Impact spreading and absorption of Newtonian droplets on topographically irregular porous materials

This paper concerns the combined influences of high impaction rates and surface geometries on the spreading–absorption behaviour of Newtonian fluid droplets under non-equilibrium conditions. A volume of fluid method is used to simulate the motion of up to 50μm diameter droplets onto surfaces comprising protuberances below 12μm in width. On both flat and systematically arranged pyramidal surfaces, spreading is an exponential function of the maximum depth of absorption. Surface spreading is furthermore a linear function of the droplet Reynolds number and a simple method of predicting spreading from the Reynolds number has been identified herein. At high impact velocities, kinetic energy driven spreading dominates over wetting driven flow; though, its relativity to absorption is a function of the droplet pressure pulse. Lateral (plan view) spreading on irregular surfaces is found to be inversely proportional to surface roughness. In this paper, surface geometry is shown to be a critical determinant of both spreading and absorption and thereby, the research herein provides details that could be useful in functionalising surfaces. Finally, new surface roughness equations derived herein make the comparison of surface spreading to surface roughness more realistic than commonly used roughness factors such as the Wenzel and RaRa roughness.