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CO2 is removed from oceanwater acidified during chloride-mediated electrochemically modulated reaction of bismuth electrodes.

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.

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.

Emulsions are widely used in agriculture where oil-based pesticides are sprayed as an emulsion. However, emulsion droplets can bounce off hydrophobic plant surfaces, leading to major health and environmental issues as pesticides pollute water sources and soils. Here, we report an unexpected transition from bouncing to sticking to bouncing as the droplet impact speed increases. We show that the physics are governed by an in situ, self-generated lubrication of the surface leading to a suction force from the nascent oil layer around the droplet. We demonstrate that this phenomenon can be controlled by a careful balance of three time scales: the contact time of the droplet, the impregnation time scale of the oil, and the oil ridge formation time scale. We lastly build a design map to precisely control the bouncing of droplets and the oil coverage of the target surface. These insights have broad applicability in agriculture, cooling sprays, combustion, and additive manufacturing.

Dust accumulation on solar panels is a major challenge, as it blocks a large portion of sunlight. Solar panels are therefore cleaned regularly using large quantities of pure water. Consumption of water for cleaning, especially in deserts, poses a substantial sustainability challenge. Here, we present a waterless approach for dust removal from solar panels using electrostatic induction. We find that dust particles, despite primarily consisting of insulating silica, can be electrostatically repelled from electrodes due to charge induction assisted by adsorbed moisture. We experimentally determine dust particle charge by conducting Stokes experiments under an electrostatic field. By considering electrostatic, van der Waals, and gravitational forces, we define the threshold electric potential for particle removal. We also demonstrate dust removal over a broad range of relative humidity, making our approach widely applicable. Last, we develop a lab-scale prototype and demonstrate up to 95% recovery of lost power output using our approach.

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.

Processes for separating oil-water mixtures are critical to operations in energy and water. However, existing separation methods pose efficiency limitations as well as environmental and safety challenges. Here, we present a low-voltage surface electrocoalescence approach that triggers coalescence of surfactant-stabilized emulsions by combining high-K dielectrics with surfactant bilayers. In this system, the high-K dielectric reduces the electrocoalescence voltage, while the surfactant bilayer functions as a self-healing, high capacitance film that prevents pinning of droplets on the dielectric surface. This high capacitance system maximizes the electric field between neighboring droplets, exerting high electrostatic pressure that overcomes the disjoining pressure between droplets, thereby enabling rapid electrocoalescence. We demonstrate electrocoalescence of surfactant-stabilized microscale droplets of saline water in oil using single volts. We expect our results may find application in the energy sector, wastewater treatment, and purification industries.

An unusual and beautiful phenomenon in which life-like crystal structures spontaneously grow from nano-engineered materials., Mineral or crystal fouling (the accumulation of precipitants on a material and damage associated with the same) is a pervasive problem in water treatment, thermoelectric power production, and numerous industrial processes. Growing efforts have focused on materials engineering strategies (e.g., superhydrophobicity) to prevent fouling. Here, we present a curious phenomenon in which crystals self-eject from heated, nanotextured superhydrophobic materials during evaporation of saline water drops. These crystal structures (crystal critters) have exceedingly minimal contact with the substrate and thus pre-empt crystal fouling. This unusual phenomenon is caused by cooperative effects of crystallization, evaporative flows, and nanoscale effects. The temperature dependence of the critter effect can be predicted using principles of mass conservation, and we demonstrate that self-propulsion can be generated via temperature gradients, which promote asymmetric growth. The insights on confinement-driven evaporative crystallization can be applied for antifouling by self-ejection of mineral foulants, for drop-based fluidic machines, or even for self-propulsion.

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., Submitted and accepted to Physical Review Letters

Corrosion is a detrimental process that can impact the performance and lifetime of many infrastructural systems. In this research, lubricant-impregnated surfaces (LIS) for corrosion protection are systematically developed and studied. Using microtextures with controlled geometry and spacing, this study shows that the corrosion resistance on LIS is greatly enhanced compared to bare iron as determined by a reduction in the corrosion current density by three orders of magnitude. Furthermore, it shows that the spreading characteristics of the lubricant are important toward ensuring effective corrosion protection. Krytox, a lubricant that covers both inside the textures as well as the top of the textures, provides two orders of magnitude greater corrosion protection as compared to silicone oil that does not cover texture tops. The practical applicability of LIS are highlighted to demonstrate corrosion protection on carbon steel in brine.

