Evaporation Kinetics and Saturated Vapor Pressure Measurement of Volatile Single Droplets Using Uniaxial Acoustic Levitation

A uniaxial acoustic levitator was used to investigate the evaporation of single droplets of water, methanol, ethanol, and n-propanol in an open, containerless environment. The temporal evolution of droplet radius was measured by optical imaging, and saturated vapor pressures were estimated from the linear 𝑟2 - time relationship using a Maxwell-based mass transfer model with a transition-regime correction. The measured vapor pressures showed reasonable agreement with literature values but were systematically lower, indicating the effect of evaporative cooling and the limitation of the quasi-isothermal assumption. Ethanol exhibited a distinct two-stage evaporation behavior, which is attributed to the combined effects of alcohol-water affinity, ambient humidity, and droplet lifetime. The later-stage behavior is inferred to result from water uptake from the surrounding air, which reduces the vapor-pressure driving force for ethanol evaporation. An apparent initial condensation rate was also estimated from the kink in the ethanol evaporation curve. These results show that acoustic levitation provides a useful non-contact platform for studying microscale droplet evaporation and vapor-pressure-related thermodynamic behavior.