Kripa K. Varanasi, Massachusetts Institute of Technology
Juergen Ruehe, University of Freiburg - IMTEK
Neelesh Patankar, Northwestern University
James Bird, Boston University
HH2: Functional Surfaces II
Tuesday PM, December 03, 2013
Hynes, Level 1, Room 103
2:30 AM - HH2.01
Dipodal Silanes: Important Tool for Surface Modification to Improve Durability
Mani Singh 1 Barry Arkles 1 Janis G Matisons 1 Yulin Pan 1
1Gelest Inc. Morrisville USAShow Abstract
Silane surface modification is an important tool applied to improve and/or change the properties of various substrates which find applications in various industries including medicine, transportation, construction, energy, biotechnology etc. Maintaining the integrity of the treated surface is very important to ensure the longevity and performance of the coating; however this is affected strongly by the wetting interactions at the interface.
Dipodal silanes possess two silicon atoms that can covalently bond to a surface (whereas conventional silane coupling agents only offer one such silicon atom for surface attachment). Dipodal silanes offer a distinctive advantage over conventional silanes in terms of maintaining the integrity of surface coatings, adhesive primers and composites in aqueous environments through improved ‘wet&’ durability. Such improved durability is associated with an increased crosslink density of the interphase formed across the surface, and its resistance to hydrolysis. Calculations based on the equilibrium constant for dissociation of the Si-O-Si bonds to silanols suggest that dipodal silanes may have 100,000 times greater resistance to hydrolysis than conventional silanes. Hydrolytic stability data for dipodal silanes with both hydrophobic alkyl functionality and hydrophilic PEG functionality will be compared to that obtained for conventional silanes, titanates and phosphonates with similar functionality. New dipodal silanes with “pendant” rather than “bridged” functionality will be introduced together with their stability data in aqueous environments. Significantly, in neutral and mildly acidic environments, dipodal silanes clearly demonstrate improved resistance to hydrolysis compared to conventional silanes, titanates and phosphonates, with several hydrophobic “pendant” dipodal silanes providing the best performance.
2:45 AM - HH2.02
Substrate Effects on the Oxidation Process of Self-Assembled Monolayers
Daniela Meroni 3 Sylvia Ardizzone 3 Ulrich S. Schubert 1 2 Stephanie Hoeppener 1 2
1Friedrich Schiller University Jena Jena Germany2Jena Center of Soft Matter (JCSM) Jena Germany3Universitamp;#224; degli Studi di Milano Milano ItalyShow Abstract
Electrochemical oxidation lithography is a scanning force lithography mode which demonstrated to be a useful tool to fabricate nanostructures with nanometer resolution. Applications of this technique include the fabrication of split-ring resonator arrays for plasmonic applications, as well as the fabrication of nanoelectronic device features, i.e., nanometric gap structures, carbon nanotube assemblies, single particle placement among others. The functionalization steps involved in the fabrication of such structures strongly depend on the wetting properties of the templates as well as on the utilization of tailor-made chemical interactions. In particular for optical applications there is a strong desire to utilize substrates others than silicon wafers for the fabrication of nanostructures. Up to now these possibilities were not at hand. Here we report on the extension of the range of suitable substrates for the electrochemical oxidation and functionalization process. Electrochemical oxidation lithography is based on the local electrochemical oxidation of a self-assembled monolayer consisting of n-octadecyltrichlorosilane (OTS) by means of a negatively biased conductive SFM tip. The self-assembly of OTS is however not limited only to the functionalization of silicon but can be applied also to a number of other technologically relevant materials, i.e., indium tin oxide, titanium dioxide and metal oxides like aluminum oxide. We investigated the peculiarities of the self-assembly of OTS on these substrates and studied in detail the characteristics of the electro-oxidation process. Essential parameters for the self-assembly as well as for the oxidation process are presented in a comparative study. In depth Scanning Kelvin Probe investigations were performed with the aim to elucidate the chemical nature of the formed species in the oxidation process. As a main result the strong shielding performance of the OTS monolayer could be demonstrated which effectively screens the surface potential of the underlying substrates; a result which has significant impact in the design and manipulation of electronic properties of nanostructured surfaces.
These results open on the one hand the spectrum of applicable substrates for the fabrication of functional nanostructures and pave on the other hand also the way towards new applications since for the first time also transparent substrates could be used to combine the advantages of electro-oxidative SFM lithography with applications which essentially require transparent substrate materials.
 T.S. Druzhinina, S. Hoeppener, U.S. Schubert, Small 2012, 8, 852-857.
 T.S. Druzinina, C. Höppener, S. Hoeppener, U.S. Schubert, Langmuir 2013, doi.org/10.1021/la4000878.
 D. Meroni, S. Ardizzone, U.S. Schubert, S. Hoeppener, Adv. Funct. Mater. 2012, 20, 4376-4382.
3:00 AM - *HH2.03
Solvent-Free Polymerization at Surfaces by Initiated Chemical Vapor Deposition (iCVD)
Karen K. Gleason 1
1MIT Cambridge, MA USAShow Abstract
The iCVD method is a powerful technology for engineering surfaces which utilizes the richness organic chemistry. In many cases, covalent bonding (grafting), can be achieved at the interface between the substrate and the film. The resultant conformal polymeric coatings, deposited without solvents, and at low substrate temperature (~room temperature), represent an enabling technology in many different fields of application.
For iCVD, one or more monomers species are feed simultaneously with an initiator into a vacuum chamber (~0.1 to 1 torr). The initiator, typically tert-butyl peroxide, thermally cracks over resistively heated filament wires (~250 to 300°C). The radicals produced initiated chain growth polymerization of the monomers on cooled surface (~25°C).
Surface Energy Control:
For poly(perfluorodecyl acrylate), p(PFDA), the degree of crystallinity and the orientation of the crystallites, parallel or perpendicular to the substrate, and the surface morphology, can be tuned using the iCVD process parameters (initiator to monomer flow rate ratio, filament temperature, and substrate temperature). Grafting is achieved by covalently bond of trichlorovinylsilane vapor to a silicon wafer. Polymerization through the surface vinyl groups creates anchoring point for synthesizing grafted polymer chains, resulting in durable superhydrophobic and oleophobic surfaces.
