Hydrophobicity of hierarchical structured super-hydrophobic surfaces with the Secondary Pillars; a lattice boltzmann method with high-density ratio

This paper studies the enhanced super-hydrophobicity on surfaces with hierarchical textures. Surface roughness increases the Contact angle(CA). Recently hierarchical structure surface, which is secondary structures established on the simple pillars or pyramids array, has gained lot of interest from researchers because hydrophobicity of hierarchical structure was found to be greater than in simple structure. In this paper, CA for two surface cases are measured with the computational fluid dynamics (CFD) modeling, known as lattice Boltzmann method (LBM). The first case is "simple structure", which is composed of square cross section pillar array. The second one is "hierarchical structure", which is composed of secondary pillar structure added on the base pillar array. Secondary pillar structure is comparatively small, about 50-100 times smaller than base pillar. Simulation of water droplets, which are statically placed on each of the surfaces, is carried out using the projection method of LBM. Projection method is used to in this study to be able to model the large density difference between air and water. Two phase immiscible fluids flow consisting of air and water (density ratio of air to water = 1:1000) is built in 3D space by using the projection method. CAs and the Cassie, Wenzel regime characteristic for hierarchical structured surface case are compared to that of simple structured surface. We were succeeded in quantifying the benefit of having hierarchical textures; the addition of the secondary structure enhances both the apparent contact angle and the robustness of the non-wetting regime.