in volume and unit-cell parameters during which multiple N2 molecules are absorbed into channels and cages (see picture). Details of the dynamic gas adsorption mechanism at the atomic level will be presented to illustrate the functional gate-control interface made of topological trifluoromethyl groups in the framework. The sequential filling of the multiple gas adsorption sites in both small and large pores within FMOF-1 and the consequent remarkable swelling of these framework cavities were captured by single-crystal diffraction measurement. The phenominal thermal breathing of FMOF-1 can be viewed in experimental density maps. Real-time 3D reciprocal space mapping technique employed at the SNS TOPAZ beam line for the analyses of guest-host interaction and structural transformation in MOFs will be introduced.Work at ORNL was supported by UT-Battelle, LLC, under contract No. DE-AC05-00OR22725 with the US Department of Energy. [1] C. Yang, X. P. Wang, and M. A. Omary Journal of the American Chemical Society 2007, 129, 15454. [2] C. Yang, X. P. Wang, and M. A. Omary Angewandte Chemie-International Edition 2009, 48, 2500-2505.
10:30 AM - RR9.4
Nanoparticle-Based Assay for Measuring Concentration-Dependent Antibody Self-Association.
Shantanu Sule 1 , Muppalla Sukumar 2 , William Weiss 2 , Anna Marie Marcelino-Cruz 1 , Tyler Sample 1 , Peter Tessier 1
1 Chemical Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 , Eli Lilly and Company, Indianapolis, Indiana, United States
Show AbstractNanoparticle aggregation represents a useful diagnostic tool in detecting highly specific and complementary intermolecular interactions, such as oligonucleotide hybridization and antibody-antigen recognition. However, its application in sensing weak protein self-interactions remains largely unexplored; these interactions regulate solubility and viscosity of monoclonal antibodies (mAbs) and other proteins widely used as therapeutic and diagnostic compounds. Herein, we describe a method (self-interaction nanoparticle spectroscopy, SINS) that employs optical signatures of metal nanoparticle aggregation to address shortcomings of conventional biophysical methods used for characterizing self-association resulting from weak protein interactions. We demonstrate that gold nanoparticles conjugated with antibodies at low protein concentrations (<40 μg/mL) display interparticle association in solution (measured by the interparticle distance-dependent Plasmon wavelength) which is well correlated with self-association of these antibodies (characterized by static light scattering measurements) at three orders of magnitude higher protein concentrations. We find that SINS measurements of multiple human mAbs reveal unique patterns of self-interaction which are strongly influenced by the solution pH and ionic strength. Furthermore, we find that a polyclonal human antibody is non-associative at all conditions evaluated in this study, suggesting that antibody self-association is more specific than previously realized. We expect our high-throughput, colorimetric assay will enable the identification of mAb variants with minimal self-association propensity, as well as identify solution conditions that minimize the association of otherwise poorly soluble mAb variants.
11:15 AM - RR9.5
Effects of Micro-Structured Surfaces on Interfacial Heat Transfer during Microdroplet Evaporation on Heated Surfaces.
