Solute deposit during evaporation of a sessile binary liquid micro-droplet on a substrate

Formation of a ring-like deposit of solute during the evaporation of a binary liquid sessile droplet on a substrate was studied both experimentally and numerically. The effects of the droplet spread radius and substrate surface temperature on ring-like deposit of solute were investigated. The ring width increased with a decrease in the spread radius and the substrate temperature. The evaporation rate distributions on the droplet surface were numerically obtained to predict the shape of the thin film of solute after the evaporation using a theoretical model for solute transfer in a sessile droplet. When the substrate temperature was low, the increase in the evaporation rate near the contact line was inhibited. As a result, the velocity in the droplet toward the contact line, which caused the solute transfer, was small and then the ring width increases.