Surface Texturing Control:
Trichlorovinylsilane has been used as an adhesion promoter between polydimethyldisiloxane (PDMS) and heavily crosslinked, hard, poly(ethylene glycol diacrylate) p(EGDA) films deposited by iCVD. This system is buckled to construct highly ordered herringbone patterns through a new sequential wrinkling strategy.
Prevention of Biofouling on Reverse Osmosis Membranes:
The amine groups present in RO membranes with maleic anhydride (MA) at the interface between a zwitterionic polymer and substrate to form covalent bonds. Without MA grafting, zwitterionic films delaminated from the membrane when placed in water. The ultrathin (~20 nm) iCVD films showed anti-fouling properties against proteins and microbes without significantly impairing water flux or salt rejection performance.
Responsive iCVD films for biotechnology:
Longitudinal tissue constructs were formed inside the microgrooves of PDMS conformally coated with iCVD poly(N-isopropylacrylamide), p(NIPAAm). Its temperature response produces both swelling and a hydrophilicity change, allowing retrieval of the tissue construct from the microgroove, useful for 3D cell culture applications in tissue engineering and drug discovery
3:30 AM - HH2.04
Bioinspired SAM Operable by Electrical Potential for Guest Capture and Release
Yoshimitsu Itoh 1 Shuo Chen 1 Takuya Masuda 2 Hidenori Noguchi 2 Kohei Uosaki 2 Takuzo Aida 1
1The University of Tokyo Bunkyo-ku Japan2National Institute of Materials Science Tsukuba JapanShow Abstract
In nature, an electrical potential generated by an ionic gradient across the cell membrane plays a vital role in regulating the behaviors of membrane proteins for functioning. Examples of such proteins include certain G-protein-coupled receptors that undergo a conformational change upon electrical perturbation in the cell membrane and alter their binding affinities towards agonist ligands. Inspired by this notion, we designed a smart surface featuring a voltage-sensing molecular monolayer of an ionic headgroup-appended oligoether. By a conformational change in response to an applied electric field, this monolayer can capture and release a dendritic ionic guest molecule. When the conformational transition is restricted by a congested molecular design, the monolayer is not operable electrically in guest release. This finding not only paves the way for new electroactive materials affording high responsiveness and broad control scopes, but also provides implications for understanding the essential role of membrane potential in the function of membrane proteins.
3:45 AM - HH2.05
A Superhydrophobic Photocatalytic Surface for Singlet Oxygen Generation
Yuanyuan Zhao 1 Yang Liu 1 QianFeng Xu 1 Mark Barahman 1 Dorota Bartusik 2 David Aebisher 2 Alexander Greer 2 Alan M. Lyons 1
1City University of New York Staten Island USA2City University of New York Brooklyn USAShow Abstract
Superhydrophobic surfaces, such as lotus leaves , exhibit extraordinary water repellent and self-cleaning properties. Recently, superhydrophobic surfaces were combined with photocatalytic materials to produce multifunctional surfaces. Reports on the preparation of superhydrophobic photocatalytic surfaces are limited and the techniques are problematic. Either they were created under extreme experimental conditions or the catalyst particles were embedded in the polymer matrix with reduced surface contact area, leading to lower photocatalytic activity. Scaling these techniques to produce large areas would be difficult. In addition, some films were not robust and lost their superhydrophobicity upon light irradiation.
Here we present a novel method for preparing superhydrophobic surfaces composed of photocatalytic particles. No chemical modification of the particle surfaces was required to achieve superhydrophobicity. Superhydrophobic surfaces were fabricated by printing polydimethylsiloxane (PDMS) into arrays of cylindrical cones that measure 400 µm base diameter, 25 µm tip diameter and 1000 µm tall on 500 µm pitch. Catalyst particles of silicon phthalocyanine dispersed in a glass matrix, which we have shown to generate singlet oxygen (1O2) under 669nm light irradiation, were adhered onto the printed PDMS posts. Superhydrophobicity can be maintained, even with hydrophilic catalytic particles, due to this significant hierarchical structure. The surface has a water contact angle of 160° and maintains superhydrophobicity in contact with water under visible irradiation from a diode laser.
Another important feature of the high aspect ratio primary roughness is that it enables easy access to the plastron, i.e. the stable layer of air under the solid-liquid interface. Printing the PDMS posts on a porous membrane, and supporting the membrane over a plenum, provides a means to control the gas composition of the plastron and thus study catalysis at the solid-liquid-gas interface. The generation of 1O2 in the plastron region and the trapping of this short-lived reactive species after it travels across gas-liquid interface and is solvated in the supported solution were demonstrated. Because 1O2 generates no waste or byproducts, the fabricated superhydrophobic and photocatalytic composite surface can be used in water purification and disinfection, such as oxidizing toxic organic molecules and deactivating bacteria, as well as the synthesis important intermediates. Quantification of 1O2 as a function of plastron gas composition and flow rate was achieved using 9, 10-anthracene dipropionic acid as the trapping agent.
1. W. Barthlott, C. Neinhuis. Planta 1997, 202, 1-8.
2. M. Barahman, A. M. Lyons, Langmuir, 2011, 27, pp 9902-9909.
3. D. Bartusik, D. Aebisher, B. Ghafari, A. M. Lyons, A. Greer, Langmuir 2012, 28, 3053.
4. D. Bartusik, D. Aebisher, A. M. Lyons, A. Greer, Environ. Sci. Technol. 2012, 46, 12098.
4:30 AM - HH2.06
A Nanofiber Materials Platform for Tuning Hydrophilicity in Membranes
Laura H. Arias Chavez 1 Yunxia Hu 1 Menachem Elimelech 1
1Yale University New Haven USAShow Abstract
Hydrophilicity is desirable for membranes in the water sector. Hydrophilic surfaces are less susceptible to fouling. They also improve water flux by facilitating more complete wetting of membrane structures to increase their effective porosity. However, most membranes are made from relatively hydrophobic polymers selected for their superior stability, mechanical integrity, and compatibility with traditional phase inversion fabrication techniques. Methods to hydrophilize hydrophobic membrane surfaces (e.g., plasma treatment, polymer chain grafting, or nanoparticle attachment) often suffer from lack of effectiveness, stability, or scalability. Additionally, they may compromise membrane performance by obstructing membrane pores or damaging structures that provide membrane selectivity.
