Wetting phenomena are ubiquitous both in nature and in various industrial processes and products. Three distinct wetting morphologies (complete wetting, partial wetting, and non-wetting) can be found in colloidal multiphase systems. Each of these three morphologies can have practical benefits, and controlling the morphology is desirable for applications ranging from oil recovery to separations. It is known that the wetting morphology depends on the balance of interfacial tensions and can thus be modified with surfactant additives. However, surfactants suffer from drawbacks such as chemical degradation under harsh application conditions, potential environmental pollution, and recovery difficulty. In this talk, I will present our recent work on colloidal particles being an attractive alternative to surfactants as wetting modifiers in multiphase systems. Our work provides a new strategy to predict and control the wetting configuration in colloidal multiphase systems, with potential benefits in a wide range of research fields, industrial processes, and commercial products.

Materials intended as tissue engineering scaffolds must be porous to accommodate cells and allow fluid flow for the transport of nutrients and removal of waste products, as both are necessary for cell survival over time. Hydrogels, freeze-dried constructs and 3D-printed materials are commonly investigated as cell scaffolds, because one can control their porosity and consequent water content to achieve suitable conditions for cell seeding.
The permeability of materials to fluid flow affects not only the change in biophysical stimuli received by cells seeded in the scaffold, but also the pressure and stresses exerted by the fluid on such cells and the scaffold&’s structure. However, the models available at present to predict the permeability of porous media are often semi-empirical and specific to a type of material, otherwise presenting parameters that need fitting to the particular results obtained. In addition, these equations do not consider the interaction between the fluid and the soft material making up the scaffold.
In the present work a general model for the permeability of porous media was investigated as a function of structure parameters such as porosity, pore size, interconnectivity, and others. The relationship between the wetting properties of the material making up the scaffold, the resulting bound-water layer, and permeability was also investigated and found to be fundamental in understanding the flow of fluid through the soft media. The proposed solutions were tested successfully against measured permeability values, through a custom-built rig that allows for the measure of flow as a result of fluid pressure, for a range of tissue engineering scaffold materials. These included nano- (hydrogels) and micro-porous (freeze-dried or 3D-printed) materials, for which an understanding of the permeability is expected to greatly improve the design of tissue scaffolds with a tailored biophysical environment.

Dispersion of colloids in ‘phobic&’ solvents is unstable and leads to irreversible aggregation. A stable dispersion in ‘phobic&’ environment requires surface tethering that matches in polarity with the environment, known as the ‘similarity rule. In this research, the ‘hedgehog&’ particles (HP) with high aspect ratio interfacial nano-topography breaks the ‘similarity rule&’ and enables stable dispersion in ‘phobic&’ solvents without the chemical camouflage. The HPs are constructed by seeding positively charged Zinc oxide (ZnO) nanoparticles on the surface of negatively charged polystyrene microspheres (mu;PS), from which ZnO nanowires (NW) are grown orthogonal to the surface. In the instance of aqueous dispersion of hydrophobic HPs, drastic reduction in the effective interaction surface area and in the solid volume fraction significantly reduces the attractive hydrophobic and van de Waals forces. Furthermore, air-pockets trapped in between the ZnO NWs due to surface super-hydrophobicity provide extra repulsion as a result of charge density build up resulting from auto-ionization at the air-liquid interface. In the case of apolar solvent dispersion of hydrophilic HPs, interfacial geometry decreases attractive interaction potential, as mentioned in the case for aqueous dispersion of h-HPs. In addition, negligible solvent ionization prolongs the electrostatic effect to longer distances, hence providing porlogned repulsion. The findings offer new perspectives and could lead to new strategies for exotic colloidal interactions and self-assembly, drug delivery and catalysis.

Amphiphilic coatings with molecular hydrophilic and hydrophobic patches have the potential to mitigate undesired bacterial adhesion and biofilm formation on wetted surfaces in a more environmentally friendly manner¹. In previous studies, the size of the patches (Hydroxyethyl methacrylate and perfluorodecylacrylate) was estimated to be sim;1.4minus;1.75 nm, but no direct observations of the molecular heterogeneity exist². Here, we investigate the amphiphilic nature with amplitude modulation atomic force microscopy (AM-AFM). High-resolution images reveal the existence of amphiphilic nanodomains (1-2 nm²). Compositional heterogeneity at the nanoscale is further corroborated by a statistical analysis on the data obtained with dynamic AM-AFM force spectroscopy. The nanoscopic results on the polymers wettability are also confirmed by contact angle measurements. The ability to visualize the amphiphilic nanodomains as well as sub-nanometer crystalline structures provides evidence for the existence of previously postulated nanostructures, and sheds light on the underlying antifouling mechanism of amphiphilic chemistry.
Reference
1 Krishnan, S. et al. Anti-Biofouling Properties of Comblike Block Copolymers with Amphiphilic Side Chains. Langmuir : the ACS journal of surfaces and colloids22, 5075-5086, (2006).
2 Baxamusa, S. H. & Gleason, K. K. Random Copolymer Films with Molecular-Scale Compositional Heterogeneities that Interfere with Protein Adsorption. Adv. Funct. Mater.19, 3489-3496, (2009).

The formation of fucoidan/chitosan-based polyelectrolyte multilayers (PEMs) has been studied with in situ Fourier transform infrared (FTIR) spectroscopy, zeta potential determination, and captive bubble contact angle measurements. Attenuated total reflectance (ATR) FTIR spectroscopy has been used to follow the sequential build-up of the multilayer, with peaks characteristic of each polymer being seen to increase in intensity with each respective adsorption stage. The sequential build-up was confirmed through measurements of the film zeta potential, with alternating negative and positive values being determined after each polymer addition.
FTIR spectral processing has allowed for the extraction of spectra from individual adsorbed layers, which have been used to provide unambiguous determination of the adsorbed mass of the PEM at each stage of formation. The PEM was seen to undergo a transition in growth regimes during build-up: from an initial supra-linear stage to a linear growth regime. In addition, the wettability of the PEM has been probed at each stage of the build-up, using the captive bubble contact angle technique. Captive bubble contact angle measurement is essential for accurate measurement of hydrophobicity of hydrated soft matter films formed via aqueous processing.
The contact angles of the PEM film were uniformly low, but showed variation in value depending on the nature of the outer polymer layer (termed an odd-even effect), and this variation correlated with the overall percentage hydration of the PEM (determined from FTIR and quartz crystal microbalance data). The nature of the hydration water within the polyelectrolyte multilayer has also been studied with FTIR spectroscopy, specifically insitu synchrotron ATR FTIR microscopy of the multilayer confined between two solid surfaces. The acquired spectra have enabled the hydrogen bonding environment of the PEM hydration water to be determined.