Ashley White 1 3 4 , Shawn Putnam 1 2 , Alejandro Briones 3 4 , James Ervin 3 4 , Larry Byrd 3 , John Jones 1
1 Air Force Research Laboratory, Materials and Manufacturing Directorate, Thermal and Materials Science Branch, Wright-Patterson AFB, Greene, Ohio, United States, 3 Thermal and Electrochemical Branch, Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, Greene, Ohio, United States, 4 University of Dayton Research Institute, University of Dayton, Dayton, Ohio, United States, 2 , Universal Technology Corporation, Dayton, Ohio, United States
Show AbstractSpray cooling is an important cooling technology that relies on droplets impinging on a heated surface and then vaporizing. However, the involved phase change mechanisms and, in particular, the influence of the surface are not well understood. Thus, fundamental evaporating droplet studies are imperative. The objective of this work is to perform an experimental investigation of the evaporation of micron-sized water droplets on heated and non-heated surfaces which have micro-pillars. Experiments were performed in which water micro-droplets impinged on either a uniformly heated Cu substrate with micron-sized pillars or a flat, uniformly-heated aluminum film deposited on glass. The single droplets were of a consistent diameter and initial velocity. For the Cu sample, the microdroplets began undergoing chaotic behavior at much lower temperatures relative to Al/glass samples (e.g., the microdroplets started bouncing, exploding, flipping, etc. at surface temperatures near TS = 125 ± 20 degrees Celsius, in comparison to TS = 230 ± 5 degrees Celsius for Al/glass). At temperatures below 100 degrees Celsius the evaporation rate on the enhanced copper surfaces is ~8 times that of Al/glass. We attribute this to a significant increase in solid-liquid-vapor contact area because the water microdroplets on the pillared Cu sample were in either 1) a partial wetting or 2) nonwetting state. In this case, vapor convection is possible through the pillar microstructure (i.e., between the microdroplet and the Cu surface). After observing these improved heat transfer characteristics on the copper pillared surfaces, we systematically varied the micro-pillar spacing on these samples. This was an effort to seek the optimal geometry to maximize the heat transfer for substrates held at different temperatures and the resulting droplet evaporation rate. Micro-droplet evaporation was studied using pillar spacing of 40, 60, 80, and 100 µm for 50 µm square pillars which had a constant height of 60 µm. High speed videos were then reviewed to determine the significant dynamics of the three phase micro-system. Thermo-capillary convection and wetting were largely influenced by the micro-pillar geometry and played varying roles on the heat transfer. Most importantly we present the role of trapped vapor-bubbles inside the evaporating microdroplets, where thermal induced vapor-bubble oscillations and thermally induced capillary waves do not significantly enhance the evaporation beyond that for systems without vapor bubble dynamics.
11:30 AM - RR9.6
Local Fields and the Origins of Electromigration in Nanomaterials.
Kirk Bevan 1 2 , Wenguang Zhu 3 2 , G. Malcolm Stocks 2 , Hong Guo 4 , Zhenyu Zhang 3 5
1 Materials Engineering, McGill University, Montreal, Quebec, Canada, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee, United States, 4 Centre for the Physics of Materials and Department of Physics, McGill University, Montreal, Quebec, Canada, 5 ICQD, University of Science and Technology of China, Hefei, Anhui, China
Show AbstractElectromigration has gained increased prominence in recent years, as the rise of nanoelectronics has given way to higher current and power densities in computing interconnects and devices. In this context we address the fundamental materials question: what drives electromigration at the nanoscale? Our understanding of the forces that drive electromigration has remained at an uneasy juncture between the mesoscopic semi-classical and atomistic quantum mechanical regimes. At the nanoscale an atomistic understanding of materials is required. Through first-principles quantum transport calculations we show that local atomic scale fields, which are not captured by semi-classical materials models, often drive nanomaterials fatigue under current stressing. We adopt a nanometer thick Ag(100) film as our model conductor, where rigorous self-consistent results are shown to follow intuitively from linear response electrostatics. In general, the findings shed new light on the origins of electromigration in nanomaterials and suggest the possibility of harnessing local applied fields to engineer electromigration at the nanoscale in a wide variety of systems.
11:45 AM - RR9.7
Release and Mass Transport of Iron Nanocrystals Encapsulated in Carbon Shells by in-situ STEM/HRTEM.
Zhenyu Liu 1 , Judith Yang 1 2
1 Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 2 Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractIn this study, we examined the dynamics of the release, migration and agglomeration of iron nanocrystals that were originally encapsulated in graphite-like carbon shells by in-situ scanning transmission electron microscopy (STEM)/high resolution transmission electron microscopy (HRTEM). Through this model system, we investigate the release kinetics of iron nanocrystal confined in a carbon shell. The iron nanocrystal particles inside the graphite-like carbon shells shrank continuously until they had completely vanished and left hollow carbon shells behind. Large iron clusters on the scale of several tens nanometers were observed on the surface of the carbon shells in the temperature range of 600~800oC. At temperatures between 800oC and 1200oC, the iron clusters on the surface began to evaporate. After heating to 1200oC, only few iron nanocrystal cores still remained. Simultaneously, the dynamics of iron nanocrystal confined in carbon nanotubes (CNTs) were investigated by in situ STEM/HRTEM. Mass transport of iron confined in CNTs induced by thermoevaportaion is quantitatively analyzed.
12:00 PM - RR9.8
Utilizing Surface Enhanced Raman Spectroscopy for the Study of Interfacial Phenomena: Probing Interactions on an Alumina Surface.
