A Study of the Velocity Field during Evaporation of Sessile Water and Water/Ethanol Drops

Many studies have investigated evaporation of sessile drops in an attempt to understand the effect of wetting on the evaporation process. Recently interest has also increased in the deposition of particles from such drops, with evaporative mass flux being deemed to be responsible for ring-like deposits, and counteraction of the mass flux by Marangoni convection explaining more uniform deposition patterns. Understanding of such deposition processes is important in biological applications, such as the Litos test-system endorsed by the Russian Ministry of Health for diagnosis of urolithiasis and the evaporation of colloidal drops for depositing and organizing proteins and DNA. In most cases where deposition from evaporating drops has been studied, velocity information is inferred from the final deposition pattern or from mathematical modeling based on simplified models of the physics of the evaporation process. In this study we have directly measured the flow velocities in the base of sessile drops, using micro particle image velocimetry, viewing the drop from below, through the cover slide. For water drops, a radial pattern of flow was observed with a maximum velocity close to but not at the pinned outer edge. For 'azeotropic' ethanol/water mixtures, the velocity field is more chaotic to begin with, passing through a phase involving three or four recirculation cells and finally having the same radial pattern as for water drops.