Phase Change-Induced Wetting Transitions on Superhydrophobic Surfaces

Superhydrophobic surfaces have many potential applications from self-cleaning textiles to ice-free aircraft. Their low affinity to water stems from a heterogeneous wetting state predicated on the stability of an intervening air layer between liquid and substrate. However, this layer is fragile and irreversible transitions to a homogeneous wetting state are possible and devastating to their performance. Recent studies have examined how pressure-based transitions, known as impalement, occur for droplets impacting superhydrophobic surfaces and have proposed solutions to mitigate against this under ambient conditions. Yet to this point, little consideration has been given to how the interplay between environmental effects and the dynamics of droplet-substrate interactions affect the behaviour of the intervening layer and the different phenomena that may result; especially in relation to phase change. In thiswork, I explore how departures from the standard ambient environments influence the local thermodynamic conditions and stability of the intervening layer, elucidating the wetting state transitions that arise from these volatile conditions. In the context of droplet impact, I investigate how the pressure and gas composition of the intervening air layer critically influence the ability of droplets to rebound from a superhydrophobic surface. Through high-speed imaging and theoretical modelling, I expose a previously unknown condensation-based wetting state transition mechanism in humid conditions and explain a trend of increasing impalement with decreasing environmental pressure. When studying the freezing of sessile droplets on superhydrophobic surfaces, in a low-pressure environment I deduce the presence of a force engendered by the evaporation difference across a droplet undergoing recalescence capable of either driving it into the texture or explosively expelling it, depending on the characteristics of the substrate in question. Exploring the complementary case of ambient pressure and low-temperature, I uncover a novel condensation-based filling mechanism induced by the temperature rise during recalescence that is endemic to freezing events on superhydrophobic surfaces that require a degree of subcooling with acute repercussions for their use in ice-repellency applications. Finally, whilst developing a superhydrophobic coating robust to impinging warm water droplets using a photothermal metasurface, I establish the theoretical foundations of condensation filling in a diffusion-limited regime based on acompetition between the filling and droplet-substrate contact times. Furthermore, I rationalise the performance of a hierarchical texture by considering the resulting shift to a nucleationlimited condensation regime. The understanding of phase change-induced wetting transitions accrued here should herald advancements in the design of superhydrophobic surfaces, broadening their working envelope and extending water repellency far beyond the state-of-the-art

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

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 repellenc y 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.ISSN:0027-8424ISSN:1091-649

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.ISSN:1936-0851ISSN:1936-086

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

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.ISSN:1745-2473ISSN:1745-248