Reducing Cancer Cell Adhesion using Microtextured Surfaces.

For the past century, trypsin has been the primary method of cell dissociation, largely without any major changes to the process. Enzymatic cell detachment strategies for large-scale cell culturing processes are popular but can be labor-intensive, potentially lead to the accumulation of genetic mutations, and produce large quantities of liquid waste. Therefore, engineering surfaces to lower cell adhesion strength could enable the next generation of cell culture surfaces for delicate primary cells and automated, high-throughput workflows. In this study, a process for creating microtextured polystyrene (PS) surfaces to measure the impact of microposts on the adhesion strength of cells is developed. Cell viability and proliferation assays show comparable results in two cancer cell lines between micropost surfaces and standard cell culture vessels. However, cell image analysis on microposts reveals that cell area decreases by half, and leads to an average twofold increase in cell length per area. Using a microfluidic-based method up to a seven times greater percentage of cells are removed from micropost surfaces than the flat control surfaces. These results show that micropost surfaces enable decreased cell adhesion strength while maintaining similar cell viabilities and proliferation as compared to flat PS surfaces.

Self-ejection of salts and other foulants from superhydrophobic surfaces to enable sustainable anti-fouling.

A recently discovered phenomenon in which crystalline structures grown from evaporating drops of saline water self-eject from superhydrophobic materials has introduced new possibilities for the design of anti-fouling materials and sustainable processes. Some of these possibilities include evaporative heat exchange systems using drops of saline water and new strategies for handling/processing waste brines. However, the practical limits of this effect using realistic, non-ideal source waters have yet to be explored. Here, we explore how the presence of various model aquatic contaminants (colloids, surfactants, and calcium salt) influences the self-ejection phenomena. Counterintuitively, we find that the addition of "contaminant" chemistries can enable ejection under conditions where ejection was not observed for waters containing only sodium chloride salt (e.g., from smooth hydrophobic surfaces), and that increased concentrations of both surfactants and colloids lead to longer ejection lengths. This result can be attributed to decreased crystallization nucleation time caused by the presence of other species in water.

Impact of Bubbles on Electrochemically Active Surface Area of Microtextured Gas-Evolving Electrodes.

The adverse effects of electrochemical bubbles on the performance of gas-evolving electrodes have been extensively studied. However, the ways in which bubbles dynamically alter the electrochemically active surface area during bubble evolution are not well understood. Here, we study hydrogen evolution at industrially relevant current densities by using controlled microtexture to examine this fundamental relationship. Surprisingly, the most densely microtextured electrodes have the lowest performance on an active surface area basis. Using high-speed imaging, we show that the benefits of microtexture to release smaller bubbles more consistently are outweighed by the inactivation induced by bubbles growing within the denser microtexture, causing these performance limitations. Additionally, we show that the area beneath adhered bubbles is electrochemically active, contrary to currently held assumptions. Our study therefore has broad implications for electrode design to avoid ineffective use of precious catalyst materials, which is especially critical for porous electrodes and three-dimensional structures with high specific surface areas.

Transient Effects Caused by Gas Depletion during Carbon Dioxide Electroreduction.

The novel use of carbon dioxide (CO2) electroreduction to generate carbon-based products which do not contribute to the greenhouse effect has promoted the vision of carbon dioxide as a renewable feedstock for future clean fuel production. Depending on the material choice for the electrocatalysis, a certain variety of products is expected from the carbon dioxide reduction reaction (CO2RR). However, as the CO2 concentration in areas close to the working electrode (relative to the diffusive boundary layer) decreases as it is being consumed and transformed into other products, the generation of H2 is favored to the detriment of CO2 electroreduction. Therefore, the extent to which H2 is produced can be used as a metric to evaluate the efficiency of CO2RR. This article proposes a model that accounts for the modes in which aqueous gas depletion evolves over time and affects the long-term CO2 electroreduction and the corresponding pH evolution near the electrode's surface. For the latter, two main contributions are distinguished: gas depletion due to CO2 consumption and ion generation in areas close to the electrocatalyst surface. pH is then suggested as an accurate and indirect means to measure CO2 concentration in a liquid electrolyte. We conclude that CO2 depletion causes a strong decay in the electrochemical reaction efficiency. In the end, we discuss several methods which may delay the onset of the adverse effects caused by gas depletion, such as the utilization of pulsed electroreduction, cycling the applied current to electrodes on and off periodically.