Eric Formo 1 , Zili Wu 1 , Shannon Mahurin 2 , Sheng Dai 1 2
1 CNMS , ORNL, Oak Ridge, Tennessee, United States, 2 Chemical Sciences, ORNL, Oak Ridge, Tennessee, United States
Show AbstractWe utilized Surface Enhanced Raman Spectroscopy (SERS) in probing interface interactions on the surface of alumina as well as investigating the acidic nature of the ultra-thin layers of alumina and supported solid acids. To accomplish this, robust SERS substrates were generated by depositing an ultra-thin protective coating of alumina on top of silver nanowires (NWs) via atomic layer deposition (ALD). The first of these in situ studies were conducted by analyzing the effects of heating a solid acid, phosphotungstic acid (PTA), on the alumina surface in either an oxygen or hydrogen environment at temperatures up to 400 oC. Interestingly, the distance-dependent decay of the enhancement factor of the SERS signal from the underlying NWs allowed us to probe with enhanced detail the interfacial region between the PTA and the alumina surface. The ability to analyze the area closest to the alumina surface was further confirmed by assembling vanadia onto the substrate, and monitoring the intensity differences between the V-O-Al and outer V=O bonds. Further, we also monitored adsorbate-interface interactions between acetonitrile and the various thicknesses of the ALD deposited alumina layers; and examined the adsorption/desorption at high temperatures of pyridine on the acidic sites of a solid acid.
12:15 PM - RR9.9
Dynamics of Spherulite Growth in PHB-V Observed Using Fast Scanning Atomic Force Microscopy.
Bede Pittenger 1 , Chanmin Su 1 , Shuiqing Hu 1 , Natalia Erina 1 , Jamie Hobbs 2
1 AFM Unit, Bruker Corporation, Nano Surfaces Business, Santa Barbara, California, United States, 2 Department of Physics and Astronomy, University of Sheffield, Sheffield United Kingdom
Show AbstractPetrochemical derived plastics have become indispensable materials in modern life. However, they are not decomposable. Poly(hydroxybutyrate) (PHB) and its derivatives are of interest because they belong to the family of bacterial thermoplastic polyesters which can be produced and degraded by microorganisms. Understanding the dynamics of phase transitions (such as melting and crystallization) in these materials is important to allow optimization of their performance and processing.Atomic Force Microscopy (AFM) has long been used to investigate dynamic processes, but the relatively low scan rate of the AFM has placed limits on the samples and conditions under which this type of study could be conducted [Hobbs1998]. By increasing the bandwidth of the AFM probe, the X, Y, and Z scanners, and the feedback loop we have developed a new fast-scanning AFM that allows scan rates of up to one frame per second, enabling real-time dynamic process monitoring. In this paper we study the dynamics of crystallization of PHB-V polymer and measure the growth kinetics of individual lamella. Additionally, we will explore how environmental variables such as temperature, lamella orientation, and proximity to adjacent spherulites can affect growth rates of spherulites and their constituent lamellae.REFERENCES:[Hobbs1998] J. K. Hobbs, T. J. McMaster, M. J. Miles, P. J. Barham, Polymer 1998, 39, 2437
12:30 PM - RR9.10
State-Selective Reaction on the Ultrathin Insulating Films.
Yousoo Kim 1 , Jaehoon Jung 1 2 , Hyung-Joon Shin 1 2 , Maki Kawai 2
1 Surface and Interface Science Laboratory, RIKEN, Wako Japan, 2 Department of Advanced Materials Science, The University of Tokyo, Kashiwa Japan
Show AbstractThe study of single molecules provides deep insights into bonding nature and underlying quantum mechanics concerning about controlling chemical reaction. The scanning tunneling microscope is a versatile and powerful tool for investigating and controlling chemistry of individual molecules on the solid surfaces. The coupling of tunneling electrons to the electronic and vibrational states of the target molecule allows us to realize mode-selective and state-selective chemistry of the individual molecules. In this talk, I will focuse on the selective control of reaction pathways by use of long lifetime of vibrationally and electronically excited states of a molecule on an insulating ultrathin metal oxide layer.[References](1) H-J. Shin et al., Nature Materials 9 (2010) 442.(2) J. Jung et al., Phys. Rev. B82 (2010) 85413.(3) J. Jung et al., J. Am. Chem. Soc. 133 (2011) 6142.