A quantitative relationship between Leidenfrost point and surface characteristics such as surface material and roughness is investigated. Based on the relationship, we have fabricated the surfaces with their Leidenfrost points (LFP) tuned by controlling surface coating and structures. As discovered by Leidenfrost, liquids placed on a hot plate levitate on the gas phase-air gap formed by the vaporization of liquids. This phenomenon is called ‘Leidenfrost effect&’. A change of LFP has attracted many researchers for several years but the ability to tune LFP is still a remaining issue. Many of previous work has progressed for various conditions so the systematic approach and analysis are needed to clearly correlate the LFP and the surface conditions. In this report, we investigate a relation of surface energy and LFP using various coating materials such as Octadecyltrichlorosilane (OTS) and 1H, 1H, 2H, 2H-Perfluorooctyltrichlorosilane (FOTS). Also, we analyze how surface roughness affects LFP via surface micro/nano structuring. The improved understanding can have potential applications such as the control of liquid droplet behavior at elevated temperatures for efficient cooling system.
This research was supported by the MSIP (Ministry of Science, ICT and Future Planning), Korea, under the “IT Consilience Creative Program” (IITP-2015-R0346-15-1008) supervised by the IITP (Institute for Information & Communications Technology Promotion)

Photocatalytic materials such as TiO₂ have attracted significant attention for their ability to alter the wettability of surfaces through external actuation. These materials can degrade adsorbed organic deposits and confer superhydrophilic properties to the surface upon irradiation with UV light. The excitation of TiO₂ by light with energy greater than the material&’s bandgap is the primary process underlying applications of titania in self-cleaning, antifogging, treatment of wastewater and most recently separation of oil-water mixtures. However, the large bandgap of TiO₂ (~ 3.2 eV for anatase and ~ 3.0 eV for rutile) limits its ability to absorb visible light or sunlight effectively. Given that only ~ 5 % of solar flux incident at the earth&’s surface lies in this spectral regime (with lambda; < 390 nm), utilization of a large fraction of sunlight for excitation process can thus be enhanced by tuning the bandgap of TiO₂ to the visible spectral regime. In this work, we have developed novel approaches to tune the bandgap of TiO₂ to be responsive in the visible spectral regime by surface modification. Under visible light irradiation, we show that these modified TiO₂ surfaces undergo hydrophobic-to-hydrophilic conversion in both air (dry) and oil (submerged) environments. We explain the mechanism behind the change in wettability in terms of the surface regeneration processes. Such unique visible-light-induced hydrophobic-to-hydrophilic conversion of our surfaces enables on-demand separation of oil-water mixtures. We anticipate that the materials that have such tunable light-responsive wettability in the visible/solar spectrum can find broad applications in oil-water separation, wastewater treatment, self-cleaning, and phase transitions.

The ability of surfaces to repel liquid jets is of broad interests and has numerous applications. Here, we will describe a broad class of materials that can repel liquids jets over a wide range of surface tension; upon impact, a liquid jet is able to bounce off such a material stably, even when the liquid wets the surface. In contrast, most jet rebound phenomena described so far are sensitive to the wetting behavior of the liquid, e.g. a superhydrophobic surface typically loses its repellence for jets of low-surface-tension liquid. We will describe a new mechanism for this stable jet rebound and outline how material properties dictate jet rebound behavior.

Surface energies of isotropic and amorphous metals have typically been measured through destructive or high-temperature methods. Low surface energy materials like polymers, ceramics and glasses use contact angles measurements as an effective surface energy calculation technique at room temperature, but such an approach is inconsistent with metal surfaces when the interacting liquid is water or an organic liquid. The strong metal inter-atomic and molecular bonds at the free surface interfere with the liquid drop surface tension and resultant contact angles. The free surfaces we seek to characterize are those of pure metals such as aluminum, copper, iron, and iron-based alloys such as Fe-Al and Fe-Ga systems. There are currently no experimental methods available to measure surface energies associated with crystallographic orientation-, morphology-, and composition-dependence of metallic solid surfaces at/near room temperature.
In this study, a liquid gallium (Ga) drop contact angle and droplet size measurement were employed. Ga, which has a melting point of ~30°C, overcomes the shortcomings of existing methods for metallic materials. The surface energy of the free metallic surface as a function of temperature, γ_SV(T), can be related to thermal-expansion-induced changes in the liquid gallium drop contact angle and the shape of the drop. A Ga drop was placed on the sample surface where the sample is heated from 30-100°C. Initial designs implemented a radiative temperature control box which enclosed an argon gas filled container where the sample resides. Improvements on the contact angle goniometer have been made to fit this experiment&’s needs. Tens of milligram Ga drops on multiple substrates (wood, polymers, glass, stone) were observed as the temperature was varied in order to validate our approach with published surface energy results. Liquid Ga contact angles have been observed on the aforementioned pure metals and alloys. A recent publication that mentions use of liquid mercury for contact angle measurement[1] provides us with alternate avenues for metal surface energy calculation.
[1] V. M. Gasanov, “MEASUREMENT OF THE EQUILIBRIUM CONTACT ANGLE OF WETTING AS A METHOD OF STUDYING THE STATE OF THE SURFACE ENERGY AT THE SOLID - LIQUID - GAS INTERFACE,” vol. 87, no. 3, pp. 599-604, 2014.

