Different modes of heat transfer and crystallization in a drop of NaCl solution: The influence of key factors on the crystallization rate and the heat transfer coefficient

Evaporation, crystallization and heat transfer in a droplet of NaCl/H2O solution have been studied experimentally. Crystallization began at the edges of the droplet at solution supersaturation S = 1.94 (S = C-sup/C-sat, where C-sup is the salt concentration in a metastable region (supersaturation), and C-sat corresponds to saturation), which is noticeably higher than the previously obtained supersaturation for the droplet, micmcapillary, and when using surfactant. When the droplet evaporated on the heated wall, the supersaturation of the salt solution was noticeably higher than when evaporating without heating. Four characteristic modes of evaporation and crystallization are distinguished. The ratio of the crystallization rate to the evaporation rate (j(cr)/j(ev)) may be both substantially greater and less than one. It is shown that the heat transfer and the crystallization rate are regulated by Marangoni convection. The data obtained are of interest for the development of modern concepts of high-temperature crystallization in the presence of intense Marangoni flow.