A study of static and dynamic robustness of hydro/omniphobic surfaces

Abstract

Liquid droplets in the Cassie-Baxter state form liquid-air interfaces that are not flat but distorted due to pressure differences across the interfaces between the asperities. These distorted interfaces play an essential role in the transition from the composite Cassie-Baxter state to the fully-wetted Wenzel state and in the determination of the robustness of the composite state. As well as the static pressure difference due to the Laplace pressure, dynamic pressure difference due to various configurations including drop impact is also a source that causes the transition with the distorted interfaces. However, there are few experimental and numerical studies that consider the details of the distorted interfaces for a wide range of liquids and there is a lack of an apriori method to evaluate the robustness of three-dimensionally complicated textures. In addition, previous studies on drop impact pressure did not cover the maximum pressure at impact in the range of low velocities (< 2 m/s). We have first investigated the shape of distorted liquid-air interfaces and their transition conditions experimentally by using droplets of various low surface tension liquids on millimeter-sized re-entrant surface topography. For the dynamic pressure difference, we proposed a modified water hammer pressure formula and compared with the experiment using a high speed camera. The static experimental results by using three dimensionally printed millimetric structures are in good agreement with our newly-developed finite element simulations. I These three-dimensional simulations of the interfacial shape provide a predictive tool for the robustness of a wide range of proposed micro-texture in terms of the breakthrough pressure at which the distorted liquid-air interface infiltrate into the space between asperities and the droplet transitions to the Wenzel state. The dynamic experimental results open a broad avenue to a novel approach to delve into the dynamic breakthrough pressure of droplets of a variety of liquids.