Evaporation of droplet in mid-air: Pure and binary droplets in single-axis acoustic levitator

Acoustic levitation method (ALM) is a container-less processing method with applications in various fields, including material processing, biology, and analytical chemistry. Because it is a container-less processing technique, ALM could prevent nucleation and contamination of materials being processed via contact with a container wall. It is well-known that evaporation of a sample is an important process in container-less processing of materials; however, the mechanism of evaporation in multicomponent droplets in a single acoustic levitator is still unclear. Thus, we evaluate and understand the evaporation of an acoustically levitated multicomponent droplet and manipulate the evaporation process of the sample in this study. Specifically, we investigate the evaporation process of pure and multicomponent droplets using container-less processing experimentally. The evaporation processes and temporal evolution of the surface temperature of a multicomponent droplet were evaluated using a high-speed camera and radiation thermometer, respectively. We used water, ethanol, methanol, hexane, acetone, pentane, and binary solutions (solution of 25 wt%, 50 wt%, and 75 wt% ethanol, methanol, and acetone, respectively) as test samples to study the effect of saturated vapor pressure on evaporation. Ethanol, methanol, and acetone droplets evaporate in two different stages. It was observed that the water vapor in the air condensed during the evaporation process of these water-soluble droplets; hence, our experimental data did not agree with the theoretical prediction in accordance with the d2 law. Nevertheless, the evaporation behavior in the first stage of evaporation was consistent with the theoretical prediction. Furthermore, for binary droplets, as the concentration of the resultant solution increased owing to evaporation, the transition time from the first to the second stage of evaporation also increased. Based on these observations, estimation equations for binary droplets were developed to ensure that the experimental and theoretical values were in good agreement.