Self-Propulsion of Boiling Droplets on Thin Heated Oil Films.

We report on the self-propulsion of boiling droplets which, despite their contact with viscous, immiscible oil films, attain high velocities comparable to those of levitating Leidenfrost droplets. Experiments and model reveal that droplet propulsion originates from a coupling between seemingly disparate short and long timescale phenomena due to microsecond fluctuations induced by boiling events at the droplet-oil interface. This interplay of phenomena leads to continuous asymmetric vapor release and momentum transfer for high droplet velocities.

Differences between Colloidal and Crystalline Evaporative Deposits.

Evaporative deposits from drops are widely studied due to their numerous applications in low-effort self-assembly, including for inkjet printing, microscale separations, and sensing/diagnostics. This phenomenon has been broadly explored for drops containing suspended colloidal particles but has been less quantified for drops with dissolved solutes. When a drop of solute/solvent mixture is evaporated on a substrate, nonvolatile solutes become supersaturated as the solvent evaporates, which then leads to crystal nucleation at the substrate-drop contact line. Emerging crystals alter the local wettability and fundamentally alter the dynamics of evaporation, which, in turn, influences the resultant evaporative deposit. Here we investigate the role of interactions between the substrate, crystals, and solution by comparing the evaporative deposition of three different salts as solutes against an evaporating colloidal solution. We show that nucleation effects can cause crystalline deposits to have a temperature relationship that is opposite to that of colloidal deposits and demonstrate how a balance between the contact-line pinning force and nucleation controls the deposit size.

Asphaltene Adsorption on Functionalized Solids.

Asphaltenes, heavy aromatic components of crude oil, are known to adsorb on surfaces and can lead to pipe clogging or hinder oil recovery. Because of their multicomponent structure, the details of their interactions with surfaces are complex. We investigate the effect of the physicochemical properties of the substrate on the extent and mechanism of this adsorption. Using wetting measurements, we relate the initial kinetics of deposition to the interfacial energy of the surface. We then quantify the long-term adsorption dynamics using a quartz crystal microbalance and ellipsometry. Finally, we investigate the mechanism and morphology of adsorption with force spectroscopy measurements as a function of surface chemistry. We determine different adsorption regimes differing in orientation, packing density, and initial kinetics on different substrate functionalizations. Specifically, we find that alkane substrates delay the initial monolayer formation, fluorinated surfaces exhibit fast adsorption but low bonding strength, and hydroxyl substrates lead to a different adsorption orientation and a high packing density of the asphaltene layer.

Evaporative Crystallization of Spirals.

Spiral motifs are pervasive in nature, art, and technology due to their functional property of providing compact length. Nature is particularly adept at spiral patterning, and yet, the spirals observed in seashells, hurricanes, rams' horns, flower petals, etc. all evolve via disparate physical mechanisms. Here, we present a mechanism for the self-guided formation of spirals from evaporating saline drops via a coupling of crystallization and contact line dynamics. These patterns are in contrast to commonly observed patterns from evaporation of colloidal drops, which are discrete (rings, concentric rings) or continuous (clumps, uniform deposits) depending on the particle shape, contact line dynamics, and evaporation rate. Unlike the typical process of drop evaporation where the contact line moves radially inward, here, a thin film pinned by a ring of crystals ruptures radially outward. This motion is accompanied by a nonuniform pinning of the contact line due to crystallization, which generates a continuous propagation of pinning and depinning events to form a spiral. By comparing the relevant timescales of evaporation and diffusion, we show that a single dimensionless number can predict the occurrence of these patterns. These insights on self-guided crystallization of spirals could be used to create compact length templates.