Fizzy water drops levitate and slide freely when placed on a superhydrophobic solid at ambient temperature., As first described by Leidenfrost, liquid droplets levitate over their own vapor when placed on a sufficiently hot substrate. The Leidenfrost effect not only confers remarkable properties such as mechanical and thermal insulation, zero adhesion, and extreme mobility but also requires a high energetic thermal cost. We describe here a previously unexplored approach using active liquids able to sustain levitation in the absence of any external forcing at ambient temperature. We focus on the particular case of carbonated water placed on a superhydrophobic solid and demonstrate how millimetric fizzy drops self-generate a gas cushion that provides levitation on time scales on the order of a minute. Last, we generalize this new regime to different kinds of chemically reactive droplets able to jump from the Cassie-Baxter state to a levitating regime, paving the way to the levitation of nonvolatile liquids.

Superhydrophobic surfaces submerged under water appear shiny due to total internal reflection of light from a thin layer of air (plastron) trapped in their surface texture. This entrapped air is advantageous for frictional drag reduction in various applications ranging from microfluidic channels to marine vessels. However, these aerophilic textures are prone to impregnation by water due to turbulent pressure fluctuations from external flows and dissolution of the trapped gas into the water. We demonstrate a novel chemical method to replenish the plastron in situ by using the decomposition reaction of hydrogen peroxide on superhydrophobic surfaces prepared with a catalytic coating. We also provide a thermodynamic framework for designing superhydrophobic surfaces with optimal texture and chemistry for underwater plastron regeneration. We finally demonstrate the practical utility of this method by fabricating periodic microtextures on aluminum surfaces that incorporate a cheap catalyst, manganese dioxide. We perform drag-reduction experiments under turbulent flow conditions in a Taylor-Couette cell (TC cell), which show that more than half of the drag increase ensuing from plastron collapse can be recovered spontaneously by injection of dilute H

Redox flow batteries are a promising technology that can potentially meet the large-scale grid storage needs of renewable power sources. Today, most redox flow batteries are based on aqueous solutions with low cell voltages and low energy densities that lead to significant costs from hardware and balance-of-plant. Nonaqueous electrochemical couples offer higher cell voltages and higher energy densities and can reduce system-level costs but tend toward higher viscosities and can exhibit non-Newtonian rheology that increases the power required to drive flow. This work uses lubricant-impregnated surfaces (LIS) to promote flow in electrochemical systems and outlines their design based on interfacial thermodynamics and electrochemical stability. We demonstrate up to 86% mechanical power savings at low flow rates for LIS compared to conventional surfaces for a lithium polysulfide flow electrode in a half-cell flow battery configuration. The measured specific charge capacity of ∼800 mAh/(g·S) is a 4-fold increas...

When molten tin droplets impact clean substrates, they either stick or spontaneously detach depending on the substrate temperature. Competition between heat extraction and fluidity controls this behaviour, forgoing the need for surface treatment.

Clathrate hydrates are icelike solid substances that can form inside oil and gas pipelines and are responsible for flow blockages, sometimes leading to catastrophic failures. Minimizing hydrate formation and adhesion on pipeline surfaces can effectively address this problem. In this paper, we achieve the lowering of the adhesion of cyclopentane hydrates by promoting a cyclopentane barrier film between the hydrate and solid surface. The presence of this interfacial liquid film depends on the relative spreading of cyclopentane on the solid surface in the presence of water. We study the role of surface chemistry and surface texture on the spreading characteristics of such interfacial films and their impact on hydrate adhesion. The use of the spreading coefficients as design parameters could take us a step closer to the development of effective passive antihydrate surfaces.

© 2020 American Chemical Society. Asphaltenes are heavy aromatic components of crude oil. Their complex chemical makeup - an aromatic core surrounded by aliphatic side chains - enables them to adhere to most surfaces. Their buildup in pipes can result in clogging and lead to interruption of production operations and expensive mechanical cleaning. We demonstrate the use of liquid-impregnated surfaces (LIS) to prevent asphaltene deposition and buildup on substrates. Indeed, these surfaces expose a liquid interface to the working fluid, which combines the benefits of a dynamic defect-free surface and tunable interfacial properties. In contrast to bulk additives that are typically mixed into the oil phase, the impregnating liquid also provides the great benefit of protecting the underlying solid surface with a stable and minimal layer of lubricant, thereby reducing costs and eliminating the need for subsequent downstream removal. We first select and confirm the thermodynamic stability of a suitable lubricant and its lack of interaction with asphaltenes. By using a carefully selected system composed of a textured and functionalized solid substrate in conjunction with a fluorinated lubricant, we show that asphaltene adsorption is prevented over long time scales. We further demonstrate the possibility of building such a system with representative industrial materials such as aluminum and expose the resulting substrate to an external shear flow to simulate pipe flow conditions.