In light of these challenges, a facile method for fabricating hydrophilic electrospun mats was developed. Stable nanofibers were electrospun from a solution of polysulfone (PSf) and polysulfone-block-poly(ethylene glycol) (PSf-b-PEG) in N,N-dimethylformamide. The ratio of PSf-b-PEG to PSf homopolymer in the electrospinning solution was varied, along with the total polymer concentration, to obtain nanofiber mats of differing composition and comparable structure. The nanofiber mats were then annealed in warm deionized water to drive the hydrophilic poly(ethylene glycol) (PEG) blocks toward the nanofiber surfaces. The water-insoluble PSf block and PSf homopolymer anchor the hydrophilic PEG block to the rest of the fiber, preventing its dissolution into aqueous environments.
As-spun and annealed fibers and fiber mats were characterized for morphological and material properties. Scanning electron microscopy shows that the fibers are cylindrical and mostly smooth with mean fiber diameters of 200 to 300 nm. Water contact angle decreased as annealing time increased, with drops from over 140° to less than 10° occurring with high PSf-b-PEG content and long annealing times. X-ray photoelectron spectroscopy revealed an increase of C-O bonds relative to C-C bonds as contact angle decreased, indicating that the reduction in contact angle was due to enrichment of PEG blocks at the fiber surface. Transmission electron micrographs and elemental mapping of the low contact angle fibers show that the fiber surface retains some PSf; the PEG enrichment that occurs does not form an exclusive PEG layer. Organic carbon concentration in the water used to anneal the fibers was low but increased with annealing time. When tested as a microfiltration membrane under gravity-driven flow, nanofiber mats rejected latex particles while allowing high water flux.
The use of PSf and PSf-b-PEG in this work demonstrates the potential this fabrication method has as a platform for producing high performance membranes. The bulk and surface properties of nanofibers can be tailored through custom pairing of other homopolymers and block copolymers to achieve application-specific membrane design requirements.
4:45 AM - *HH2.07
Osmostic Flows in Nanochannels and Application to Energy Harvesting
Lyderic Bocquet 1 2
1University Lyon 1 Villeurbanne France2MIT Cambridge USAShow Abstract
Osmosis describes the flow of water across semipermeable membranes powered by the chemical free energy contained in salinity gradients. It is a fundamental transport process for water in all living systems, and its applications are countless. While osmosis can fundamentally be expressed in simple terms via the van&’t Hoff ideal gas formula for the osmotic pressure, it is a complex phenomenon taking its roots in the subtle interactions occurring at the scale of the membrane nanopores.
I will first discuss some molecular views of osmosis, allowing to rationalize the osmotic pressure across nanochannels. I will then show that specific designs of nanofluidic devices allow to build an osmotic diode, leading to water flow rectification. This osmotic diode functionality opens new opportunities for water purification and complex flow control in nanochannels.
I will then report experiments on fluid transport at the nanoscales, in particular across nanopores, nanochannels and nanotubes. I will first demonstrate osmotically driven flow in nanochannels, as well as flow rectification in asymmetric nanochannels in line with the theoretical predictions for the osmotic diode.
Finally I will focus on the study of osmotic transport inside a single Boron-Nitribe nanotube. This trans-membrane nanofluidic device was developped using nanomanipulation tools using nanoscale building blocks. Experiments show unprecedented energy conversion from salt concentration gradients. Applications in the field of osmotic energy harvesting will be discussed.
References: see http://www-lpmcn.univ-lyon1.fr/~lbocquet
laquo; Giant osmotic energy conversion measured in a single transmembrane boron-nitride nanotube raquo;, A. Siria, P. Poncharal, A.-L. Biance, R. Fulcrand, X. Blase, S. Purcell, and L. Bocquet, Nature 494 455-458 (2013)
laquo; Nanofluidic osmotic diodes raquo;, C. Picallo, S. Gravelle, L. Joly, E. Charlaix and L. Bocquet, submitted (2013)
laquo; Soft nanofluidic transport in a soap film raquo;, O. Bonhomme, O. Liot, A.-L. Biance, and L. Bocquet, Phys. Rev. Lett. 110 054102 (2013)
laquo; Nanofluidics, from bulk to interfacesraquo;, L. Bocquet , E. Charlaix, Chemical Society Reviews 39, 1073 - 1095 (2010)
5:15 AM - HH2.08
Mechanical Stability of Superhydrophobic Surfaces
Juergen Ruehe 1 Jonas Groten 1 Vitaliy Kondrashov 1
1University of Freiburg Freiburg GermanyShow Abstract
Materials with superhydrophobic properties are usually generated by covering the surfaces with hydrophobic nanoscale rough features. A major problem, however, for any practical application of such strongly water-repellent surfaces is the mechanical fragility of the nanostructures. Even moderate forces caused by touching or rubbing the surfaces are frequently enough to destroy the nanostructures and lead to the loss of the superhydrophobic properties. In this paper we study the mechanical stability of superhydrophobic surfaces with three different topographies i.e. with nano- and microscale features and surfaces carrying a combination of both. The surfaces are generated by silicon etching and subsequent coating with a monolayer of fluoropolymer (PFA). We perform controlled wear tests on the different surfaces and discuss the impact of wear on the wetting properties of the different surfaces.
A second system exhibits two roughness levels: silicon microcones surrounded by superhydrophobic silicon nanograss. The fabrication process of the surface structure is a mask-free process where both microcones and nanograss are simultaneously fabricated by the Deep Reactive Ion Etching (DRIE) technique in the overpassivation regime. Varying the process parameters, microcones of different size and density were fabricated, while the nanograss size and distribution was kept constant.
When strong shear stress is applied to such substrates, the microcones take the load and prevent contact of the shearing surface with mechanically instable silicon nanostructures. As result, the surfaces can withstand strong shear forces without noticeable loss in superhydrophobicity. Even when the shear stress is so very large that large scale mechanical damage happens the surface remains superhydrophobic (albeit with slight pinning caused by released hydrophilic areas from broken microstructures. Higher densities of microstructures make the surfaces much stronger against high shear forces, while causing slightly stronger pinning. Thus designing the surface structure allows to precisely tailor the mechanical and wetting properties of such nanostructures surfaces.
