Two types of Cassie-to-Wenzel wetting transitions on superhydrophobic surfaces during drop impact

Abstract

Despite the fact that superhydrophobic surfaces possess useful and unique properties, their practical application has remained limited by durability issues. Among those, the wetting transition, whereby a surface gets impregnated by the liquid and permanently loses its superhydrophobicity, certainly constitutes the most limiting aspect under many realistic conditions. In this study, we revisit this so-called Cassie-to-Wenzel transition (CWT) under the broadly encountered situation of liquid drop impact. Using model hydrophobic micropillar surfaces of various geometrical characteristics and high speed imaging, we identify that CWT can occur through different mechanisms, and at different impact stages. At early impact stages, right after contact, CWT occurs through the well established dynamic pressure scenario of which we provide here a fully quantitative description. Comparing the critical wetting pressure of surfaces and the theoretical pressure distribution inside the liquid drop, we provide not only the CWT threshold but also the hardly reported wetted area which directly affects the surface spoiling. At a later stage, we report for the first time to our knowledge, a new CWT which occurs during the drop recoil toward bouncing. With the help of numerical simulations, we discuss the mechanism underlying this new transition and provide a simple model based on impulse conservation which successfully captures the transition threshold. By shedding light on the complex interaction between impacting water drops and surface structures, the present study will facilitate designing superhydrophobic surfaces with a desirable wetting state during drop impact.

Drop impact on inclined superhydrophobic surfaces

“…We believe that it is because the transition mechanism changes at high tilting angles, and we demonstrated that transitions in this range are not associated with traditional momentum transfer. We do not fully understand the observed wetting mechanism, but we speculate the observation agrees with what a recent study by Lee et al [33] has suggested. Further research will need to be conducted to substantiate this hypothesis.…”

“…3, the wetting transition at the tilting angle of 75°is observed not at the location of impact (red solid line), but at the downstream where the recoil occurs (red dashed line). Recently Lee et al reported that there are two types of distinct wetting transitions depending on the original kinetic energy and the solid fraction of the superhydrophobic surface [33]. Type 2 transition in their report is believed to occur during the recoil phase of the impacting drop, which matches our observation in Fig.…”

“…In both cases, the driving force for the transition is pressure, which, for the case of dynamic impact, originates from the transfer of momentum of the drop [18,26,27]. For example, a superhydrophobic surface may or may not repel an incoming drop depending on the relation between dynamic pressure (qV 2 ; we refer readers to reference for the derivation of dynamic pressure from momentum transfer [33]) and the Laplace pressure created by the curvature and surface tension of water-vapor interface. As the Laplace pressure P Laplace ¼ jr (j is the curvature of the interface), and the maximum curvature of a water meniscus inside a pore is inversely proportional to the size of the pore (% 1=a; a represents the nominal size of air pockets or the interspacing between textures), one can define a dimensionless parameter by taking the ratio between two pressure terms.…”

The springtail cuticle as a blueprint for omniphobic surfaces

“…10). [172][173][174][175] Furthermore, droplets that are not repelled, but stuck onto the surface, continuously enhance their inner pressure due to evaporation accompanied by shrinkage of the droplet size. In nature, the liquidrepellent cuticular surfaces of plants and animals mainly have to resist the kinetic energy of impacting rain droplets that exhibit an inner hydrostatic pressure in the range from 10 4 to 10 5 Pa or droplet formation from condensation at high relative humidity.…”

Droplet manipulation on a structured shape memory polymer surface

“…It is important to note that the threshold h ori value should be higher than h men in real experiments due to the water hammer effect during dispensing a water droplet on the SMP surface. 46 The apparent contact angle of a water droplet is defined as the contact angle of a water droplet on the structured SMP surface. Theoretically, the apparent contact angle (θ * ) is calculated from the intrinsic contact angle (θ Y ) and geometric parameters such as the solid fraction and roughness ratio of a surface.…”