Imparting scalephobicity with rational microtexturing of soft materials.

Crystallization fouling, a process where scale forms on surfaces, is widespread in nature and technology, negatively affecting energy and water industries. Despite the effort, rationally designed surfaces that are intrinsically resistant to it remain elusive, due in part to a lack of understanding of how microfoulants deposit and adhere in dynamic aqueous environments. Here, we show that rational tuning of coating compliance and wettability works synergistically with microtexture to enhance microfoulant repellency, characterized by low adhesion and high removal efficiency of numerous individual microparticles and tenacious crystallites in a flowing water environment. We study the microfoulant interfacial dynamics in situ using a micro-scanning fluid dynamic gauge system, elucidate the removal mechanisms, and rationalize the behavior with a shear adhesive moment model. We then demonstrate a rationally developed coating that can remove 98% of deposits under shear flow conditions, 66% better than rigid substrates.

Engineering Electrode Rinse Solution Fluidics for Carbon-Based Reverse Electrodialysis Devices.

Natural salinity gradients are a promising source of so-called "blue energy", a renewable energy source that utilizes the free energy of mixing for power generation. One promising blue energy technology that converts these salinity gradients directly into electricity is reverse electrodialysis (RED). Used at its full potential, it could provide a substantial portion of the world's electricity consumption. Previous theoretical and experimental works have been done on optimizing RED devices, with the latter often focusing on precious and expensive metal electrodes. However, in order to rationally design and apply RED devices, we need to investigate all related transport phenomena─especially the fluidics of salinity gradient mixing and the redox electrolyte at various concentrations, which can have complex intertwined effects─in a fully functioning and scalable system. Here, guided by fundamental electrochemical and fluid dynamics theories, we work with an iron-based redox electrolyte with carbon electrodes in a RED device with tunable microfluidic environments and study the fundamental effects of electrolyte concentration and flow rate on the potential-driven redox activity and power output. We focus on optimizing the net power output, which is the difference between the gross power output generated by the RED device and the pumping power input, needed for salinity gradient mixing and redox electrolyte reactions. We find through this holistic approach that the electrolyte concentration in the electrode rinse solution is crucial for increasing the electrical current, while the pumping power input depends nonlinearly on the membrane separation distance. Finally, from this understanding, we designed a five cell-pair (CP) RED device that achieved a net power density of 224 mW m-2 CP-1, a 60% improvement compared to the nonoptimized case. This study highlights the importance of the electrode rinse solution fluidics and composition when rationally designing RED devices based on scalable carbon-based electrodes.

Freezing-induced wetting transitions on superhydrophobic surfaces.

Supercooled droplet freezing on surfaces occurs frequently in nature and industry, often adversely affecting the efficiency and reliability of technological processes. The ability of superhydrophobic surfaces to rapidly shed water and reduce ice adhesion make them promising candidates for resistance to icing. However, the effect of supercooled droplet freezing-with its inherent rapid local heating and explosive vaporization-on the evolution of droplet-substrate interactions, and the resulting implications for the design of icephobic surfaces, are little explored. Here we investigate the freezing of supercooled droplets resting on engineered textured surfaces. On the basis of investigations in which freezing is induced by evacuation of the atmosphere, we determine the surface properties required to promote ice self-expulsion and, simultaneously, identify two mechanisms through which repellency falters. We elucidate these outcomes by balancing (anti-)wetting surface forces with those triggered by recalescent freezing phenomena and demonstrate rationally designed textures to promote ice expulsion. Finally, we consider the complementary case of freezing at atmospheric pressure and subzero temperature, where we observe bottom-up ice suffusion within the surface texture. We then assemble a rational framework for the phenomenology of ice adhesion of supercooled droplets throughout freezing, informing ice-repellent surface design across the phase diagram.

Transparent sunlight-activated antifogging metamaterials.