A superamphiphobic surface, trapping air in microscopic protrusions, repels both high and low surface tension liquids (> ~25 mN/m) with low roll-off angle (< 10^o) and high receding contact angle (> 150^o). [1] Due to the low adhesion of the liquid to the surface, the shape of the droplet is determined by minimizing the interfacial energy between liquid and air. Recently, using this unique surface property, a new concept of polymer microparticle synthesis from polymer powder or directly from monomers on a superamphiphobic surface, based on the soot-templating method, was demonstrated. [2] Inspired from those previous researches, in this present study, we introduce a strategy to fabricate mesoporous supraparticles from nanoparticle (NP) dispersion on the superamphiphobic surface. The strong liquid repelling property of superamphiphobic surface allows the spherical shape of NP dispersion drop on the surface and the contact edge movement of drop during evaporation. Therefore drops can keep the spherical shape during evaporation and form mesoporous particles from drops of NP dispersion after water evaporation. Thanks to the small contact area of fabricated mesoporous particles by moving contact edge of drops, fabricated particles could be detached and collected from the surface. This is environmental friendly particle formation method because over the whole particle fabrication process, water is the only wasting material. In addition, mesoporous particles with various sizes (few micron to 600 mu;m), materials (e.g. TiO₂, SnO₂, and ZnO), and structures (e.g. heterogeneous particle and core/shell particle) were produced by only simple parameter alternation e.g. drop volume, and concentration and types of NPs. Furthermore in order to develop for the multiple particle formation process, a multi-nanodroplet-dispensing machine was employed producing large amounts of smaller than 10 µm-size mesoporous particles continuously for various scientific applications.
[1] X. Deng, L. Mammen, H.-J. Butt, D. Vollmer, Science 2012, 335, 67.
[2] X. Deng, M. Paven, P. Papadopoulos, M. Ye, S. Wu, T. Schuster, M. Klapper, D. Vollmer, H.-J. Butt, Angew. Chem. Int. Ed. 2013, 52, 11286.

Microscale technologies offer the capability to generate in vitro artificial cellular microenvironments that recapitulate the spatial, biochemical and biophysical characteristics of the native extracellular matrices and enable systematic, quantitative, and high-throughput studies of cell fate in their respective environments. We developed a microfluidic platform for the generation of two-dimensional arrays of micrometer-size cell-laden hydrogel modules (HMs) for cell encapsulation and culture. Fibroblast cells (NIH 3T3) and non-adhesive T cells (EL4) encapsulated in HMs showed high viability and proliferation. The platform was used for real-time studies of the effect of spatial constrains and structural and mechanical properties of HMs on cell growth, both on the level of individual cells. Due to the large number of cell-laden HMs and stochastic cell distribution, cell studies were conducted in a time- and labor efficient manner. The platform has a broad range of applications in the exploration of the role of chemical and biophysical cues on individual cells, studies of in vitro cell migration, and the examination of cell-extracellular matrix and cell-cell interactions.

With the advent of nanotechnology, physical properties of nanomaterials and nanostructures have been investigated by a large variety of theoretical and experimental techniques in order to better understand their structural, electronic, optical and thermal properties.
One of the fundamental questions in nanoscience is how structural and dynamic properties on the macroscale behave when dimensions are reduced to the nanoscale. One example of this is drops [1]. Drops, by definition, are structures composed of a small amount of liquid, completely surrounded (or almost completely) by free surfaces.
The impact dynamics of drops on solid or liquid surfaces are key elements on a series of important phenomena on basic science and technological applications. Examples of this are ink-jet printers, rapid cooling by liquid spray, etc.
An example of how the knowledge on this area is yet incipient is the wettability of graphene [2, 3], one of the most investigated materials. There are conflicting data even for fundamental aspects as hydrophobicity [4].
In this work we report a study on the dynamical properties of the splash of nanodroplets on graphene substrates by means of molecular dynamics simulations. Preliminary results show that when a droplet of about 15000 atoms is deposited at the surface, the liquid assumes a hemispherical configuration. Simulations with impact velocities of 100, 500 and 1000 m/s show varied splash patterns as the velocity increases on the graphene sheet, with the drop initially spreading on the surface but then retracting and maintaining a shape similar to that of the deposition only for velocities of 100 and 500 m/s. The contact angle of droplet deposition and impact (87 and 85 degrees, respectively) are strongly suggestive (at least for the droplet sizes considered here) of a hydrophilic behavior.
[1] A. L. Yarin, Annu. Rev. Fluids Mech. 38, 159 (2006).
[2] K. S. Novoselov et al., Science 306, 666 (2004).
[3] Editorial, Nature Mater. 12, 865 (2013).
[4] Z. Li et al., Nature Mater. 12, 925 (2013).

Dynamic wetting transitions play an important role during the formation and transport of liquid dispersions in microgeometries. The development of methods for finely handling viscous materials at the small-scale requires a better understanding of the interactions between multi-fluids and solid surfaces under confined microflows. Multiple viscous substances can indeed adopt complex flow arrangements and exhibit a variety of wetting and lubrication instabilities based on a combination of fluid properties, flow parameters, and geometry. Here, we conduct a systematic study to elucidate the influence of large viscosity coefficients during liquid/liquid multiphase flows in microchannels. We experimentally measure the natural spreading behavior of model fluid pairs on flat borosilicate glass surfaces over a range of viscosity contrasts using high-speed goniometry and probe their forced dynamic wetting properties when injected into microfluidic devices. Scaling laws are developed based on droplet and carrier fluid viscosities to predict wetting/lubrication transitions of microfluidic segmented flows and two-fluid separated flows. A variety of small-scale flow regimes and dynamic fluid arrangements are also characterized, including an intriguing mode of droplet formation where a wetting tongue forms a rivulet that regularly emits droplets and the occurrence of cornered droplets in liquid/liquid systems. This study shows the possibility to dynamically structure thick fluids and engineer multi-fluid materials using novel microflow configurations.

Airborne hydrocarbon is a common contaminant for surface analysis. However, its impact on the wettability has often be neglected. This presentation will highlight the impact of airborne hydrocarbon on the wettability measurements. As an example, we show that graphitic surfaces can be contaminated by airborne hydrocarbon within minutes of air exposure. Such contamination increases the hydrophobicity of the surface: although graphite have long been believed to be hydrophobic, we show that its hydrophobicity is actually caused by adsorption of airborne hydrocarbon; a clean graphitic surface is in fact mildly hydrophilic.