Reducing the contact time of a bouncing drop.

Surfaces designed so that drops do not adhere to them but instead bounce off have received substantial attention because of their ability to stay dry, self-clean and resist icing. A drop striking a non-wetting surface of this type will spread out to a maximum diameter and then recoil to such an extent that it completely rebounds and leaves the solid material. The amount of time that the drop is in contact with the solid--the 'contact time'--depends on the inertia and capillarity of the drop, internal dissipation and surface-liquid interactions. And because contact time controls the extent to which mass, momentum and energy are exchanged between drop and surface, it is often advantageous to minimize it. The conventional approach has been to minimize surface-liquid interactions that can lead to contact line pinning; but even in the absence of any surface interactions, drop hydrodynamics imposes a minimum contact time that was conventionally assumed to be attained with axisymmetrically spreading and recoiling drops. Here we demonstrate that it is possible to reduce the contact time below this theoretical limit by using superhydrophobic surfaces with a morphology that redistributes the liquid mass and thereby alters the drop hydrodynamics. We show theoretically and experimentally that this approach allows us to reduce the overall contact time between a bouncing drop and a surface below what was previously thought possible.

Crystallization-Induced Fouling during Boiling: Formation Mechanisms to Mitigation Approaches.

Boiling is significantly altered by the presence of dissolved salts. In particular, salts whose solubility decreases with temperature have the tendency to crystallize and adhere to the heat transfer surface and adversely affect the thermal performance. Scaling due to the precipitation of such salts poses serious operational and safety challenges in several practical applications, including heat exchangers, pipelines, and desalination. Here, we study the effect of dissolved salts on the dynamics of pool boiling and its impact on the heat transfer coefficient and critical heat flux (CHF). We find that even undersaturated conditions can lead to crystallization and scale buildup on the boiling surface and dramatically lower heat transfer performance. For example, the CHF for a salt solution that is 75% of the saturation concentration is found to be at least 2 times lower than that for deionized water. Using simultaneous high-speed optical and infrared imaging, we determine the interdependence between crystallization-induced scale formation and bubble evolution dynamics, including bubble nucleation, growth, and departure. We find that salt crystallizes in a "coffee-ring" pattern due to evaporation at the contact line of the bubble. On the basis of the role of the microlayer and triple contact line on scale formation, we propose manipulating surface wettability as a means to avoid scale formation and the associated decrease in the heat transfer coefficient. Surfaces with hybrid wettability are demonstrated as a means to mitigate the reduction in the heat transfer coefficient and CHF in the presence of dissolved salts.

Evaporative Crystallization in Drops on Superhydrophobic and Liquid-Impregnated Surfaces.

Mineral-fouling induced corrosion and deterioration of marine vessels, aircraft, and coastal structures is due in part from structural intrusion of crystals grown from ocean-generated saline drops. As such, much work has explored surface treatments that induce hydrophobicity or introduce barriers for antifouling and corrosion prevention; however, the efficacy of these strategies will be altered by the underlying substrate texture. Here, we study the behavior of evaporating saline drops on superhydrophobic and liquid-impregnated surfaces as a function of surface texture. On superhydrophobic surfaces, four disparate regimes (which are not observed for particle-laden drops) emerge as a function of the substrate solid fraction: Cassie-pinning, Cassie-gliding, Cassie-Wenzel transition, and Wenzel. These regimes control the morphology of the resultant crystal deposits. In contrast to the superhydrophobic surfaces, spreading liquid-impregnated surfaces demonstrate minimal influence of solid fraction on evaporative crystallization. The area, area localization, timescale of evaporation, and deposit morphology are all normalized by the presence of the lubricating layer, thus introducing an efficient method of eliminating crystal "coffee rings" as well as reducing the potential for fouling and corrosion.