Counteracting surface fogging to maintain surface transparency is important for a variety of applications including eyewear, windows and displays. Energy-neutral, passive approaches predominantly rely on engineering the surface wettability, but suffer from non-uniformity, contaminant deposition and lack of robustness, all of which substantially degrade durability and performance. Here, guided by nucleation thermodynamics, we design a transparent, sunlight-activated, photothermal coating to inhibit fogging. The metamaterial coating contains a nanoscopically thin percolating gold layer and is most absorptive in the near-infrared range, where half of the sunlight energy resides, thus maintaining visible transparency. The photoinduced heating effect enables sustained and superior fog prevention (4-fold improvement) and removal (3-fold improvement) compared with uncoated samples, and overall impressive performance, indoors and outdoors, even under cloudy conditions. The extreme thinness (~10 nm) of the coating-which can be produced by standard, readily scalable fabrication processes-enables integration beneath other coatings, rendering it durable even on highly compliant substrates.

Ultrathin Durable Organic Hydrophobic Coatings Enhancing Dropwise Condensation Heat Transfer.

Organic hydrophobic layers targeting sustained dropwise condensation are highly desirable but suffer from poor chemical and mechanical stability, combined with low thermal conductivity. The requirement of such layers to remain ultrathin to minimize their inherent thermal resistance competes against durability considerations. Here, we investigate the long-term durability and enhanced heat-transfer performance of perfluorodecanethiol (PFDT) coatings compared to alternative organic coatings, namely, perfluorodecyltriethoxysilane (PFDTS) and perfluorodecyl acrylate (PFDA), the latter fabricated with initiated chemical vapor deposition (iCVD), in condensation heat transfer and under the challenging operating conditions of intense flow (up to 9 m s-1) of superheated steam (111 °C) at high pressures (1.42 bar). We find that the thiol coating clearly outperforms the silane coating in terms of both heat transfer and durability. In addition, despite being only a monolayer, it clearly also outperforms the iCVD-fabricated PFDA coating in terms of durability. Remarkably, the thiol layer exhibited dropwise condensation for at least 63 h (>2× times more than the PFDA coating, which survived for 30 h), without any visible deterioration, showcasing its hydrolytic stability. The cost of thiol functionalization per area was also the lowest as compared to all of the other surface hydrophobic treatments used in this study, thus making it the most efficient option for practical applications on copper substrates.

Bistability of Dielectrically Anisotropic Nematic Crystals and the Adaptation of Endothelial Collectives to Stress Fields.

Endothelial monolayers physiologically adapt to flow and flow-induced wall shear stress, attaining ordered configurations in which elongation, orientation, and polarization are coherently organized over many cells. Here, with the flow direction unchanged, a peculiar bi-stable (along the flow direction or perpendicular to it) cell alignment is observed, emerging as a function of the flow intensity alone, while cell polarization is purely instructed by flow directionality. Driven by the experimental findings, the parallelism between endothelia is delineated under a flow field and the transition of dual-frequency nematic liquid crystals under an external oscillatory electric field. The resulting physical model reproduces the two stable configurations and the energy landscape of the corresponding system transitions. In addition, it reveals the existence of a disordered, metastable state emerging upon system perturbation. This intermediate state, experimentally demonstrated in endothelial monolayers, is shown to expose the cellular system to a weakening of cell-to-cell junctions to the detriment of the monolayer integrity. The flow-adaptation of monolayers composed of healthy and senescent endothelia is successfully predicted by the model with adjustable nematic parameters. These results may help to understand the maladaptive response of in vivo endothelial tissues to disturbed hemodynamics and the progressive functional decay of senescent endothelia.

Microscale investigation on interfacial slippage and detachment of ice from soft materials.

Surface icing is detrimental to applications ranging from transportation to biological systems. Soft elastomeric coatings can engender remarkably low ice adhesion strength, but mechanisms at the microscale and resulting ice extraction outcomes need to be understood. Here we investigate dynamic ice-elastomer interfacial events and show that the ice adhesion strength can actually vary by orders of magnitude due to the shear velocity. We study the detailed deformation fields of the elastomer using confocal traction force microscopy and elucidate the underlying mechanism. The elastomer initially undergoes elastic deformation having a shear velocity dependent threshold, followed by partial relaxation at the onset of slip, where velocity dependent "stick-slip" micropulsations are observed. The results of the work provide important information for the design of soft surfaces with respect to removal of ice, and utility to fields exemplified by adhesion, contact mechanics, and biofouling.