5:30 AM - HH2.09
Customized Wetting Behavior on Metallic Surfaces by a One-Step Laser Micromachining Process
Jorge Lehr 1 Anne Kietzig 1
1McGill University Montreal CanadaShow Abstract
The wettability of surfaces is an important aspect of modern materials engineering. The ability to control the wetting behavior is a key feature in fields like surface coating, adhesives and microfluidic applications. We present a new method using femtosecond-laser micromachining to produce controlled wettability in a one-step process on metallic surfaces. Previously separated processing steps have been merged into a single-step, which results in better precision and a drastic reduction of process time. Thereby, we benefit from the femtosecond laser&’s property to alter both the surface topology and the chemical reactivity of a solid surface. Our method bases on the phenomenon that surfaces, which are chemically activated by femtosecond-laser micromachining, react with oxygen containing gases or gas mixtures to satisfy the oxygen deficit that originates from the exposure to femtosecond-laser irradiation. Modifying the surface chemistry consequently increases or decreases the original contact angle of the metal.
We have conducted the micromachining process on titanium (99.9%) under controlled conditions in different background media like oxygen, carbon dioxide and water, which serve not only as chemical reactant for the activated metallic surface but also contribute to the formation of the microstructure. Thereby, the background media influence the effective laser energy exposure of the target surface. Environmental parameters like temperature and pressure have been varied to study their influence on the outcome of the experiments. We have been able to render surfaces either hydrophilic or hydrophobic to different extents. Furthermore, we have controlled the formation of the nano- and microstructure via the parameters of the laser micromachining process, which has allowed us to reach superhydrophobic and superhydrophilic wetting behavior.
With this method we open a new field of possibilities to customise the wetting behavior of metals for various engineering applications to reduce consumption of energy and raw materials.
HH3: Poster Session I: Functional Surfaces
Kripa K. Varanasi
Tuesday PM, December 03, 2013
Hynes, Level 1, Hall B
9:00 AM - HH3.01
Electrospun Fiber Membranes for Membrane Desalination
Fei Guo 1 Gregory Rutledge 1
1Massachusetts Institute of Technology Cambridge USAShow Abstract
Electrospinning is a simple and useful method for producing various fiber assemblies by controlling the polymer solution and electrospinning parameters. The fibers can be spun into nonwoven structures having high porosity, small pore size and high surface-to-volume ratio. These electrospun membranes more hydrophobic than the corresponding cast films due to the air trapped in the voids of the membranes. High porosity combined with high hydrophobicity is desirable for membrane distillation desalination, in which water is transported through the membrane preferentially as vapor, due to a modest temperature differential across the membrane. In this work, various polymer fiber membranes are fabricated by the electrospinning technique. The membrane structure, porosity, hydrophobicity, and the effect of post-spin treatment, such as thermal annealing and modification of surface chemistry by initiated chemical vapor deposition, are studied. The electrospun fiber membranes are tested for desalination using the air gap membrane distillation configuration. The effect of membrane properties and the operating conditions on the membrane distillation performance are discussed.
9:00 AM - HH3.02
Wetting States: Occurrence, Feasibility and Quantification
Anjishnu Sarkar 1 Anne Kietzig 1
1McGill University Montreal CanadaShow Abstract
The contact angle for a surface-liquid combination is governed by energy minimized states, also known as wetting states. One indicator of a wetting state is the penetration depth of the liquid inside the valleys of a rough surface. A clear quantification of the wetting thermodynamics, and explicit formulations for the contact angle have been limited to wetting states with complete penetration (Wenzel) and no penetration (Cassie). Over the past decade, several theories have been established which hint at an intermediate wetting state characterized by a partial penetration of the liquid inside the roughness valleys. This work aims at understanding the occurrence and thermodynamics of an intermediate wetting state. Assuming forced wetting arising at the apex of the roughness valley, the occurrence of a wetting state is realized by balancing the pressures acting on and away from the given surface with a square pillar topology. It is found that impact below a threshold velocity leads to the formation of an intermediate state. The feasibility of one wetting state over the other is governed by the surface characteristics. The principle of energy minimization is used to introduce the general equation of wettability, which computes the contact angle for any wetting state. A wetting parameter is incorporated, which forms the fingerprint of a wetting state. The wetting parameters corresponding to zero and complete penetration respectively return the Cassie and Wenzel equations. The intermediate wetting state is expressed by an implicit solution that is in good agreement with energy minimization. The physics of an intermediate wetting state is revisited, and a mathematical tool is provided which promises significant convenience in contact angle calculations.
9:00 AM - HH3.03
The Effect of Wettability on Hydrazine-Processed CZTS Thin Films
Zhaoning Song 1 Adam B. Phillips 1 Yao Xie 1 Rajendra R. Khanal 1 Michael J. Heben 1
1The University of Toledo Toledo USAShow Abstract
With 15% CuInGaSe2(CIGS) and 11% efficiency Cu2ZnSn(S,Se)4(CZTS) solar cells reported, hydrazine-based spin coating technique has proven to be an effective method for fabricating Cu chalcogenide thin films and devices. During the solution-based process, drops of hydrazine solution interact with the substrate and, ideally, wet, spread, and form a coherent film as the solvent evaporates. One of the crucial factors that affect the film quality and device performance is the wettability of the hydrazine-CZTS solution on the substrate. Here we report a study on the wetting and spreading of hydrazine-CZTS slurries on various solid surfaces. Using surface tension calculations as our guide, we were able to alter the wetting behavior by controlling the solution composition and varying the hydrophobicity of the substrate surface through thin film addition. As predicted by our model, we were able to achieve improved wettability and, subsequently, smoother films.
To characterize the wettability and adhesion of hydrazine-CZTS slurries on various substrates, the surface tension was measured using a static contact angle method. 8 µL hydrazine-CZTS slurry was placed on various surfaces including soda-lime glass, Si, transparent conducting oxides (ITO, SnO2, ZnO), wide band gap chalcogenides (CdS, In2S3, ZnS), metals (Mo, Cu, Au, Al), carbon single-walled nanotubes (SWNTs), and CZTS thin films.The contact angle was measured macroscopically by using image analysis of a droplet on the substrate surface. The work of adhesion between the solid-liquid interface and the spreading coefficient for the droplets were determined using Young&’s equation. The work of adhesion and spreading coefficient for Mo and the CZTS slurry interface were measured to be 133.2 mJ/m2 and -0.37 mJ/m2, respectively. In comparison, those values for soda-lime glass and the CZTS slurry interface were 124.5 mJ/m2 and -9.24 mJ/m2. These results indicate that Mo surface is more easily wet by the CZTS slurry than the glass surface. A simple 2-D numerical model was developed to simulate the static wetting. The model indicates that the wettability of the surface can be increased by reducing the surface tension through surface modification, meaning a partially-wetting material such as glass can be become completely wetting. This was experimentally verified by adding a SWNT or metal film to the glass substrate. Additionally, the wettability between the CZTS slurry and the substrate can be controlled by changing the equilibrium of wetting through modifification of the CZTS slurry composition.