This paper is associated with a video winner of a 2018 APS/DFD Gallery of Fluid Motion Award for work presented at the DFD Gallery of Fluid Motion. The original video is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2018.GFM.V0097

Stable surfaces with high boiling heat flux are critical to many thermal and energy conversion systems, and it is well-known that the microscale texture and wettability of a surface influences its critical heat flux (CHF). We investigate pool boiling on microstructured ceramic-coated carbon nanotube (CNT) surfaces. CNT microstructures are patterned with precise dimensions over large areas, and a ceramic coating by atomic layer deposition (ALD) imparts stability in the presence of capillary forces and thermal stresses that occur during boiling, achieving a measured CHF as high as 245 W cm–2. We also show that the nanoporosity of the ceramic–CNT microstructures has a negligible influence on the CHF because surface rewetting is dominated by microscale imbibition. The high CHF values achieved on our surfaces are attributed to the micropatterning and the nanoscale surface texture of the CNTs, which accelerate liquid imbibition upon bubble departure. Our findings also suggest further enhancements in CHF can be made by optimizing the microstructure pattern and improving its wettability. Therefore, micropatterned ceramic−CNT composites are a potentially attractive substrate for industrial applications of pool boiling., Air Force Office of Scientific Research (FA9550-16-1-0011)

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.

A common problem which we encounter on a daily basis is dispensing of yield stress fluids such as condiments, lotions, toothpaste, etc. from containers. Beyond consumer products, assuring the flow of yield stress fluids such as crude oil, mud, blood, paint, pharmaceutical products, and others, is essential for the respective industries. Elimination of wall-induced friction can lead to significant savings in the energy required for flow of yield stress fluids, as well as associated product loss and cleaning costs. Lubricant-impregnated surfaces (LIS) have been shown to change the dynamic behavior of yield stress fluids and enable them to flow without shearing. Despite the wide applicability of this technology and its general appeal, the fundamental physics governing the flow of yield stress fluids on LIS have not yet been fully explained. In this work, we study the mobility of yield stress fluids on LIS, and explain the relationship between their macroscale flow behavior and the microscale properties of LIS. We show that for yield stress fluids the thermodynamic state of an LIS can be the difference between mobility and immobility. We demonstrate that LIS can induce mobility in yield stress fluids even below their yield stress allowing them to move as a plug without shearing with an infinite slip length. We identify different mobility mechanisms and establish a regime map for drag reduction in terms of the shear stress to yield stress ratio and the microscopic properties of the LIS. We demonstrate these regimes in a practical application of pipe flow thereby providing key insights for the design of LIS to induce mobility of yield stress fluids in a broad range of practical applications.