Enhanced Atmospheric Water Harvesting with Sunlight-Activated Sorption Ratcheting.

The global challenge of clean water scarcity needs to be confronted with novel sustainable, climate neutral solutions, over the entire spectrum of possible clean water availability. Atmospheric moisture represents a major untapped resource that can be harvested by sorbents, enabling water production in dry inland regions where it is needed. While benefiting from the utilization of an important renewable energy source, solar-driven, sorbent-based atmospheric water harvesting systems are inseparably based on a single water harvesting cycle per day, which severely limits the daily water productivity and the competitiveness of this very promising technology. Here, we rationally design an atmospheric water harvesting strategy, using durable hydrogel sorbents, that operates with sorption "ratcheting"─a large sequence of rapid adsorption and subsequent desorption steps─activated by direct sunlight. Employing theoretical considerations, we tailor the ratcheting timescales to the inherent sorption properties of the hydrogels, optimally exploiting their natural harvesting capabilities, while maintaining the sustainable utility of the daily cycle. Amplified by the favorable sorption properties and ratcheting stability of the sorbent, this strategy demonstrates an impressive ∼80% increase in water harvesting yield over the daily cycle systems. The generic nature of the ratcheting concept shows great potential to advance the water harvesting capabilities of a range of related systems.

Exploiting radiative cooling for uninterrupted 24-hour water harvesting from the atmosphere.

Atmospheric water vapor is ubiquitous and represents a promising alternative to address global clean water scarcity. Sustainably harvesting this resource requires energy neutrality, continuous production, and facility of use. However, fully passive and uninterrupted 24-hour atmospheric water harvesting remains a challenge. Here, we demonstrate a rationally designed system that synergistically combines radiative shielding and cooling-dissipating the latent heat of condensation radiatively to outer space-with a fully passive superhydrophobic condensate harvester, working with a coalescence-induced water removal mechanism. A rationally designed shield, accounting for the atmospheric radiative heat, facilitates daytime atmospheric water harvesting under solar irradiation at realistic levels of relative humidity. The remarkable cooling power enhancement enables dew mass fluxes up to 50 g m-2 hour-1, close to the ultimate capabilities of such systems. Our results demonstrate that the yield of related technologies can be at least doubled, while cooling and collection remain passive, thereby substantially advancing the state of the art.

Leidenfrost droplet trampolining.

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.

Superhydrophobic surfaces for extreme environmental conditions.

Superhydrophobic surfaces for repelling impacting water droplets are typically created by designing structures with capillary (antiwetting) pressures greater than those of the incoming droplet (dynamic, water hammer). Recent work has focused on the evolution of the intervening air layer between droplet and substrate during impact, a balance of air compression and drainage within the surface texture, and its role in affecting impalement under ambient conditions through local changes in the droplet curvature. However, little consideration has been given to the influence of the intervening air-layer thermodynamic state and composition, in particular when departing from standard atmospheric conditions, on the antiwetting behavior of superhydrophobic surfaces. Here, we explore the related physics and determine the working envelope for maintaining robust superhydrophobicity, in terms of the ambient pressure and water vapor content. With single-tier and multitier superhydrophobic surfaces and high-resolution dynamic imaging of the droplet meniscus and its penetration behavior into the surface texture, we expose a trend of increasing impalement severity with decreasing ambient pressure and elucidate a previously unexplored condensation-based impalement mechanism within the texture resulting from the compression, and subsequent supersaturation, of the intervening gas layer in low-pressure, humid conditions. Using fluid dynamical considerations and nucleation thermodynamics, we provide mechanistic understanding of impalement and further employ this knowledge to rationally construct multitier surfaces with robust superhydrophobicity, extending water repellency behavior well beyond typical atmospheric conditions. Such a property is expected to find multifaceted use exemplified by transportation and infrastructure applications where exceptional repellency to water and ice is desired.