In order to investigate the effect of wettability on the quality of deposited thin films, surface roughness and morphology of CZTS films deposited on various substrates were characterized using AFM and SEM. Consistent with expectation, smoother films were found for surfaces with higher wettability. These results will provide valuable guidance for the solution-processed deposition of hydrazine-based CZTS thin films.
9:00 AM - HH3.04
Manipulation of Wetting Directions Using Nanostructures with Asymmetric Surface Energies
Changquan Lai 1 Carl V. Thompson 1 2 Wee Kiong Choi 1 3
1National University of Singapore Singapore Singapore2Massachusetts Institute of Technology Cambridge USA3National University of Singapore Singapore SingaporeShow Abstract
The spreading of a liquid on a solid surface, also known as wetting, usually takes place isotropically. However, if a surface can cause wetting to occur in certain specific directions only, benefits such as reduction in drag forces can arise. This has important implications for applications in the field of microfluidics, biosensing etc. and is the reason behind the heightened interest in surfaces that can produce directional wetting in recent years. Previous attempts at engineering surfaces that can induce directional wetting had mainly focused on the use of structurally anisotropic micro-/nano-structures such as bent or slanted nanowires. In this study, it is shown that directional wetting can also be achieved with nanoscale surface energy anisotropy. This surface energy anisotropy was generated by depositing a metal at an oblique angle onto an array of polymeric nanostructures fabricated with laser interference lithography and plasma etching. Because the polymer is more hydrophobic than the deposited metal, each nanostructure in the array had a face that was more hydrophilic than the other. When a water droplet was placed on such a surface, it was always found to wet preferentially in the direction of the hydrophilic face. Depending on the shape of the nanostructure, which can be controlled by the fabrication process, wetting can be made uni-, bi- or tri-directional. A model is proposed to suggest the basis for directional wetting observed in this study and is validated against measurements of contact angles and spreading anisotropy displayed by the droplet. The insights obtained in this study contribute to the understanding of wetting on heterogeneous surfaces and expand the capability of researchers to engineer functional surfaces for the control of wetting directions.
9:00 AM - HH3.05
Wettability Study of Vibrating Microstructure Patterned Surfaces
Pengtao Wang 1 Junwei Su 1 Hongwei Sun 1 Wendy Dai 2 George Cernigliaro 2
1Umass Lowell Lowell USA2MicroChem Newton USAShow Abstract
Microscale Polymethyl Methacrylate (PMMA) pillars with different height, spacing and molecular weight (MW) were fabricated on a quartz crystal microbalance (QCM) surface using nanoimprinting lithography (NIL) technology with Polydimethylsiloxane (PDMS) as the molding material. The patterned surfaces were treated with oxygen plasma and POTS (Perfluorinated Octyl Trichlorosilane) vapor to modulate surface wettability. The wettability of micropillars was initially characterized by contact angle measurement. Thereafter, a technique using laser scanning confocal microscope (LSCM) with immersion lens was developed to accurately measure the interface between pillars and liquid. The QCM impedance spectra for different water-micropillar interactions were obtained and correlated with their respective hydrophobicity and pillar parameters. Water was found always in the Wenzel state (fully penetrating) for micropillars of low height, and gradually shifts to the Cassie state (non-penetrating) with increasing pillar height and reduced pillar spacing. The results also show that hydrophobic surfaces in Wenzel state have similar responses to the hydrophilic surfaces while a significantly weaker response from QCM was found for the hydrophobic surfaces. Furthermore, the responses for hydrophobic surfaces are highly dependent on the height and spacing of micropillars, as well as the penetration depth of water. More importantly, the vibrating QCM surface results in micropillar vibration on the surface and, in turn, leads to changes in interactions between pillars and water. The effects of pillar height and molecular weight on surface hydrophobicity were studied in detail to clarify this interesting phenomenon.
9:00 AM - HH3.06
Mussel-Inspired Surface Chemistry with Controlled Molecular-Level Adhesion for Developing a Nitric Oxide Nano-Depot
Seonki Hong 1 Haeshin Lee 1
1Korea Advanced Institute of Science and Technology Daejeon Republic of KoreaShow Abstract
Here we report a mussel-inspired, material-independent surface chemistry with controlled adhesive properties in a molecular-level for developing a platform to deliver the smallest therapeutic molecule, nitric oxide (NO). Poly(dopamine) (pDA) and poly(norepinephrine) (pNE) coating is both an emerging mussel-inspired surface functionalization techniques that can be applicable to a variety of materials regardless of their chemical environment of surfaces with its unique material-independent adhesion properties.[1,2] The surface modification mechanism of pDA and pNE are known as following the similar 5,6-dihydroxyindole (DHI) derivative chemical pathway,[3,4] and the secondary amine involved in DHI can be useful for an excellent NO-loading molecular platform. We prepared surface conjugated diazeniumdiolates groups (NO precursor) onto the secondary amines involved in the respective pDA or pNE layers, and the spontaneous dissociation of one diazeniumdiolates releases two molecules of NO gas molecules when water molecule is accessed to the surfaces. We found that significant large amount of NO was attached and released from the pNE modified surface compared to pDA modified one. The difference in molecular adhesion of NO precursor is arising from the existence of a unique by-product called 3,4-dihydroxybenzaldehyde (DHBA), unavoidably generated during the pNE surface functionalization. DHBA can interact with NE by Schiff-base formation followed by auto-reduction of imine bond, resulting DHBA-NE dimer. This key leads to the nanoscale-smooth coating morphology of pNE, which can be a big advantage over pDA, especially in applications involving nano-scale substrates. Not only for controlling the morphology, the integration of DHBA-NE on pNE coating layer is essential that additional secondary amine involved in DHBA-NE can generate an excess amount of NO-loading molecular platform for NO storage and delivery, which can be potentially useful for biomedical applications.