Design of a spaceflight biofilm experiment.

Biofilm growth has been observed in Soviet/Russian (Salyuts and Mir), American (Skylab), and International (ISS) Space Stations, sometimes jeopardizing key equipment like spacesuits, water recycling units, radiators, and navigation windows. Biofilm formation also increases the risk of human illnesses and therefore needs to be well understood to enable safe, long-duration, human space missions. Here, the design of a NASA-supported biofilm in space project is reported. This new project aims to characterize biofilm inside the International Space Station in a controlled fashion, assessing changes in mass, thickness, and morphology. The space-based experiment also aims at elucidating the biomechanical and transcriptomic mechanisms involved in the formation of a "column-and-canopy" biofilm architecture that has previously been observed in space. To search for potential solutions, different materials and surface topologies will be used as the substrata for microbial growth. The adhesion of bacteria to surfaces and therefore the initial biofilm formation is strongly governed by topographical surface features of about the bacterial scale. Thus, using Direct Laser-Interference Patterning, some material coupons will have surface patterns with periodicities equal, above or below the size of bacteria. Additionally, a novel lubricant-impregnated surface will be assessed for potential Earth and spaceflight anti-biofilm applications. This paper describes the current experiment design including microbial strains and substrata materials and nanotopographies being considered, constraints and limitations that arise from performing experiments in space, and the next steps needed to mature the design to be spaceflight-ready.

Inverted Leidenfrost-like Effect during Condensation.

Water droplets condensing on solidified phase change materials such as benzene and cyclohexane near their melting point show in-plane jumping and continuous "crawling" motion. The jumping drop motion has been tentatively explained as an outcome of melting and refreezing of the materials surface beneath the droplets and can be thus considered as an inverted Leidenfrost-like effect (in the classical case vapor is generated from a droplet on a hot substrate). We present here a detailed investigation of jumping movements using high-speed imaging and static cross-sectional cryogenic focused ion beam scanning electron microscope imaging. Our results show that drop motion is induced by a thermocapillary (Marangoni) effect. The in-plane jumping motion can be delineated to occur in two stages. The first stage occurs on a millisecond time scale and comprises melting the substrate due to drop condensation. This results in droplet depinning, partial spreading, and thermocapillary movement until freezing of the cyclohexane film. The second stage occurs on a second time scale and comprises relaxation motion of the drop contact line (change in drop contact radius and contact angle) after substrate freezing. When the cyclohexane film cannot freeze, the droplet continuously glides on the surface, resulting in the crawling motion.

How droplets nucleate and grow on liquids and liquid impregnated surfaces.

Condensation on liquids has been studied extensively in context of breath figure templating, materials synthesis and enhancing heat transfer using liquid impregnated surfaces. However, the mechanics of nucleation and growth on liquids remains unclear, especially on liquids that spread on the condensate. By examining the energy barriers of nucleation, we provide a framework to choose liquids that can lead to enhanced nucleation. We show that due to limits of vapor sorption within a liquid, nucleation is most favoured at the liquid-air interface and demonstrate that on spreading liquids, droplet submergence within the liquid occurs thereafter. We provide a direct visualization of the thin liquid profile that cloaks the condensed droplet on a liquid impregnated surface and elucidate the vapour transport mechanism in the liquid films. Finally, we show that although the viscosity of the liquid does not affect droplet nucleation, it plays a crucial role in droplet growth.

Hydrophobicity of rare-earth oxide ceramics.