Electrocatalytic reduction of CO2 (CO2RR) is an attractive method of converting CO2 to solar fuels. Much research in this field has been focused on developing novel catalysts to enhance the activity and product distribution (selectivity). Copper has been studied widely as it can produce both one carbon (C1) products such as methane and formate, and products with two or more carbons (C2+) such as ethylene, ethanol, and n-propanol1. Copper is not immune from the competing hydrogen evolution reaction: the poor solubility of CO2 in water (~34 mM at ambient conditions) limits the CO2RR current density, and hydrogen generation is favored if the aqueous CO2 concentration becomes locally undersaturated close to the catalyst during CO2RR2. This limitation is even more pronounced for nanotextured copper, because the increased active surface area leads to faster depletion of local CO2 thereby promoting hydrogen evolution over CO2RR. Hence, overcoming these CO2 availability limitations can enable higher CO2RR activity while reducing co-evolution of hydrogen. In this work, we develop a gasphilic CO2 trap that increases gas-liquid mass transfer and maintains supersaturated CO2 concentration around the catalyst during CO2RR. Gasphilic surfaces need a special combination of surface chemistry and texture to capture bubbles and form a sheet of gas underwater which is called a plastron3. By creating pyramidal textures, CO2 bubbles are efficiently captured within the textures and form a CO2 plastron. When this plastron is placed proximal to both smooth and nanostructured copper catalysts during CO2RR, the current density is enhanced and maintained throughout the reaction, when compared to two commonly used methods of CO2 delivery: headspace and bubbling in the bulk electrolyte. The plastron allows for quick replenishment of CO2 during the CO2RR reaction in the vicinity of the catalyst. As a result, the H2 Faradaic efficiency is reduced from 33% to 13% on smooth copper, and from 62% to 33% on nanostructured copper. This is accompanied by an increased production of C2+ products including ethylene, ethanol and propanol, as well as acetone and acetate at Faradaic efficiencies exceeding 1%. We highlight the importance of this catalyst-proximal plastron approach by comparing against recent aqueous-phase CO2RR studies, and discuss how this approach can inform optimal design of continuous CO2RR systems such as gas-diffusion electrodes. [1] Nitopi, S., Bertheussen, E., Scott, S. B., Liu, X., Engstfeld, A. K., Horch, S., Seger, B., Stephens, I. E. L., Chan, K., Hahn, C., Nørskov, J. K., Jaramillo, T. F. & Chorkendorff, I. Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte. Chem. Rev. 119, 7610–7672 (2019). [2] Lobaccaro, P., R. Singh, M., Lee Clark, E., Kwon, Y., T. Bell, A. & W. Ager, J. Effects of temperature and gas–liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO2 reduction electrocatalysts. Physical Chemistry Chemical Physics 18, 26777–26785 (2016). [3] Panchanathan, D., Rajappan, A., Varanasi, K. K. & McKinley, G. H. Plastron Regeneration on Submerged Superhydrophobic Surfaces Using In Situ Gas Generation by Chemical Reaction. ACS Appl. Mater. Interfaces 10, 33684–33692 (2018).

Ice adhesion on superhydrophobic surfaces can significantly increase in humid environments because of frost nucleation within the textures. Here, we studied frost formation and ice adhesion on superhydrophobic surfaces with various surface morphologies using direct microscale imaging combined with macroscale adhesion tests. Whereas ice adhesion increases on microtextured surfaces, a 15-fold decrease is observed on nanotextured surfaces. This reduction is because of the inhibition of frost formation within the nanofeatures and the stabilization of vapor pockets. Such "Cassie ice"-promoting textures can be used in the design of anti-icing surfaces.

Summary Electrochemical reduction of carbon dioxide (CO2RR) is an attractive approach toward converting CO2 to solar fuels. Novel catalyst chemistries and morphologies may provide high selectivity to both one-carbon (C1) and two-carbon (C2) products over hydrogen; however, the limited aqueous solubility of CO2 restricts the CO2RR current density. Here, we demonstrate how gasphilic bubble-trap surfaces enhance mass transfer and enrich and maintain CO2 concentration during CO2RR by forming a catalyst-proximal plastron layer, consequently increasing the CO2RR activity on both smooth and nanostructured copper as compared to the conventional headspace or bubbling modes of CO2 delivery. The H2 Faradaic efficiency is suppressed from 33% to 13% on smooth copper and from 62% to 33% on nanostructured copper, accompanied by enhanced formation of C2+ products, including ethylene, propanol, and ethanol, and >1% acetone and acetate. We highlight the importance of the catalyst-proximal plastron approach by comparing against recent aqueous-phase CO2RR studies.

Highly concentrated biological drug formulations would offer tremendous benefits to global health, yet they cannot be manually injected using commercial syringes and needles due to their high viscosities. Current approaches to address this problem face several challenges such as crosscontamination, high cost, needle clogging, and protein inactivation. This work reports a simple method to enhance formulation injectability using a core annular flow, where the transport of highly viscous fluids through a needle is enabled by coaxial lubrication by a less viscous fluid. A phase diagram to ensure optimally lubricated flow while minimizing the volume fraction of lubricant injected is established. The technique presented here allows for up to a 7x reduction in injection force for the highest viscosity ratio tested. The role of buoyancy-driven eccentricity in governing nominal pressure reduction is also examined. Finally, the findings are implemented into the development of a double barreled syringe that significantly expands the range of injectable concentrations of several biologic formulations.

