A Lattice Boltzmann Approach for Directional Transporting of Droplets Using Heterogeneous Surface Roughness

Fluid droplet on a surface with roughness has been simulated to investigate the hydrophobicity of surface and also measure the increase in contact angle (CA). Surface roughness increases the area of solid-liquid interface and this increase in composite interface makes the water to repel solid surface, thus causing an increase of the CA. Recently heterogeneous structure surface, which is pillar or rib structures with gradually changing pitches in certain direction, has gained lot of interest from researchers because wetting characteristics of those structures allow droplet movement without external forces. In this paper, droplet movement for heterogeneous surface cases are simulated with the computational fluid dynamics (CFD) modeling, known as Lattice Boltzmann method (LBM). First part of the study concentrates on droplet transportation. Half of the surface is more-hydrophobic region, textured with microscopic pillars and the other part of surface is less-hydrophobic, textured or smooth surface. Second part of the study concentrates on droplet breakup. More-hydrophobic textured band is located at center of less-hydrophobic textured surface. To see the effect of surface structure only, we choose same chemical property for all surfaces. Water droplets are spatially placed on border line of the different textures of surface. The simulations are carried out using projection method of LBM. Projection method has been 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 Projection method. This method can calculate solid-liquid-gas composite interface.