Ring to Mountain Transition in Deposition Pattern of Drying Droplets.

When a droplet containing a nonvolatile component is dried on a substrate, it leaves a ringlike deposit on the substrate. We propose a theory that predicts the deposit distribution based on a model of fluid flow and the contact line motion of the droplet. It is shown that the deposition pattern changes continuously from a coffee ring to volcanolike and to mountainlike depending on the mobility of the contact line and the evaporation rate. An analytical expression is given for the peak position of the distribution of the deposit left on the substrate.