Velocity measurement during evaporation of seeded, sessile drops on heated surfaces

Abstract

Evaporation of sessile drops has been studied extensively 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 Marangoni convection counteracting this mass flux explaining more uniform deposition patterns. Understanding of such deposition processes is important in ink-jet printing and other micro-scale deposition technologies, where the nature of deposition can have a dramatic effect on the quality or effectiveness of the finished product. 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-PIV, viewing the drop from below, through the cover slide. The images obtained have also enabled us to observe the formation of holes in the liquid film during the latter stages of evaporation.