In this work, we found that stainless steels with different crystal structures generated two new surface micro-nano morphologies after anodization. The surface morphologies were observed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM). Element identification and quantitative analysis of each element were carried out using energy dispersive X-ray spectroscopy (EDS). Anodized stainless steel with a face-centered cubic crystal structure (SS-fcc) exhibits 45° grooves along the crystal edges while stainless steel with a body-centered cubic structure (SS-bcc) has wedges protruding out of the anodized surface. The crystal structures and the percentage of crystal planes of SS-fcc and SS-bcc were characterized using X-ray diffraction (XRD) before and after anodization. It was observed that different crystal facets of SS-fcc and SS-bcc disappear at different rates during anodization. This explains the different micro-nano morphologies observed during the anodization process. It is concluded that different crystal planes have different anodization properties and the degree of disappearance of the crystal plane is consistent with its energy order. Keywords: Anodization, Stainless steel, Crystal structure, Micro-nano morphology

Many natural surfaces such as butterfly wings, beetles' backs, and rice leaves exhibit anisotropic liquid adhesion; this is of fundamental interest and is important to applications including self-cleaning surfaces, microfluidics, and phase change energy conversion. Researchers have sought to mimic the anisotropic adhesion of butterfly wings using rigid surface textures, though natural butterfly scales are sufficiently compliant to be deflected by capillary forces exerted by drops. Here, inspired by the flexible scales of the Morpho aega butterfly wing, synthetic surfaces coated with flexible carbon nanotube (CNT) microscales with anisotropic drop adhesion properties are fabricated. The curved CNT scales are fabricated by a strain-engineered chemical vapor deposition technique, giving ≈5000 scales of ≈10 µm thickness in a 1 cm2 area. Using various designed CNT scale arrays, it is demonstrated that the anisotropy of drop roll-off angle is influenced by the geometry, compliance, and hydrophobicity of the scales; and a maximum roll-off anisotropy of 6.2° is achieved. These findings are supported by a model that relates the adhesion anisotropy to the scale geometry, compliance, and wettability. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex multidirectional patterns, suggest further opportunities to create engineered synthetic scale surfaces.

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. Keywords: Bacteria; Fungi; Pseudomonas aeruginosa; Penicillium rubens; Direct laser-interference patterning (DLIP); Lubricant-impregnated surface (LIS), United States. National Aeronautics and Space Administration (Grant 80NSSC17K0036)

When a drop impacts a stationary drop on a non-wetting surface, the two drops coalesce, spread on the surface and retract, sometimes bouncing off. We measure the maximum diameter and retraction time for a wide range of parameter values and generalize known single drop models to drop-on-drop impacts.

Factors governing atomization of droplets impacting a mesh, starting with the elementary unit of a single hole, are investigated. It is shown how this process can be used to generate finely controlled sprays with micrometric droplet sizes and low kinetic energy, as is critical for agricultural applications.

We have developed a scalable, efficient photothermal trap for deicing by localized light absorption and heat spreading at the ice-substrate interface., Ice buildup is an operational and safety hazard in wind turbines, power lines, and airplanes. Traditional deicing methods, including mechanical and chemical means, are energy-intensive or environmentally unfriendly. Superhydrophobic anti-icing surfaces, while promising, can become ineffective due to frost formation within textures. We report on a “photothermal trap”—a laminate applied to a base substrate—that can efficiently deice by converting solar illumination to heat at the ice-substrate interface. It relies on the complementing properties of three layers: a selective absorber for solar radiation, a thermal spreader for lateral dispersal of heat, and insulation to minimize transverse heat loss. Upon illumination, thermal confinement at the heat spreader leads to rapid increase of the surface temperature, thereby forming a thin lubricating melt layer that facilitates ice removal. Lateral heat spreading overcomes the unavoidable shadowing of certain areas from direct illumination. We provide a design map that captures the key physics guiding illumination-induced ice removal. We demonstrate the deicing performance of the photothermal trap at very low temperatures, and under frost and snow coverage, via laboratory-scale and outdoor experiments.

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.

Laser gas assisted texturing of silicon wafer surface is carried out. Morphological and metallurgical changes in the treated layer are examined using the analytical tools. Microhardness and fracture toughness of the laser treated surface are measured using the indentation technique while residual stress formed is determined from the X-ray diffraction data. The hydrophobicity of the textured surfaces are assessed incorporating the contact angle data and compared with those of as received workpiece surfaces. Environmental dust accumulation and mud formation, due to air humidity, at the laser treated and as received workpiece surfaces are simulated and the effect of the mud residues on the properties of the laser treated surface are studied. The adhesion work due to the presence of the mud on the laser treated surface is also measured. It is found that laser textured surface composes of micro/nano poles and fibers, which in turn improves the surface hydrophobicity significantly. In addition, formation of nitride species contributes to microhardness increase and enhancement of surface hydrophobicity due to their low surface energy. The mud residues do not influence the fracture toughness and microhardness of the laser textured surface; however, they reduced the surface hydrophobicity significantly.