Dynamical measurements of receding contact angle for evaporating drops

We study the receding contact angle of drops during evaporation. Drops of a dyed solution are allowed to evaporate for some time τ before they are removed via a small hole in the substrate. We record the contact angle as the fluid is removed and extract the receding contact angle — that is, the angle at which the drop first depins — as a function of τ and dye concentration. For low τ (i.e., immediate drop removal), the receding contact angle depends only weakly on concentration. For τ on the order of a few minutes, the receding contact angle decreases with a growing dependence on concentration. This suggests that the concentration of solute near the drop edge grows due to the flows induced by evaporation as in the coffee-ring effect. Surprisingly, even pure water exhibits a significant τ-dependent receding contact angle.