Natural convection in evaporating sessile drops with solidification

Abstract

Transient natural convection and internal solidification in a minute drop evaporating on a horizontal plate is investigated by the laser shadowgraph-schlieren method and direct microscopic photography. The drop is point-cooled at the center of the base by the tip of a metal wire connected to a block of dry ice. A thermocapillary-induced, doughnut-shaped vortex region exists around the solid phase during the course of internal solidification. The genesis of a thermocapillary-induced vortex ring and internal solidification can be predicted from the time history of drop temperatures. The internal solidification suppresses the surface rippling of an evaporating drop, thus retarding the evaporation rate. Empirical expressions are obtained which describe the effects of the Marangoni, Prandtl and Stefan numbers and the degree of supercooling on the solid growth velocity.