Hydrophobic materials that are robust to harsh environments are needed in a broad range of applications. Although durable materials such as metals and ceramics, which are generally hydrophilic, can be rendered hydrophobic by polymeric modifiers, these deteriorate in harsh environments. Here we show that a class of ceramics comprising the entire lanthanide oxide series, ranging from ceria to lutecia, is intrinsically hydrophobic. We attribute their hydrophobicity to their unique electronic structure, which inhibits hydrogen bonding with interfacial water molecules. We also show with surface-energy measurements that polar interactions are minimized at these surfaces and with Fourier transform infrared/grazing-angle attenuated total reflection that interfacial water molecules are oriented in the hydrophobic hydration structure. Moreover, we demonstrate that these ceramic materials promote dropwise condensation, repel impinging water droplets, and sustain hydrophobicity even after exposure to harsh environments. Rare-earth oxide ceramics should find widespread applicability as robust hydrophobic surfaces.

Drag reduction using lubricant-impregnated surfaces in viscous laminar flow.

Lubricant-impregnated surfaces (LIS), where micro/nanotextured surfaces are impregnated with lubricating liquids, have received significant attention for their robust, superslippery properties. In this study, we systematically demonstrate the potential for LIS to reduce drag in laminar flows. We present a scaling model that incorporates the viscosity of the lubricant and elucidates the dependence of drag reduction on the ratio of the viscosity of the working fluid to that of the lubricant. We experimentally validate this dependence in a cone and plate rheometer and demonstrate a drag reduction of 16% and slip length of 18 µm in the case where the ratio of working fluid viscosity to lubricant viscosity is 260.

Different shades of oxide: from nanoscale wetting mechanisms to contact printing of gallium-based liquid metals.

Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because, despite having surface tension 10 times higher than water, they strongly adhere to a majority of substrates. This unusually high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In this work, we demonstrate that, dependent on dynamics of formation and resulting morphology of the liquid metal-substrate interface, GaInSn adhesion can occur in two modes. The first mode occurs when the oxide shell is not ruptured as it makes contact with the substrate. Because of the nanoscale topology of the oxide surface, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate's surface energy or texture. In the second mode, the formation of the GaInSn-substrate interface involves rupturing of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We demonstrate that in this latter mode GaInSn adhesion is dominated by the intimate contact between new oxide and substrate. We also show that by varying the pinned contact line length using varied degrees of surface texturing, the adhesion of GaInSn in this mode can be either decreased or increased. Lastly, we demonstrate how these two adhesion modes limit microcontact printing of GaInSn patterns but can be exploited to repeatedly print individual sub-200 nm liquid metal drops.

Ice adhesion on lubricant-impregnated textured surfaces.

Ice accretion is an important problem and passive approaches for reducing ice-adhesion are of great interest in various systems such as aircrafts, power lines, wind turbines, and oil platforms. Here, we study the ice-adhesion properties of lubricant-impregnated textured surfaces. Force measurements show ice adhesion strength on textured surfaces impregnated with thermodynamically stable lubricant films to be higher than that on surfaces with excess lubricant. Systematic ice-adhesion measurements indicate that the ice-adhesion strength is dependent on texture and decreases with increasing texture density. Direct cryogenic SEM imaging of the fractured ice surface and the interface between ice and lubricant-impregnated textured surface reveal stress concentrators and crack initiation sites that can increase with texture density and result in lowering adhesion strength. Thus, lubricant-impregnated surfaces have to be optimized to outperform state-of-the-art icephobic treatments.

Mechanism of frost formation on lubricant-impregnated surfaces.

Frost formation is a major problem affecting a variety of industries including transportation, power generation, construction, and agriculture. Currently used active chemical, thermal, and mechanical techniques of ice removal are time-consuming and costly. The use of nanotextured coatings infused with perfluorinated oil has recently been proposed as a simple passive antifrosting and anti-icing method. However, we demonstrate that the process of freezing subcooled condensate and frost formation on such lubricant-impregnated surfaces is accompanied by the migration of the lubricant from the wetting ridge and from within the textured substrate to the surface of frozen droplets. For practical applications, this mechanism can comprise the self-healing and frost-repelling characteristics of lubricant impregnated-surfaces, regardless of the underlying substrate's topography. Thus, further research is necessary to develop liquid-texture pairs that will provide a sustainable frost suppression method.