Evaporation and crystallization of NaCl-water droplets suspended in air by acoustic levitation

Evaporation of salt-water solutions is common in industrial processes and nature. Evaporation kinetics for multicomponent droplets have been investigated previously but most experimental systems have droplets contacting a solid substrate. Here, drying of acoustically levitated salt-water solutions is exam-ined. The results show that the addition of NaCl to water raises the evaporation rate of water from the droplet. Since water evaporates from the surface of the droplet, the NaCl concentration is maximum there and reaches the solubility limit first yielding crystals at the droplet interface. The crystals fall to the base of the droplet forming a 'cup' within which the droplet sits and further evaporates. The experimental results are compared to theory which shows that the diameter versus time behaviour is matched for the pure water case but the NaCl solution cases require an enhanced water vapour-in-air diffusion coef-ficient to match the experimental data.(c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

The evaporation process and crystal formation of salt-water solutions were studied in this paper, showing that NaCl can enhance the evaporation rate of water within droplets and lead to crystal formation. Experimental results matched theory better for pure water cases compared to NaCl solution cases, which required an enhanced water vapour-in-air diffusion coefficient.