Effect of solution flow field on evaporation-induced NaCl crystallization in acoustically levitated droplets

Acoustically levitated droplets have exhibited great potential in crystallization studies due to their versatility with various solution types and the simplicity of the apparatus. By separating the precursor solution and the solid surface, the crystallization process could be observed and analyzed in a contactless environment. The decoupling of the crystallization system and surface physical/chemical properties greatly boosts the in situ investigation of early-stage nucleation and crystal growth mechanisms. However, the interaction between the precursor solution and applied acoustic field during the crystallization process is often neglected, which imposes significant influences on the crystal products. In this paper, visual experiments were carried out to study the NaCl crystallization process in acoustically levitated droplets. Image processing was employed to acquire the evaporation rate of the droplets, and particle image velocimetry analysis was used to characterize the flow field. Effects of droplet size and initial NaCl concentration were investigated, and the crystallization behaviors in the levitated droplet and sessile droplet were compared. The results indicate that the acoustic field introduced a forced convection of fluid within the levitated droplets, influencing the evaporation rate, supersaturation degree, and the morphology of the crystal product. The obtained mechanism is important for the application of acoustically levitated droplets and can be further applied to other crystallization research based on the acoustic levitation systems.