Phase Change-Induced Wetting Transitions on Superhydrophobic Surfaces

Abstract

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.