Radial wicking of droplets exhibit universal drying dynamics

When a drop contacts and absorbs in a thin porous surface, it can wick radially outward. This phenomenon is exploited in cooling textiles, but also complicates forensic stain analysis. The distance that the liquid spreads and the time it takes to evaporate are coupled, yet the consequence of this coupling on the wicking dynamics is not well understood. Here we measure and model how evaporation can reverse the direction of the liquid front so that the wetted patch shrinks after reaching a maximum diameter. We show that both this maximum diameter and the time to fully evaporate can be predicted with a single dimensionless parameter based on the substrate and liquid properties. Yet regardless of the system properties, we uncover a universal dynamic in which drops expand for approximately 1/4 of their lifetime and shrink for the rest. Counter-intuitively, we find that neither the evaporative cooling nor the stain area scales linearly with the initial droplet volume.