Transparent Photothermal Metasurfaces Amplifying Superhydrophobicity by Absorbing Sunlight.

Imparting and maintaining surface superhydrophobicity has been receiving significant research attention over the last several years, driven by a broad range of important applications and enabled by advancements in materials and surface nanoengineering. Researchers have investigated the effect of temperature on droplet-surface interactions, which poses additional challenges when liquid nucleation manifests itself, due to ensuing condensation into the surface texture that compromises its antiwetting behavior. Maintaining surface transparency at the same time poses an additional and significant challenge. Often, the solutions proposed are limited by working temperatures or are detrimental to visibility through the surface. Here we introduce a scalable method employing plasmonic photothermal metasurface composites, able to harvest sunlight and naturally heat the surface, sustaining water repellency and transparency under challenging environmental conditions where condensation and fogging would otherwise be strongly promoted. We demonstrate that these surfaces, when illuminated by sunlight, can prevent impalement of impacting water droplets, even when the droplet to surface temperature difference is 50 °C, by suppressing condensate formation within the texture, maintaining transparency. We also show how the same transparent metasurface coating could be combined and work collaboratively with hierarchical micro- and nanorough textures, resulting in simultaneous superior pressure-driven impalement resistance and avoidance of water nucleation and related possible frosting in supercooled conditions. Our work can find a host of applications as a sustainable solution against impacting water on surfaces such as windows, eyewear, and optical components.

Wetting transitions in droplet drying on soft materials.

Droplet interactions with compliant materials are familiar, but surprisingly complex processes of importance to the manufacturing, chemical, and garment industries. Despite progress-previous research indicates that mesoscopic substrate deformations can enhance droplet drying or slow down spreading dynamics-our understanding of how the intertwined effects of transient wetting phenomena and substrate deformation affect drying remains incomplete. Here we show that above a critical receding contact line speed during drying, a previously not observed wetting transition occurs. We employ 4D confocal reference-free traction force microscopy (cTFM) to quantify the transient displacement and stress fields with the needed resolution, revealing high and asymmetric local substrate deformations leading to contact line pinning, illustrating a rate-dependent wettability on viscoelastic solids. Our study has significance for understanding the liquid removal mechanism on compliant substrates and for the associated surface design considerations. The developed methodology paves the way to study complex dynamic compliant substrate phenomena.

Transparent Metasurfaces Counteracting Fogging by Harnessing Sunlight.

Surface fogging is a common phenomenon that can have significant and detrimental effects on surface transparency and visibility. It affects the performance in a wide range of applications including windows, windshields, electronic displays, cameras, mirrors, and eyewear. A host of ongoing research is aimed at combating this problem by understanding and developing stable and effective antifogging coatings that are capable of handling a wide range of environmental challenges "passively" without consumption of electrical energy. Here we introduce an alternative approach employing sunlight to go beyond state-of-the-art techniques, such as superhydrophilic and superhydrophobic coatings, by rationally engineering solar absorbing metasurfaces that maintain transparency, while upon illumination induce localized heating to significantly delay the onset of surface fogging or decrease defogging time. For the same environmental conditions, we demonstrate that our metasurfaces are able to reduce defogging time by up to 4-fold and under supersaturated conditions inhibit the nucleation of condensate outperforming conventional state-of-the-art approaches in terms of visibility retention. Our research illustrates a durable and environmentally sustainable approach to passive antifogging and defogging for transparent surfaces. This work opens up the opportunity for large-scale manufacturing that can be applied to a range of materials, including polymers and other flexible substrates.

3D-Printed Surface Architecture Enhancing Superhydrophobicity and Viscous Droplet Repellency.

