Leaving a Mark: How Droplets Spread, Stain, and Fade away

When a drop contacts and absorbs in a thin porous surface, it can wick outward, leading to a stain whose size depends on how far the drop spreads. A non-volatile drop is predicted to wick indefinitely; however, if the drop is volatile, the effect of evaporation on the extent of spreading 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 demonstrate that the dynamics collapse to a universal curve independent of the substrate and drying time, and we discuss how this property, along with related scalings, can be exploited in a variety of applications ranging from forensics to firefighting.