Although there is already a rich body of literature on the subject of drop impacts on solid surfaces, the vast majority consider only a normal angle of incidence, and impact velocities limited by the terminal velocity of the droplets, around 4 m/s. These reports have been pivotal in understanding the fundamentals of the impact process, but they cannot accurately represent the impact process involved in engineering problems such as rain erosion on turbine blades and fast-moving vehicles, or ice accretion on airplane wings. Hence, the objective of this study is to comprehensively determine what impact behaviours are possible in these applications, and to predict the conditions under which they occur. To accomplish this, we performed high-speed (up to 27 m/s, We>9000), oblique drop impacts with surface tilt angles ranging from 0° to 90°. These high velocities were achieved by applying the double-diaphragm ballistics method to accelerate sample surfaces, in synchronization with a drop-on demand generator. Using this experimental setup, we tested smooth, rough, and textured (superhydrophobic) PTFE and aluminum surfaces, which allowed us to isolate the role of the surface chemistry and structure on the behaviour, leading to several novel discoveries. By comparing the aluminum and PTFE surfaces, we found that the inherent surface chemistry affects the normal splashing threshold of impacting droplets, so that for surfaces of equivalent structure, splashing is encouraged on the more hydrophobic surface. Comparing between the smooth and superhydrophobic surfaces, we found that the observed impact behaviours were more symmetrical on the superhydrophobic surfaces, with very few instances of one-sided splashing, whereas we observed asymmetrical splashing under a wide range of conditions on the smooth surfaces. We relate this difference to the low liquid-solid surface fraction of superhydrophobic surfaces, which reduces the drag of liquids flowing over them. In the case of oblique drop impacts, this effect causes the behaviour to be governed almost exclusively by the normal component of the impact velocity, since the droplets can spread across the surfaces laterally without being greatly affected by the tangential motion of the surface. Finally, we found that on superhydrophobic surfaces, the pinning transition from rebounding to partial rebounding behaviour is dependent only on the normal impact velocity, and is independent of the tangential component. As a result, in highly oblique impacts on our superhydrophobic surfaces, we observed a new impact behaviour that we call the stretched rebound, in which the droplet rebounds from the surface during the spreading phase, instead of after receding.

Water hydration is ubiquitous to biological molecules and surfaces at the nanoscale and critical to the mediation of all biological processes¹. However, the lack of methods with sufficient resolution and/or sensitivity under moisturized environments results to the wetting of biological surfaces not well understood². Here, we discuss the mechanisms leading to height formation in ambient atomic force microscopy AFM and use them to show how the apparent height of DNA molecules is affected by their local hydrophilicity/hydrophobicity relative to the supporting surface. In addition, we provide an interpretation behind the loss of apparent height that occurs in the AFM operated under attractive regime irrespectively of stiffness. Our study on apparent height measurements is consistent with partial hydration of the molecule relative to the supporting surface. Our approach to nanoscale water affinity mapping could impact the study of biological processes where the role of water films and fluidity is relevant such as protein folding and proteinminus;DNA interactions.
Reference
1 Cheung, M. S., García, A. E. & Onuchic, J. N. Protein folding mediated by solvation: Water expulsion and formation of the hydrophobic core occur after the structural collapse. Proceedings of the National Academy of Sciences99, 685-690, (2002).
2 Luger, K., Mader, A. W., Richmond, R. K., Sargent, D. F. & Richmond, T. J. Crystal structure of the nucleosome core particle at 2.8[thinsp]A resolution. Nature389, 251-260, (1997).

Capillary interactions resulting from liquid bridges or interfacial deformation around wettable particles have been a topic of many investigations, but their extraordinary potential in the making of responsive nanoscaled structures has largely escaped attention. We will present a new class of materials where ultraflexible filaments are assembled from magnetic nanoparticles wetted by liquid and bound by capillarity. The superparamagnetic nanoparticles used as structural units are covered by condensed, surface-anchored lipid films, which on contact form nanocapillary liquid bridges between them. Initial burst of magnetic field collects the particles into filaments by magnetophoresis. After switching off the field the particles retain their structural arrangement by a soft attractive potential induced by the liquid-like bridges. The soft and reversible capillary binding on contact and the fluidity of the nanobridges provide extraordinary high flexibility and reconfigurability of the filaments (Nature Materials, 2015). The formation of liquid menisci was correlated to the thermodynamic phase of the lipid shell around the particles, its fluidity and nanoparticle wettability. The estimates of the magnitude of the nanocapillary force are in reasonable agreement with the experiment. The liquid bridges allow for easy particle rolling and sliding; thus the resulting ultrahigh flexibility was measured to be orders of magnitude higher than any other linear structures reported to date. These new soft and magnetically responsive structures can be dynamically reconfigured and programmed. This soft, "snappable" capillary binding is an unconventional way of assembling self-repairing networks and potentially making magnetically self-repairing gels and other types of reconfigurable soft matter.

An experimental investigation of time dependent wetting behaviors of water droplet is presented on rough metal substrates which the mean roughness of samples(sample#8544;#12289;#8545;) are 1.435mu;m and 5.721mu;m, respectively, in order to study the influence of rough structures on time dependent wetting of metal substrates. AISI 304L stainless steel samples were polished to investigate the impact of roughness on wettability property on metal substrates. The static contact angles and contact angles evolution over time were measured with a video contact angle analyzer. The results show that different mean roughness corresponds to different wetting behavior of water droplet on rough metal substrates, The hydrophobic property increases with increase of surface roughness, the contact angle of sample#8544;with low roughness is 48.8°, and the contact angle of sample#8545;with high roughness is 62.1°. And the contact angle decreases obviously with the time changing, it due to the state of wetting transitions. The droplet spread on flat substrates with low roughness slowly, the pace of expansion is slower with low roughness than that on the rough metal surfaces. When time t=0s, the wettability state is Cassie state, then the droplet spreads over time, the contact angle decreases progressively. The length reduction between ethanol / water compound droplet is obviously faster than water droplet, the decrease of contact angle with ethanol / water compound droplet is faster than contact angle with water droplet, the wetting speed of ethanol / water compound droplet is faster than water droplet, they are all due to the volatile liquid. The state change into almost completely Wenzel state but can not reach the Wenzel perfect wetting state. Indeed, little air bubbles still get trapped at the interface. Compare with Cassie state, Wenzel wetting state is more sensitive to the change of surface roughness.The deeper depth of rough surface, the more gas expelled out of the gap, the more contact angle decease, and these variations are accompanied by modifications of the time dependent wetting behaviors. They are all agreement with the experiment results. The specific process between Wenzel-Cassie wetting state transition on rough patterned hydrophilic or hydrophobic material in detail needs further scrutiny.