Macrotextured superhydrophobic surfaces can reduce droplet-substrate contact times of impacting water droplets; however, surface designs with similar performance for significantly more viscous liquids are missing, despite their importance in nature and technology such as for chemical shielding, food-staining repellency, and supercooled (viscous) water droplet removal in anti-icing applications. Here, we introduce a deterministic, controllable, and upscalable method to fabricate superhydrophobic surfaces with a 3D-printed architecture, combining arrays of alternating surface protrusions and indentations. We show a more than threefold contact time reduction of impacting viscous droplets up to a fluid viscosity of 3.7 mPa·s, which equals 3.7 times the viscosity of water at room temperature, covering the viscosity of many chemicals and supercooled water. On the basis of the combined consideration of the fluid flow within and the simultaneous droplet dynamics above the texture, we recommend future pathways to rationally architecture such surfaces, all realizable with the methodology presented here.

Cascade Freezing of Supercooled Water Droplet Collectives.

Surface icing affects the safety and performance of numerous processes in technology. Previous studies mostly investigated freezing of individual droplets. The interaction among multiple droplets during freezing is investigated less, especially on nanotextured icephobic surfaces, despite its practical importance as water droplets never appear in isolation, but in groups. Here we show that freezing of a supercooled droplet leads to spontaneous self-heating and induces strong vaporization. The resulting, rapidly propagating vapor front causes immediate cascading freezing of neighboring supercooled droplets upon reaching them. We put forth the explanation that, as the vapor approaches cold neighboring droplets, it can lead to local supersaturation and formation of airborne microscopic ice crystals, which act as freezing nucleation sites. The sequential triggering and propagation of this mechanism results in the rapid freezing of an entire droplet ensemble, resulting in ice coverage of the nanotextured surface. Although cascade freezing is observed in a low-pressure environment, it introduces an unexpected pathway of freezing propagation that can be crucial for the performance of rationally designed icephobic surfaces.

Desublimation Frosting on Nanoengineered Surfaces.

Ice nucleation from vapor presents a variety of challenges across a wide range of industries and applications including refrigeration, transportation, and energy generation. However, a rational comprehensive approach to fabricating intrinsically icephobic surfaces for frost formation-both from water condensation (followed by freezing) and in particular from desublimation (direct growth of ice crystals from vapor)-remains elusive. Here, guided by nucleation physics, we investigate the effect of material composition and surface texturing (atomically smooth to nanorough) on the nucleation and growth mechanism of frost for a range of conditions within the sublimation domain (0 °C to -55 °C; partial water vapor pressures 6 to 0.02 mbar). Surprisingly, we observe that on silicon at very cold temperatures-below the homogeneous ice solidification nucleation limit (<-46 °C)-desublimation does not become the favorable pathway to frosting. Furthermore, we show that surface nanoroughness makes frost formation on silicon more probable. We experimentally demonstrate at temperatures between -48 °C and -55 °C that nanotexture with radii of curvature within 1 order of magnitude of the critical radius of nucleation favors frost growth, facilitated by capillary condensation, consistent with Kelvin's equation. Our findings show that such nanoscale surface morphology imposed by design to impart desired functionalities-such as superhydrophobicity-or from defects can be highly detrimental for frost icephobicity at low temperatures and water vapor partial pressures (<0.05 mbar). Our work contributes to the fundamental understanding of phase transitions well within the equilibrium sublimation domain and has implications for applications such as travel, power generation, and refrigeration.

Metasurfaces Leveraging Solar Energy for Icephobicity.

