Marangoni flow and free convection during crystallization of a salt solution droplet

The behavior of evaporation and crystallization of a drop of an aqueous solution of LiBr salt at the moment of rapid motion of the crystalline crust has been experimentally investigated. With the use of non-contact method of Particle Image Velocity (PIV) a technique was developed for measuring the instantaneous velocity field inside the droplet during crystallization. Data on the convection dynamics inside the droplet during crystallization have been obtained for the first time. It is shown that reliable measurement of the rate of convection in a drop is possible even through a thick crust of crystallohydrates, which is formed on the surface. Previously it was thought that the characteristic size of the convective vortex is comparable to the drop base radius R. The novelty of the work lies in the fact that a group of vortices, whose dimensions are much smaller than the radius R, is formed at crystallization. It is shown that even when the crystalline crust covers 80-90 % of the surface of the entire droplet, appreciable convective motion is realized inside the fluid, which must be considered for correct modeling of heat transfer and evaporation. Most modern technologies associated with the growth of crystals inside solutions often face problems of heat exchange control and the need to create a uniform temperature field inside the liquid. These studies help to understand the mechanisms of convection during crystallization and suggest ways to control crystallization.