Design of Surface Hierarchy for Extreme Hydrophobicity

Abstract

An extreme water-repellent surface is designed and fabricated with a hierarchical integration of nano- and microscale textures. We combined the two readily accessible etching techniques, a standard deep silicon etching, and a gas phase isotropic etching (XeF2) for the uniform formation of double roughness on a silicon surface. The fabricated synthetic surface shows the hallmarks of the Lotus effect: durable super water repellency (contact angle>173 degrees) and the sole existence of the Cassie state even with a very large spacing between roughness structures (>1:7.5). We directly demonstrate the absence of the Wenzel's or wetted state through a series of experiments. When a water droplet is squeezed or dropped on the fabricated surface, the contact angle hardly changes and the released droplet instantly springs back without remaining wetted on the surface. We also show that a ball of water droplet keeps bouncing on the surface. Furthermore, the droplet shows very small contact angle hysteresis which can be further used in applications such as super-repellent coating and low-drag microfludics. These properties are attributed to the nano/micro surface texture designed to keep the nonwetting state energetically favorable.

Design of a robust superhydrophobic surface: thermodynamic and kinetic analysis

“…Existing approaches at surface design involve b/a ratio less than the critical limit 12,17,19,21,39,40 Although the critical limit is wellknown, the domains of dependent parameters, namely c/a ratio and θY have not yet been investigated. It can be shown that the critical limit assumes a positive real value, if and only if the Young's contact angle exceeds 90° (equation 8).…”

“…. In recent times, θY as high as 154.6° have been reported, in the process of incorporating nanostructures on micropillars 17,41,42 .…”

“…the antiwetting pressure (Pantiwetting). The antiwetting pressure (Pantiwetting) corresponds to the energy difference between the homogeneous and the heterogeneous wetting regimes 16,17,40 . The antiwetting pressure denotes the force per unit area offered to a water droplet as it transitions from a depinned metastable Cassie state to a Wenzel state.…”

Metal oxide thin films and nanostructures for self-cleaning applications: current status and future prospects

“…If W is the pillar width then the roughness factor and contact areas can be expressed as r = 1 + 4R/(W/a) and R = 1/(b/a + 1) 2 [21]. These equations lead to the conclusion that a composite interface consisting of a hierarchical roughness (such as a periodic structure of micro/nanopillars) is ideal for…”

“…In addition, for practical applications, the surfaces should also exhibit low contact angle hysteresis along with selfcleaning properties [19][20][21]. The schematic representation of a water droplet over smooth, microstructure, nanostructure and hierarchical structure is shown in Figure 4.…”