The separation of oil/water emulsions has become one of the most urgent global environment problems because of increased oil pollution caused by petrochemical, textile, and food industries. Traditional hydrophobic/oleophilic absorbent material and filtration membranes are frequently used in practical applications, in which the problems they suffer from are pore clogging and surface fouling by oil. Therefore, the development of advanced material that can selectively absorb water and completely repel oil is highly desirable. Underwater superoleophobic phenomenon provides an opportunity to design a surface with high water affinity and low oil adhesion. Studying the wetting mechanism of an underwater oil droplet on structured surfaces is significant for the preparation of surfaces with controlled wettablility. In this manuscript, wetting behavior of an under oil droplet on structured surfaces was investigated by molecular dynamics simulations and experiments. Surfaces with different wetting properties and pillared structures were simulated, in which the wetting properties were expressed by the water contact angles of their corresponding flat surfaces, and the structures were modified by varying the pillar area ratio. Wetting states and contact angles of the oil droplet on different surfaces were studied. The relation between the wetting of two-liquid system (underwater oil) and the wetting of one liquid (only water or oil) was derived and compared with the Young's Equation. Furthermore, an underwater superoleophobic mesh was prepared by experimental deposition of hydrophilic nanoparticles on a stainless-steel mesh. The wetting behavior of underwater oil droplets in the experiment was in good agreement with that in the simulation.

Omniphobic surfaces have shown great promise for reducing drag in microfluidic devices, in pipelines, and on ship hulls. Traditionally, the contact angle a fluid drop makes with a surface has been used to characterize the repellency of the surface. A yield-stress drop however does not reach its equilibrium state; for instance a “drop” of toothpaste takes a complex shape on your toothbrush. What is its contact angle? How does the drop&’s surface tension and yield-stress interplay to pin the drop? We present a theory to predict the drag reduction of yield-stress fluids and methods for accurately decoupling contributions from surface tension and yield-stress. Experiments conducted on omniphobic surfaces in a parallel plate rheometer reveal an apparent reduction in yield-stress independent of surface tension. By examining a drop sliding down an inclined plane, the coupling of surface tension and yield-stress allows the angle at which the drop begins to slide to be computed. Furthermore, we demonstrate application of omniphobic surface technology in enabling the operation of a semi-solid flow cell in which the reactants exhibit yield-stress rheology.

In nature, superhydrophobic surfaces of the water bug Notonecta glauca and the floating fern Salvinia passively fix and retain an air film underwater, thus giving these species the ability to effectively move on and in water. The underwater air film minimizes the water-solid contact area and significantly reduces the frictional drag between water and solid. Superhydrophobic and air-retaining properties of natural surfaces originate from the dense layers of micro- and nanoscale hairs covering them. Inspired by the natural surfaces, we developed a nanofur material covered by a layer of densely packed randomly distributed high aspect ratio nano- and microhairs with different size microcavities at their base. Polymeric nanofur is fabricated using a highly scalable hot pulling method in which softened polymer is locally elongated with a heated sandblasted steel plate.
The major problem of the air-retaining surfaces is the instability of the retained air film under external stimuli such as pressure or flow. To investigate the robustness of the underwater air film on the irregular surface topography of the nanofur, we analyzed the airminus;water interface above the nanofur underwater at different applied pressures. The changes in the air-covered area were estimated using reduced reflectivity of the airminus;water interface collapsed under hydrostatic pressure. The air layer retained by the nanofur withstands similar applied pressures as the air layer of Salvinia molesta leaves, and much higher pressures than the one on Lotus leaves. As a result of the nanofur topography consisting of different size microcavities surrounded by hairs, we observed a gradual decrease in the area covered by air. The experimentally measured results are consistent with theoretically predicted critical applied pressures. Furthermore, we studied the intensity profiles of the air/water interface using confocal microscopy to estimate the long term stability of the air layer and the behavior of the air/water interface above microcavities under pressure. Finally, the air-retention of the nanofur results in significant drag reduction, which was observed in microfluidic channels lined with nanofur.

In this talk, I will demonstrate how to alter macroscopic flows via interfacial modifications at micro scales. Three flow scenarios using soft interfaces are scrutinized and presented, namely forced wetting of drop impact, microfluidic slippage on a superhydrophobic surface, and electrically-driven convection upon a charge-selective surface. In drop impact on a solid substrate, the impact outcomes can be tuned by varying the microstructures. In microfluidic laminar flow, our pore-scale measurements reveal that the geometry of the gas-liquid interface strongly influences the hydrodynamic slippage, i.e., drag reduction, using hydrophobic micro-structures. In electrodialysis using an ion-selective membrane, micro-vortices set in under sufficiently large electric forcing, sustaining ion transport across the charged interface. From these results, we learn that interfacial conditions play an important role, thereby offering controls of flow motions on soft interfaces.

The use of electrowetting-on-dielectric (EWOD) and dielectrophoresis (DEP) techniques to form and manipulate volumes (tens of micro-liter) of dissimilar fluid droplets (oils and water) into single and double emulsions is described: one phase of the emulsion contains the chemicals needed to form a polymer and is combined with a second immiscible phase to form a double emulsion. The double emulsion is subsequently formed into a millimeter-size spherical polymer (or polymer foam) shell using the DEP force induced by an electric field. While there are many different uses for polymeric shells, the application of interest here is for the shell to contain the cryogenic hydrogen fuel needed for laser-driven fusion experiments. This application requires precise control of the droplets&’ volumes and the ability to make polymers with different chemical compositions and structures, including foam materials with varying densities.
To date, experiments with EWOD and DEP microfluidic techniques used a very limited set of liquids and have focused on using electrical forces to control fluid motion. Expanding the technique to make a product with this process requires adding chemicals to fluids that are needed to facilitate the chemistry, but these chemicals and fluids may diminish the efficacy of the EWOD/DEP techniques. This is the situation for our application: the desired material for the shells is a water-based polymer foam (resorcinol formaldehyde), which requires the presence of a surfactant in the water phase to stabilize the water/oil emulsion. The lower surface tension of the surfactant-containing water phase (<20 mN/m) greatly affects the wettability of the teflon-coated dielectric surface, which reduces the effectiveness of the EWOD/DEP process. This effect can be partially offset by using high voltages (up to 430 Vrms) and high frequencies (10 kHz) to extract and transport droplets, but there is an upper limit to this approach and alternative strategies are needed. The surfactant alters the wettability of the dielectric surface, is affected by the electric field, and helps stabilize thin fluid membranes that form when the droplet is moved (the “soap-bubble” effect); this behavior will be discussed in detail.
This presentation will provide an overview of the processes described above with an emphasis on the competing electrical and surface-energy parameters for using electric-field techniques to manipulate fluid droplets for subsequent materials processing.
This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Numbers DE-NA0001944 and DE-NA0001369, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

