The wicking of volatile drops on thin porous substrates

A liquid droplet deposited onto a thin porous substrate can penetrate into the media and then spontaneously spread radially outward. The spreading of a non-volatile drop follows power-law dynamics set by a balance of capillarity and viscosity. However, if the drop is volatile, the precise effect of evaporation on the wicking dynamics is not well understood. Here we measure and model how evaporation limits the extent that the droplet spreads, reversing the direction of the liquid front so that the droplet shrinks in size after reaching a maximum diameter. Through a series of systematic experiments, we investigate the evaporation rate and the wicking dynamics, both of which can be predicted with relatively simple models. These results provide insight into the size of stains deposited by droplets on porous substrates such as paper or fabric.