Inhibiting ice accumulation on surfaces is an energy-intensive task and is of significant importance in nature and technology where it has found applications in windshields, automobiles, aviation, renewable energy generation, and infrastructure. Existing methods rely on on-site electrical heat generation, chemicals, or mechanical removal, with drawbacks ranging from financial costs to disruptive technical interventions and environmental incompatibility. Here we focus on applications where surface transparency is desirable and propose metasurfaces with embedded plasmonically enhanced light absorption heating, using ultrathin hybrid metal-dielectric coatings, as a passive, viable approach for de-icing and anti-icing, in which the sole heat source is renewable solar energy. The balancing of transparency and absorption is achieved with rationally nanoengineered coatings consisting of gold nanoparticle inclusions in a dielectric (titanium dioxide), concentrating broadband absorbed solar energy into a small volume. This causes a > 10 °C temperature increase with respect to ambient at the air-solid interface, where ice is most likely to form, delaying freezing, reducing ice adhesion, when it occurs, to negligible levels (de-icing) and inhibiting frost formation (anti-icing). Our results illustrate an effective unexplored pathway toward environmentally compatible, solar-energy-driven icephobicity, enabled by respectively tailored plasmonic metasurfaces, with the ability to design the balance of transparency and light absorption.

Spontaneous self-dislodging of freezing water droplets and the role of wettability.

Spontaneous removal of liquid, solidifying liquid and solid forms of matter from surfaces, is of significant importance in nature and technology, where it finds applications ranging from self-cleaning to icephobicity and to condensation systems. However, it is a great challenge to understand fundamentally the complex interaction of rapidly solidifying, typically supercooled, droplets with surfaces, and to harvest benefit from it for the design of intrinsically icephobic materials. Here we report and explain an ice removal mechanism that manifests itself simultaneously with freezing, driving gradual self-dislodging of droplets cooled via evaporation and sublimation (low environmental pressure) or convection (atmospheric pressure) from substrates. The key to successful self-dislodging is that the freezing at the droplet free surface and the droplet contact area with the substrate do not occur simultaneously: The frozen phase boundary moves inward from the droplet free surface toward the droplet-substrate interface, which remains liquid throughout most of the process and freezes last. We observe experimentally, and validate theoretically, that the inward motion of the phase boundary near the substrate drives a gradual reduction in droplet-substrate contact. Concurrently, the droplet lifts from the substrate due to its incompressibility, density differences, and the asymmetric freezing dynamics with inward solidification causing not fully frozen mass to be displaced toward the unsolidified droplet-substrate interface. Depending on surface topography and wetting conditions, we find that this can lead to full dislodging of the ice droplet from a variety of engineered substrates, rendering the latter ice-free.

Imparting Icephobicity with Substrate Flexibility.

Ice accumulation hinders the performance of, and poses safety threats for, infrastructure both on the ground and in the air. Previously, rationally designed superhydrophobic surfaces have demonstrated some potential as a passive means to mitigate ice accretion; however, further studies on material solutions that reduce impalement and the contact time for impacting supercooled droplets (high viscosity) and can also repel droplets that freeze during surface contact are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface micro/nanotexture on enhancing both icephobicity and the repellency of viscous droplets (typical of supercooled water). We first investigate the influence of increased viscosity (spanning from 0.9 to 1078 mPa·s using water-glycerol mixtures) on impalement resistance and the droplet-substrate contact time after impact. Then we examine the effect of droplet partial solidification on recoil and simulate more challenging icing conditions by impacting supercooled water droplets (down to -15 °C) onto flexible and rigid surfaces containing ice nucleation promoters (AgI). We demonstrate a passive mechanism for shedding partially solidified (recalescent) droplets-under conditions where partial solidification occurs much faster than the natural droplet oscillation-which does not rely on converting droplet surface energy into kinetic energy (classic recoil mechanism). Using an energy-based model (kinetic-elastic-capillary), we identify a previously unexplored mechanism whereby the substrate oscillation and velocity govern the rebound process, with low areal density and moderately stiff substrates acting to efficiently absorb the incoming droplet kinetic energy and rectify it back, allowing droplets to overcome adhesion and gravitational forces, and recoil. This mechanism applies for a range of droplet viscosities, spanning from low- to high-viscosity fluids and even ice slurries, which do not rebound from rigid superhydrophobic substrates. For a low-viscosity fluid, i.e., water, if the substrate oscillates faster than the droplet spreading and retraction, the action of the substrate is decoupled from the droplet oscillation, resulting in a reduction in the droplet-substrate contact time.