Large number physiological, pathological, and technological events necessitate description of the behavior of the soft, charged interfaces. Some of the examples include adhesion of a bacterium of the Streptococcus strain to its pathological site, regulation of the trans-membrane potential of the cell membrane for controlling the intracellular influx of ions and nutrients, attachment of MS2 bacteriophage virus to the bacteria, engineering metal and non-metal nanoparticles with grafted layers of polyelectrolytes (PEs) for applications ranging from advanced drug delivery and oil recovery to water harvesting and nano-manufacturing, fabrication of “smart” PE-grafted nanochannels for sensing and manipulating ion and liquid transport, etc. In this talk, we shall present our recent theoretical findings on different issues related to the thermodynamics, transport, and adhesion at such soft, charged interfaces. Our theory approximates the soft, charged interfaces as a layer of PEs grafted on a rigid solid and in contact with an electrolyte solution. The important issues that will be highlighted are (a) Prediction of a new functional form of the distribution of the monomers of a single grafted PE in a PE layer with pH-dependent charge density, (b) Scaling Laws for PE-grafted nanochannels, (c) Comparison of the electroosmotic and ionic currents in PE-grfated nanochannels, (d) Under-water adhesion on PE-grafted interfaces. We anticipate that our findings will provide important breakthroughs and motivate new experiments to better understand the science and engineering of soft, charged interfaces.

Microfluidics offers a tight control over fluid flows, allowing the production of drops with a very narrow size distribution. To achieve this high level of control over the drop formation, the wettability of the channels must be carefully tuned. I will show examples, where adjustments in the channel wettability enable the production of drops with sizes ranging from 100 nm to 100 µm through unconventional mechanisms. For example, strongly non-wetting channel walls enable the production of single emulsion drops through a mechanism that is truly scalable. By contrast, strongly wetting channels allow production of double emulsion drops with very thin shells; these double emulsions can be used as templates to make capsules with shells as thin as a few nm and as thick as a few µm. In addition, I will show how the wetting properties of channel walls affect the formation of air-borne, sub-mm sized drops; these very small drops can be used as templates to make nanoparticles of a variety of different compositions whose size and structure can be closely controlled.

Swimming at low Reynolds number (micro-organisms) requires movements or deformations that are not time-reversible. For an introduction see: E. M. Purcell, "Life at low Reynolds number" (American Journal of Physics 45, 3-11, 1977).
Here we discuss a simple realization of a two-rotor swimmer made of two interacting liquid drops that spontaneously counter-rotate in an applied electric field. The self-propulsion is due to the local electro-hydrodynamic flow instability around the drop pair, and the direction of propulsions can take any angle with respect to the applied electric field. Systems of several interacting rotors will also be discussed.

A number of physiological processes in living organisms involve the selective “catch and release” of biomolecules. Inspired by these biological processes, we use computational modeling to design synthetic systems that can controllably catch, transport, and release specific molecules within the surrounding solution, and, thus, could be harnessed for effective separation processes within microfluidic devices. Our system consists of an array of oscillating, microscopic fins that are anchored onto the floor of a microchannel and immersed in a flowing bilayer fluid. The oscillations drive the fins to repeatedly extend into the upper fluid and then tilt into the lower stream. The fins exhibit a specified wetting interaction with the fluids and specific adhesive interactions with nanoparticles in the solution. With this setup, we determine conditions where the oscillating fins can selectively bind, and thus, “catch” target nanoparticles within the upper fluid stream and then release these particles into the lower stream. We isolate the effects of varying the wetting interaction and the fins&’ oscillation modes on the effective extraction of target species from the upper stream. Our findings provide fundamental insights into the system&’s complex dynamics and yield guidelines for fabricating devices for the detection and separation of target molecules from complex fluids.

Capacitive deionization (CDI) takes advantage of electric double layer storage to desalinate brackish water. CDI can be a competitive technology for brackish water treatment due to its higher energy efficiencies compared to reverse osmosis and its more inherent resistance to fouling. In addition, low current and power requirements can allow for developing compact and off-grid CDI systems. However, CDI is still a developing technology where designing porous materials that maximize adsorption capacity and salt removal rates is still needed. In this study we used vertically-aligned carbon nanotube (VA-CNT) electrodes, with minimal tortuosity, to investigate the role of porous geometry on diffusion resistance, salt adsorption rate and capacity. These electrodes were grown to 1 mm heights using thermal chemical vapor deposition and were delaminated to separate it from the corrosive catalyst. Three-electrode testing was used to investigate the intrinsic material capacitance, where the specific capacitance was 60 F/g. We characterized the geometry of the material using impedance spectroscopy and determined pore diameters (CNT inter-spacing) averaging 74 nm with distributions of 25 nm. Meanwhile, we developed a flow-cell prototype to study salt adsorption in these VA-CNT electrodes. Through these flow cell experiments, we demonstrated adsorption capacities > 20 mg salt/g carbon material, which is competitive with existing materials. We also investigated the charge efficiency, a comparison between salt adsorption and charge accumulation, giving insight into double layer charging dynamics. These flow cells show charge efficiencies of up to 50%. Our work provides insights into the relationship between electric double layer capacitance and ion storage through a desalination framework and promises to guide the optimization of CDI systems in the future.

