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.

Inspired by nature, microtextured and nanotextured surfaces have demonstrated unique droplet repellent properties (1), which are beneficial for self-cleaning (2), antiicing (3), and condensation enhancement (4). For many practical applications, repelling an impacting water droplet is important. Much work has been performed to understand how surface topography and composition stabilize the Cassie–Baxter wetting state (5), a prerequisite for high droplet repellency, under static (6, 7) and dynamic (8–24) conditions to preclude transitioning to the Wenzel wetting state (25). To prevent the Cassie–Baxter to Wenzel wetting-state transition, hereafter defined as impalement, the capillary (antiwetting, surface property) pressure must exceed the wetting (droplet) pressure (9, 17). In previous research, the latter has been attributed to the dynamic pressure (17), effective water hammer (10, 11, 16, 23), and deformation of the droplet by the compressed air layer leading to a ring-shaped pressure maximum (13, 26).It is established that the use of hierarchical surface texture and low-surface energy coatings are key components for achieving liquid repellency and preventing impalement (13, 15, 16, 27). Much of this understanding is based on work conducted under ambient conditions; however, work on droplet mobility that departs from ambient environmental conditions (28–31) [i.e., substrate cooling (13, 32), supercooled droplet impact (33, 34), ambient pressure reduction (35–37), and droplet heating (38)] is yielding new insight into, and unveils new requirements for, the rational design of superrepellent surfaces. Mechanisms for the loss of superrepellent behavior include condensation-based impalement in the presence of hot vapor (warm droplets) (28, 38), increased droplet viscosity (33), and rapid recalescent freezing inhibiting droplet recoil (cold droplets) (32, 34). Therefore, in addition to wettability, it is necessary to investigate important aspects such as nonstandard atmosphere environments and nucleation (3, 39, 40)—which affect the intervening gas-layer dynamic state during droplet impact and enhance droplet–substrate adhesion—to enable surface texture tailoring to counter such effects to preserve superhydrophobicity (38, 41–46). While the effects of droplet and environmental temperature (hot or cold) on superhydrophobicity for impacting droplets are being better understood, research into the effect of the environmental gas pressure, an equally important counterpart, is comparatively scant (35–37), as is the combined effect with humidity. Both are very important for defining the thermodynamic state of the intervening gas layer during droplet impact, necessary to determine its behavior. Previous work has demonstrated that decreasing the environmental pressure influences the droplet impact dynamics on smooth and rough surfaces (e.g., prompt vs. thin-sheet splashing) (26, 47–50), which may alter impact and recoil dynamics on superhydrophobic surfaces; however, this remains to be seen.Here, we examine, experimentally and theoretically, the combined effects of reducing environmental pressure and varying humidity on droplet impact and recoil from superhydrophobic surfaces, identify impalement mechanisms, and armed with this knowledge, rationally nanoengineer robust superhydrophobic surfaces that can repel impacting droplets across a wide range of environmental conditions. We demonstrate that the likelihood of impalement on textured surfaces increases as the ambient pressure decreases and provide rationales to explain this. Additionally, through variation of the relative humidity, we report, and theoretically underpin, a hitherto unknown mechanism for wetting-state transition through supersaturation of, and subsequent condensation within, the air layer resulting from the pressure increase beneath an impacting droplet. Finally, we demonstrate an alternative coating capable of resisting impalement within the working envelope experimentally explored, based on our accrued knowledge. We believe that our observations will have profound implications for all applications of superhydrophobicity in both low-pressure, such as those involving ice accretion on aircraft through superior repellency of supercooled drops, and naturally humid environments, including for self-cleaning materials such as textiles.