 H. Lee, S. Dellatore, W. Miller, P. B. Messersmith, Science 2007, 426-430.
 S. M. Kang, J. Rho, I. S. Choi, P. B. Messersmith, H. Lee, J. Am. Chem. Soc. 2009, 131, 13224-13225.
 S. Hong, Y. S. Na, S. Choi, I.-T. Song, W. Kim, H. Lee, Adv. Funct. Mater. 2012, 22, 4711-4717.
 M. d'Ischia, A. Napolitano, A. Pezzella, P. Meredith, T. Sarna, Angew. Chem., Int. Ed. 2009, 48, 3914-3921.
 S. Hong+, J. Kim+, Y. S. Na, J. Park, S. Kim, K. Singha, G.-I. Im, W. J. Kim, H. Lee, Angew. Chemie. Int. Ed. 2013, accepted, DOI: 10.1002/anie.201301646.
9:00 AM - HH3.07
Transparent Water-Repelling Multiscale Structures Inspired by Termite Wing
Sanghyuk Wooh 1 Jai Hyun Koh 1 Sujin Lee 1 Hyunsik Yoon 2 Kookheon Char 1
1Seoul National University Seoul Republic of Korea2Seoul National University of Science amp; Technology Seoul Republic of KoreaShow Abstract
Nature is a great source of inspiration for creating unique structures with special functions. The representative examples of water-repelling surfaces in nature, such as lotus leaves, rose petals, and insect wings, consist of an array of bumps (or long hairs) and nanoscale surface features with different dimension scales. On the other hand, low optical transparency due to the Mie scattering from micro-roughness with different dimension scales has recently been considered as a crucial issue for recent interest in as eye glasses, solar cell panels and car windows which need both transparency and superhydrophobic property. Herein, we introduced a method of realizing highly transparent multi-dimensional hierarchical structures inspired by termite wings along with water-repellancy of the surfaces with different drop impact scenarios. The multi-dimensional hierarchical structures were fabricated by soft imprinting method with TiO2 nanoparticle pastes. In order to achieve the enhanced hydrophobicity, fluorinated moieties were adsorbed to the patterned surfaces to lower the surface energy. As a result, super-hydrophobic surfaces (above 176° in water contact angle with a hysteresis less than 2°) with high transparency (above 90 % in transmittance) in visible region were realized. Moreover, it has been shown that UV light was effectively blocked by TiO2 and different dyes were also easily incorporated inside the mesoporous structure, resulting in photocromic functional films.
9:00 AM - HH3.08
Fabrication of Superhydrophobic Stainless Steel Surface with Hierarchical Structure
Heon-Yul Ryu 1 Si-Hyeong Cho 1 Dong-Ho Han 2 Jin-Goo Park 1 2
1Hanyang University, South Korea Ansan Republic of Korea2Hanyang University, South Korea Ansan Republic of KoreaShow Abstract
Stainless steel is widely known corrosion resistant metal. Its resistance to corrosion and staining makes it an ideal material for many applications such as architecture, industrial establishments and medical instruments. The corrosion resistance of stainless steel comes from sufficient chromium to form a passive film of chromium oxide. With its intrinsic corrosion resistance, the wettability of metal surface is also important property which depends on the chemical compositions and geometrical microstructures. Even though the fabrication of superhydrophobic surface has been researched for a long time, it is difficult to fabricate an engineering superhydrophobic surface on stainless steel due to the chemical degradation of surface and its difficulty to fabricate microstructure on the surface. Recently, superhydrophobic surfaces on stainless steel have been studied by sol-gel coating, dip-coating, electroless plating, Teflon-like coating with plasma and laser machining. The structure made by etching method is sturdy while coated films can be fallen off from the stainless steel substrate. With laser machining, very small structure can be fabricated but it is not cost effective due to its high cost and low throughput.
In this study, a method was developed to fabricate a superhydrophobic surface on stainless steel. Chemical wet etching and electrochemical etching were used to make nano scale roughness and micro structure on the surface. FeCl3 which is generally used as an etchant for stainless steel was used for chemical wet etching. Electrochemical etching process uses electrochemical reaction at the interface between metal and electrolyte. With this process, it is possible to fabricate micro structure without burr on the surface and to control the dimension of structure. SAM (self assembled monolayer) method was also used to deposit hydrophobic layer on the micro/nano hierarchical structure. A low surface energy of SAM layer on the structure makes the surface superhydrophobic. The fabricated stainless steel was analyzed by FE-SEM, 3D-profiler and FT-IR spectrometer. The characteristics of superhydrophobic surface were analyzed by measurements of the static/dynamic contact angle and sliding angle.
The superhydrophobic surface of stainless steel has been successfully developed with hierarchical structure and liquid SAM coating. The effect of dimension to hydrophobicity of metal surface was calculated with different microstructure which has various pattern size and pitch.
9:00 AM - HH3.09
Superhydrophobic Low Refractive Index Thin Film with Nanopores Composed Bio-Based Polymer by Layer-by-Layer and Application to Transparent SLIPS
Kengo Manabe 1 Shigo Nishizawa 1 Seimei Shiratori 1
1Keio University Yokohama JapanShow Abstract
The development of multilayer thin films with nanopores containing biomolecules has attracted much attention in material and surface science as one of the optical applications and surface wettability control via LBL (Layer-by-Layer) self-assembly method. Also, chitin nanofibers (CHINFs) have attracted much attention because of their high mechanical strength. The shells of crustaceans are expected to be particularly useful for materials applications, because they are made from mineral salts, protein, and chitin; it is known that mineral salts can be removed using HCl, and proteins can be removed using NaOH. The CHINFs surface is transformed from chitin to chitosan by deacetylation, which results in a positive charge on the CHINFs, due to the presence of amine groups. In this study, we fabricated antireflective film with nanopores for robust functional surface composed bio-based polymer and added hydrophobic performance by gas-phase process. To fabricate a low refractive index film, therefore, porosity was introduced into a thin film. The porous thin film was obtained by increasing the number of airspaces inside the membrane. Then, by depositing the deacetylated CHINFs, Poly (acrylic acid) (PAA) and SiO2 nanoparticles using the LBL method, the lower refractive index layer was fabricated. (CHINFs/PAA) films indicated that the highest transmittance was 96% and the lowest refractive index was 1.29. The transmittance decreased by only 1.3% after abrasion of 200 g/cm2 with cotton. (CHINFs/SiO2) films showed that the highest transmittance was 97% and the lowest refractive index was 1.20. After adding hydrophobic performance by gas-phase process, the film indicated that the highest transmittance was 97%, the lowest refractive index was 1.23 and the water contact angle is 153°. In addition, we fabricated transparent SLIPS (Slippy Liquid Infused Surface) film by putting a drop of lubricant oil (Krytox 103) on the films and the highest transmittance of this SLIPS film was 98%.