Electrokinetic cell separation methods have been previously established to have greater efficiency when compared to traditional flow cytometry methods. It has been shown by many researchers that buffer solutions in which cells are suspended in have enormous effects on producing required dielectrophoretic (DEP) forces to separate and manipulate cells. The goal of this project is to design and test tailored ionic liquid compositions for enhancing DEP forces on cells while creating an environment for preserving their integrity. We analyzed three methylimidazolium based ionic liquids (ILs) with low half maximal inhibitory concentration was used as suspension medium for cell separation. These dicationic ionic liquids possess slight electrical and structural differences with high thermal stability. The three trypes of buffers of which two ionic liquids and one standard sucrose media were tested for cytotoxicity and their ability to enhance DEP signals from Saos-2 (osteosarcoma, bone cancer cells) MC3T3-E1 (osteobalsts, healthy bone cells). The effects of varying concentration of ILs and their contribution towards DEP signals from cells were studied. We were able to achieve improved system resolution of manipulating cells with no harmful effects on cells. The studies showed that cancerous cells (Saos-2) demonstrated two orders higher tolerance and maximum lateral displacement due to DEP forces than healthy bone cells.

We have measured DNA translocation through a glass nanopore functionalized with probe DNA molecules. These DNA-functionalized nanocapillaries selectively facilitate the translocation of target ssDNAs that are complementary to the probe ssDNAs. In addition, translocation of the complementary target ssDNA exhibits two appearances of translocation speed, such as fast and slow translocation, whereas that of non-complementary target ssDNA yields only fast translocation events. These observations suggest that the complementary and non-complementary target ssDNAs may be discriminated due to interactions between target and probe ssDNAs. The temperature dependence measurements of DNA translocation show that slow translocation events are ascribed to the interaction between probe and target ssDNA. This confirms their dwell time is dependent on the base-pair binding strength. These results demonstrate that mere single-base different ssDNA groups are selectively sortable by using the DNA-functionalized nanocapillaries.

Piezoelectric sensors have been explored recently because of their application for flexible, wearable, and multifunctional electronics. However, there are some of drawbacks to realize such unique devices by utilizing solid conducting nanomaterials. In this study, the microfluidic system with ionic liquids is applied as an alternative approach to fabricate a highly sensitive and transparent strain sensor. The mixture of ionic liquids can match its refractive index with the elastomeric microchannel material, thereby making the fluidic strain sensor transparent. The resistance of the ionic liquids filled in the channel is highly responsive to various deformation of the sensor system, such as stretching, bending, twisting, and surface pressing, without a significant hysteresis. Our microfluidic strain sensor system shows great promise for realizing highly sensitive, transparent, and wearable motion detecting sensors.

We developed a label-free antimicrobial peptide (AMP) assisted microcantilever sensor array for rapid detection of Gram-positive and Gram-negative (Gram(+/-)) bacteria strains. Leucocin A (LeuA) and Colicin V (ColV) are functionalized onto gold coated microcantilever sensor array as the sensing layer using cysteamine linker to yield peptide density of ~4.8 nmol cm^-2. This sensor array was able to demonstrate explicit fast-recognition to the bacteria strains with at high sensitivity to 1 cell mL^-1 with continuous in-line detection. The limit of detection (LOD) for both Gram(+/-) bacteria is from 10³-10⁶ cfu mL^-1. This sensor has a sensitivity comparable with the antibody sensors[1] but a much higher stability due to the property of AMPs. The sensing array was also capable to distinguish an artificial contaminated water sample with mixed L. monocytogenes and E.coli. Furthermore, fragments AMPs were also used for pathogenic bacteria detection with similar sensitivity on microcantilever array.[2] Thus, the AMPs assisted microcantilever array is a rapid, sensitive BioMEMS sensor platform with required specificity. This study demonstrates the potential of using AMPs with microcantilever arrays for selective detection of in contaminated water and food products.
Reference:
[1] M.S. Mannoor, S. Zhang, A.J. Link, M.C. McAlpine, Proc. Natl. Acad. Sci. 107 (2010) 19207.
[2] S. Azmi, K. Jiang, M. Stiles, T. Thundat, K. Kaur, ACS Comb. Sci. 17 (2015) 156.

The functionality of materials is largely determined by the mechanisms that take place at sub-micron length scales and at interfaces. In order to understand these complex material systems and further improve them, it is necessary to measure and map variations in properties and functionality at the relevant physical, chemical, and temporal length scales. The goal of multimodal imaging is to transcend the existing analytical capabilities for nanometer scale spatially resolved material characterization at interfaces through a unique merger of advanced scanning probe microscopy, mass spectrometry and optical spectroscopy. Combining atomic force microscopy (AFM) and mass spectrometry (MS) onto one platform has been demonstrated by our group as a method for high resolution spot sampling and imaging of substrates. To advance this basic approach and to expand its capabilities we now have incorporated Band-Excitation (BE) to allow us to measure nanomechanical properties of soft materials by measuring the contact resonance frequency shift. In this presentation, I will discuss the benefits of a multimodal imaging system and demonstrate our results for polymeric systems and bacterial colonies. I will also talk about future developments to incorporate spectroscopic measurements into the platform.
This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, United States Department of Energy. ORNL is managed by UT-Battelle, LLC for the U.S. Department of Energy under contract DE-AC05-00OR22725.

In designing biomimetic polymer coatings, it is important to measure their physico-chemical properties in the environment for which they are intended. Generally, such coatings perform their function in the aqueous environment of physiological fluids, where interfacial charge development is inevitable. Thus the interaction of the coating with its surroundings and its subsequent functioning are highly sensitive to electrostatic effects.The glycocalyx, a highly negatively charged structure composed of glycosaminoglycans, proteins and carbhohydrate residues, resides as a membranous coating on endothelial cells. It is known to be electrokinetically active and can generate streaming current when a physiologically relevant electrolyte solution such as blood flows across its surface. By using streaming current to direct information exchange between the cell and its environment, it can influence biological functions such as erythrocyte mobility. It also has the interesting sieve-like property of repelling invasion by pathogens while selectively permitting ion transport and protein uptake. In this work, we present a model surface that mimics the function of the endothelial glycocalyx by resisting non-specific protein adsorption while retaining the ability to bind to target molecules via electrostatically mediated interactions. We accomplish this by designing a composite surface consisting of two elements. The first element, the underlying “rigid” substrate is a copolymer formed by the chemical vapor deposition (CVD) copolymerization of two paracyclophane-based monomers, one co-monomer consisting of an atom transfer radical polymerization (ATRP) initiator and the other co-monomer containing an ionizable amine moiety. The protonated amine functions as a positively charged affinity site that binds to negatively charged species of interest such as DNA, viral proteins, bacteria etc. The second element, the top layer, consists of a “soft” polymer brush grafted from the initiation sites on the copolymer using ATRP, and serves as the non-fouling background. The density of the affinity sites relative to that of the ATRP initiation sites and the resulting isoelectric point (IEP) can be tuned by modifying the CVD co-polymerization conditions. We quantified the surface charge of this hybrid surface using streaming current and examined the effects of several design parameters- the permeability of the polymer brush, the surface concentration of the amine, brush thickness, pH and ionic strength. To check whether the observed binding affinity of a model ligand follows a trend similar to that of our electrokinetic measurements, preliminary adsorption studies were performed. We conclude that streaming current measurements present an interesting platform to probe the structure and function of glycocalyx mimicking surfaces by shedding light on the effect of electrostatic, structural and hydrodynamic properties on their interfacial behaviour.

