Abstract

The spreading dynamics of a drop deposited on a superhydrophilic micropillar array are qualitatively different from those on a smooth surface in that a thin fringe layer rapidly wicks into microstructures while the bulk collapses in the center. Here we experimentally measure the temporal evolution of the shapes of water and silicone oil drops on the superhydrophilic surfaces, and find various power laws that describe the spreading dynamics. The bulk radius increases initially but shrinks in the late stages for drainage of its volume by wicking. The fringe film tends to grow diffusively for the entire timespan of spreading with the effective diffusivity being a function of surface tension, pillar height, viscosity, and surface roughness. The entire footprint is shown to grow like t1/4, t being time, for the entire time range, which allows us to estimate the time for complete drop spreading.