Microlayered flow structure around an acoustically levitated droplet under a phase-change process

The acoustic levitation method (ALM) has found extensive applications in the fields of materials science, analytical chemistry, and biomedicine. This paper describes an experimental investigation of a levitated droplet in a 19.4-kHz single-axis acoustic levitator. We used water, ethanol, water/ethanol mixture, and hexane as test samples to investigate the effect of saturated vapor pressure on the flow field and evaporation process using a high-speed camera. In the case of ethanol, water/ethanol mixtures with initial ethanol fractions of 50 and 70 wt%, and hexane droplets, microlayered toroidal vortexes are generated in the vicinity of the droplet interface. Experimental results indicate the presence of two stages in the evaporation process of ethanol and binary mixture droplets for ethanol content >10%. The internal and external flow fields of the acoustically levitated droplet of pure and binary mixtures are clearly observed. The binary mixture of the levitated droplet shows the interaction between the configurations of the internal and external flow fields of the droplet and the concentration of the volatile fluid. Our findings can contribute to the further development of existing theoretical prediction. High-speed visualizations of acoustically levitated droplets reveal differences in flow field interactions depending on droplet composition. Koji Hasegawa and colleagues from Kogakuin University and the University of Tsukuba in Japan have used a high-speed camera system to observe the evolution of internal and external flow fields in droplets levitated using acoustic standing waves. They found that droplets of pure water develop simple toroidal vortices at the base, while hexane and binary mixtures of water and ethanol develop microlayered vortices near the droplet interface. The results show that the flow fields of an acoustically levitated droplet affect the evaporation process, and that the concentration of volatile fluid affects the configuration of the flow fields. This experimental insight is expected to help in the development of improved contactless sample manipulation at the micro- and nanoscale.