The phenomenon of colloidal particle adsorption at interfaces between two immiscible liquids has been investigated for more than a century, with a recent increased interest¹. Drops fully covered by particles, so called Pickering emulsion drops, are (among other applications) used to stabilize emulsions and they are also ideal templates for producing particles and advanced capsules². Recent studies show how electrohydrodynamic circulation flows in drops can assemble solid or fluid colloidal particle films on drop surfaces^3,4.
Here, we report investigations of both the rheology and dynamics of Pickering drops subjected to DC E-fields. Like leaky-dielectric drops in E-fields, free surface charges accumulate at colloidal capsule surfaces and create an electric stress forcing the capsule to deform. By using different particle packing and drop geometries, we consider how stiffness and shape of Pickering droplets may influence the macroscopic emulsion yield stress. Although there have been some studies on the response of particle covered drops or bubbles to shear flow⁵ or other mechanical forces⁶, there are very few reports on the plasticity of Pickering drops. We demonstrate plasticity of leaky-dielectric capsules by the utilization of a uniform DC E-field which offers good control of the compressive stress exerted on capsules as the applied stress can easily be adjusted, reversed or turned off.
We find that there is a capsule geometry dependent critical E-field corresponding to a deformation yield point of the colloidal capsule above which a capsule plastically deforms. Below this yield point, we observe electro-orientation, weak capsule deformation and crumpled and folded states. The crumpling of the Pickering drop surface enables the capsule to accommodate the compressive electric stress. The results are discussed in relation to standard rheological models, including a simple model to estimate the electric properties of the particle layer.
References:
1 Zeng, C., Bissig, H. & Dinsmore, A. D. Particles on droplets: From fundamental physics to novel materials. Solid State Commun.139, 547-556 (2006).
2 Rozynek, Z., Mikkelsen, A., Dommersnes, P. & Fossum, J. O. Electroformation of Janus and patchy capsules. Nature communications5 (2014).
3 Dommersnes, P. et al. Active structuring of colloidal armour on liquid drops. Nat. Commun.4, 2066 (2013).
4 Rozynek, Z., Dommersnes, P., Mikkelsen, A., Michels, L. & Fossum, J. O. Electrohydrodynamic controlled assembly and fracturing of thin colloidal particle films confined at drop interfaces. Eur. Phys. J. Special Topics223, 1859-1867 (2014).
5 Ha, J. W. & Yang, S. M. Electrohydrodynamic effects on the deformation and orientation of a liquid capsule in a linear flow. Phys Fluids12, 1671-1684 (2000).
6 Subramaniam, A. B., Abkarian, M., Mahadevan, L. & Stone, H. A. Colloid science: non-spherical bubbles. Nature438, 930, (2005).

The ability to control the shape of liquid metals on the sub-mm scale is important for many applications, including flexible electronics, optics, metamaterials, and microfluidics. While mercury has drawn much of the focus of liquid metal research, its toxicity and incompatibility with microfluidic systems limits its potential for applications. Gallium and gallium-based alloys, however, are perfectly suitable for use with microfluidics, but have traditionally been avoided due to a passivating surface oxide layer that limits their use in hydrodynamics and electrochemistry.
Here, we present a method to control the interfacial energy of a liquid metal via application of an applied voltage on its surface. This approach can tune the interfacial tension of a metal significantly, rapidly, and reversibly using low voltages (< 1 V). This drastic change in surface tension has been harnessed to induce a variety of previously unidentified electrohydrodynamic phenomena, including: injection and withdrawal of liquid metal in microfluidic channels at low voltages; directional control of liquid metal in an open channel; and the formation of liquid metal fibers beyond the Rayleigh instability limit. Furthermore, the phenomena can be made to occur on any substrate, and in a wide variety of electrolytes.
These properties can be harnessed to manipulate liquid metal alloys based on gallium, which may enable shape-reconfigurable metallic components in electronic, electromagnetic, and microfluidic devices without the use of toxic mercury. The results also suggest that oxides—which are ubiquitous on most metals and semiconductors—may be harnessed to lower interfacial energy between dissimilar materials.

Automated microfluidic systems allow to controllably assemble multiple droplets of complicated and previously unattainable morphologies. In the talk I will demonstrate the techniques for generation of multiple Janus droplets, i.e., arbitrarily long chains of alternating immiscible segments and for generation of arbitrary sequences of multiple encapsulated microdroplets with online and individual control over the number of cores and volumes of all the constituents (cores and shells). I will also discuss the process of folding of the one dimensional multiple droplet assemblies into the three dimensional structures—mesoscale “atoms”. Finally, I will discuss potential applications of the custom tailored multiple microdroplets and the microfluidic systems that generate these droplets in research on reaction-diffusion dynamic chemical systems.

The creation of new colloidal materials involves the design of functional building blocks. Here we introduce a microfluidic method for designing building blocks one by one, at high throughput, with a broad range of shapes. The method exploits a coupling between hydrodynamic interactions and depletion forces that controls the configurational dynamics of droplet clusters traveling in microfluidic channels. Droplet clusters can be cross-linked in situ with UV. By varying the flow parameters, clusters are prescribed a given size, geometry, chemical and/or magnetic heterogeneities enabling local bonding. We produce compact structures (chains, triangles, diamonds, tetrahedrons,...) and non compact structures, such as crosses and T, difficult to obtain with current techniques. Size dispersions are small (2 %) and throughputs are high (25000/hours). The work opens a new pathway, based on microfluidics, for designing colloidal building blocks with a potential to enable the creation of new materials.