9:00 AM - HH3.10
Electrospinning of UV-Responsive Poly(azobenzene)
Phillip Burkholder 1 Ruya Ozer 1
1University of Tulsa Tulsa USAShow Abstract
Polymers bearing azobenzene functionality have been gaining increasing attention because of their ability to alter physical properties or create mechanical action of bulk polymers in response to changes in environmental conditions. Generally, azobenzenes are attached to the side chains of polymeric precursors followed by casting to generate thin films for further studies. Our approach presented here differs from the previous works in that it utilizes self-polymerization of hydroxyl and/or acrylate functionalized azobenzene moieties and assembly of stimuli-responsive materials using electrospinning technique. We hypothesize that increased surface area and porosity of electrospun nanofibers of azobenzene functionality will greatly improve the photomechanical response factor of the resulting materials. The polymerization reaction was carried out without using a crosslinker in inert atmosphere at 70 °C for 24 hours. Reaction progress and the product was monitored using 1H NMR. The resulting polymer was then dissolved in DMF at approximately 75% wt% which was determined to be have the ideal viscosity for electrospinning. The dissolved polymer was loaded into a syringe and electrospun with an applied voltage of 15kV at 2 mu;L/m for 30 minutes. The resulting deep orange mat exhibits reversible changes in hydrophobicity and color when exposed to 365 nm light. The resulting materials were examined using TGA, DSC, 1H NMR, wettability , viscosity, and gel chromatography.
9:00 AM - HH3.11
Environment-Dependent Interfacial Strength Using First Principles Thermodynamics: The Example of the Pt-HfO2 Interface
Yenny Cardona Quintero 1 Rampi Ramprasad 1 Venkatesh Botu 1
1University of Connecticut Storrs USAShow Abstract
Interfaces between metal and metal oxides are encountered in several applications, including surface protection, catalysis and electronics. In each one of these applications, the interfacial mechanical strength plays a decisive role in the stability and reliability of the systems. However, there are several factors that affect this property including interfacial composition, atomic level structure and processing conditions (temperature and pressure). Several theoretical and experimental methods have been developed to determine the interfacial mechanical strength. Among the theoretical methods, density functional theory (DFT) is a popular technique in the calculation of the work of separation (Wsep) of metal-metal oxide interfaces. However, DFT only provides information at 0 K situations, which implies that the direct quantitative comparison of the computed results with the experimental Wsep is not always possible.
In this work, we used DFT methods to calculate the Wsep of the Pt-HfO2 interface at different interfacial O coverages (0, 0.25, 0.5, 0.75 and 1 OML) and several cleavage planes of the heterostructure. Using first principles thermodynamics, we combined the information of the lowest Wsep of the Pt-HfO2 system for different O configurations and propose a parameter-free method to determine the average work of separation as a function of temperature and O pressures. The average work of separation obtained for the Pt-HfO2 heterostructure is in excellent agreement with the experimental value reported in the literature, measured at high temperatures and low O pressures. The methodology used in this work can be extended to different metal-metal oxide interfaces to determine the adhesive properties as a function of temperature and pressure.
9:00 AM - HH3.12
Polymers/Nano-Metals Interfaces: Wetting and Embedding Kinetics at the Nanoscale
Francesco Ruffino 1 2 Vanna Torrisi 3 4 Giovanni Marletta 3 4 Maria Grazia Grimaldi 1 2
1Universitamp;#224; di Catania Catania Italy2MATIS-IMM-CNR Catania Italy3Universitamp;#224; di Catania Catania Italy4CSGI Firenze ItalyShow Abstract
In recent years, nanocomposite materials fabricated by metal nanoparticles (NPs) and thin metal nano-grained films deposited on or embedded in soft polymeric substrates have emerged in the developing nanotechnonolgy applications (i. e. organic transistors, light-emitting diodes, solar cells etc.). To generate metal NPs or nano-grained metallic films on or near a polymer surface, ion implantation, evaporation, and sputtering methods have been utilized. To precisely control the surface properties of polymer nanocomposite films, it is essential to understand how NPs interact with the surface. In the present work, we report on an atomic force microscopy study of the wetting, adhesion and embedding kinetics properties (as a consequence of the long-range mobility of the polymeric chains above the glass transition temperature), under thermal processes, of Au and Ag NPs sputter-deposited on two amorphous soft polymeric layers: poly(methyl methacrylate) (PMMA) and polystyrene (PS). We studied in details the embedding process as a function of the annealing temperature and time so to evaluate the embedding rate both for Au and Ag in PMMA and PS. Also the NPs embedding statistics was studied analyzing the number of the embedded NPs as a function of the annealing time. The quantification of all these parameters, allow us to propose the embedding mechanism of deposited metal NPs on soft polymeric films, as an effective way to generate, in a controlled way, metal/polymer nanocomposites.
9:00 AM - HH3.13
Dielectric Response Variation and van der Waals-London Dispersion Interaction
Jaime C. Hopkins 1 Daniel M. Dryden 2 Wai-Yim Ching 3 Roger H. French 2 V. Adrian Parsegian 1 Rudolf Podgornik 1
1University of Massachusetts- Amherst Amherst USA2Case Western Reserve University Cleveland USA3University of Missouri-Kansas City Kansas City USAShow Abstract
Since the creation of the Lifshitz theory of van der Waals-London dispersion (vdW-Ld) forces, accurate knowledge of dielectric response functions has been the key requirement for reliable computation. The connections between material properties, their processing into a form appropriate for computation, and -most important- the propagation of spectral details into specifics of vdW-Ld forces have no general solutions. They must, necessarily, be carefully recognized case-by-case for different materials, media, and geometries. Among the many electronic properties requiring careful attention are excitonic many-body effects for which it is not clear how the changes in optical spectra will affect the overall magnitude of the vdW-Ld interaction between materials.
