Pinning of Sliding Droplets on Microstructured Surfaces

Most often, the structured packing surfaces build into absorption columns contain a texture with dimensions in the same order of magnitude as the liquid film thickness. While letting a droplet flow down such structured surfaces, frequently it halts midstream on the plate. We perform numerical simulations to asses the conditions of surface structures which lead to the pinning of a sliding droplet. A thermodynamic consistent Cahn-Hilliard-Navier-Stokes model is adopted to calculate the two phase flow. Boundary conditions which allow for dynamic contact angle hysteresis and slip between the moving contact lines and the solid surface are applied. As the resulting model forms a very tightly coupled and nonlinear system of equations the discretization and solution strategy is carefully selected to allow efficient and accurate simulations. The model and the solution strategy are validated against analytical and experimental data of static and dynamic wetting of droplets. We present results of gravity-driven droplets on inclined surfaces with structures in the size of the droplets and discuss the observed requirements for pinning a sliding droplet to a surface.