Excitonic effects in low-dimensional systems, such as graphene and carbon nanotubes, tend to introduce additional absorption peaks in the energy range below 5 eV and to shift slightly the positions of some other peaks. An important question in this respect is how the changes in optical spectra due to excitons affect the overall Hamaker coefficients and the trends in the vdW-Ld interaction between different materials. Because the vdW-Ld interaction is non-local in frequency, being a functional (or more precisely a discrete sum over the Matsubara frequencies) of the frequency-dependent dielectric response, it is reasonable to expect that the interaction will depend more on the global properties of the dielectric response set by sum rules rather than on specific details of spectra.
A related problem is connected with refractive index-matching, sometimes used to modify the inter-particle interactions through their vdW-Ld component. In the index-matching method, one assumes that the vdW-Ld interaction stems only from the optical frequency regime; if this part of the spectra of interacting bodies and the intervening medium is made to coincide, its contribution to the vdW-Ld interaction will vanish identically. Again, because of the non-local nature of the vdW-Ld interaction, one should expect that even for refractive-index-matched situations the vdW-Ld interactions will depend primarily on the global properties of the dielectric response and much less on the presence or absence of a specific peak.
We have embarked on a detailed investigation of how changes in the spectral properties of the dielectric response function over a finite interval of frequencies alter the strength of vdW-Ld interactions expressed as the corresponding Hamaker coefficient. We consider dielectric spectra that differ only by the presence or absence of a single (exciton) peak and observe the consequence of this difference on the Hamaker coefficient for the interaction of two planar semi-infinite regions. We analyze specifically the changes of all the Matsubara components to the interaction free energy in order to monitor the effect of the dielectric response variation on the vdW-Ld interaction energy functional.
9:00 AM - HH3.14
Water Repelling Property of Electrospun Polylactic Acid Membrane Treated with Fluorinated Silane
Guang Li 1 Matteo Chiesa 1
1Masdar Institute of Science and Technology Abu Dhabi United Arab EmiratesShow Abstract
Membrane Distillation (MD) is a thermally-driven separation process, in which only vapor molecules transfer through a microporous hydrophobic membrane . In sea water desalination using MD, it is highly desired that the membrane structure has a low affinity for water, which requires the membrane to exhibit high hydrophobicity to minimize water adhesion . The extensive use of PTFE (polytetrafluoroethylene) and PVDF (polyvinylidenefluride) in MD has raised the attention to their environmental impact due to membrane disposal. Due to its biodegradability and hydrophobicity, polylactic acid (PLA) is a potential candidate for MD desalination [2, 3]. In this study, PLA membrane is fabricated by electrospinning. The morphology of the membrane can be controlled by adjusting the electrospinning process parameters and by applying post-heating treatments. Furthermore, the thickness of the membranes and the presence of beaded fibers, fused fibers and porous fibers can be adjusted. The morphology of the obtained membrane is investigated by means of scanning electron microscope. The static and dynamic contact angle (CA) is measured by sessile drop technique with water as the probe liquid. To enhance the hydrophobicity of the PLA membrane and obtain improved water repelling properties, a fluorinated silane (trichloro(1H,1H,2H,2H-perfluorooctyl)silane) is coated on the membrane under modest vacuum (asymp;10 kPa) by vapor deposition. The treated membranes are found to have unaffected morphology and structural characteristics and higher CA compared with those of the untreated membrane. Thus treating membranes of different thickness and morphology with fluorinated silane yields even higher hydrophobicity which can be favorable for applications like anti-wetting and self-cleaning, etc. Meanwhile, keeping in mind that CA measurement only yields the macroscopic hydrophobicity characteristics of the membrane, the microscopic water repelling property inside the pores in the membrane is investigated by means of force spectroscopy in atomic force microscopy (AFM) .
1. Alkhudhiri, A., N. Darwish, and N. Hilal, Membrane distillation: A comprehensive review. Desalination, 2012. 287: p. 2-18.
2. Gross, R.A. and B. Kalra, Biodegradable polymers for the environment. Science, 2002. 297(5582): p. 803-807.
3. Li, L., R. Hashaikeh, and H.A. Arafat, Development of eco-efficient micro-porous membranes via the electrospinning and annealing of poly (lactic acid). Journal of Membrane Science, 2013.
4. Buruaga, L., et al., Production of hydrophobic surfaces in biodegradable and biocompatible polymers using polymer solution electrospinning. Journal of Applied Polymer Science, 2011. 120(3): p. 1520-1524.
5. Santos, S., et al., Measuring the true height of water films on surfaces. Nanotechnology, 2011. 22(46): p. 465705.
9:00 AM - HH3.15
Influence of Surface Treatment on Adhesion of iCVD PGMA Thin Films for Wafer-Level Bonding
Vijay Jain Bharamaiah Jeevendrakumar 1 Bruce Adair Altemus 2 Adam James Gildea 2 Magnus Bergkvist 1
1College of Nanoscale Science and Engineering, University at Albany Albany USA2Tokyo Electron U.S Holdings, Inc Austin USAShow Abstract
In this work, we investigate the effect of various surface treatments on the bonding performance of initiated chemical vapor deposition (iCVD) polyglycidylmethacrylate (PGMA) thin film adhesives.
iCVD is a conformal, solvent-less, vapor phase polymerization and deposition technique that eliminates surface tension effects related to solvent phase processing. Moreover, iCVD has several advantages compared to traditional vapor-phase polymerization techniques like plasma-enhanced chemical vapor deposition (PECVD) and hot-wire chemical vapor deposition (HWCVD). It is a low-energy technique that results in higher retention of polymer functional groups compared to high-energy PECVD and HWCVD processes. Furthermore, iCVD has minimal influence on the substrate chemistry, unlike PECVD which involves constant bombardment of surface with high-energy ions.
The gas-phase processing, substrate compatibility, and retention of reactive group functionality, enable use of iCVD-PGMA films as thin film adhesives. In semiconductor processes potential applications include thermo compression bonding for wafer thinning, 3D integration, and packaging of MEMS/ NEMS devices. In preliminary work we studied iCVD-PGMA for use in