Physics Of Two-Stage Evaporation Of Volatile Droplets Suspended In An Ultrasonic Levitator

Abstract

The evaporation process of mono-component droplets in an ultrasonic levitator is experimentally monitored, focusing on three types of volatile liquids: methanol, ethanol, and 2-propanol. This investigation reveals a ‘two-stage’ evaporation process characterized by an initial rapid evaporation stage followed by a significantly slower evaporation stage; a pattern consistently observed across all three volatile liquids. However, for droplets of comparable size, methanol droplets evaporate faster in the rapid stage than ethanol and iso-propanol droplets and enter the slow evaporation stage earlier. On the other hand, during the slow evaporation stage, 2-propanol droplets exhibit the lowest evaporation rate compared to methanol and ethanol droplets. To explain the two-stage evaporation and the varying evaporation characteristics of mono-component droplets with different volatile components, the streaming patterns within the levitator, with and without a suspended droplet, are visualized and quantified using Particle Imaging Velocimetry (PIV). Two acoustic-induced jet flows, originating from the transducer and reflector of the levitator and considered as Eckart streaming, are observed within the empty levitator. These jet flows move toward the levitated droplet, influencing its evaporation process. Additionally, Stefan flow, carrying vapor away from the surface of the volatile droplet, is clearly observed due to rapid evaporation. The competition between Stefan flow and Eckart streaming ultimately determines the external streaming patterns around the investigated droplets. A theory combining the external streaming patterns and vapor enrichment is proposed to interpret the two-stage evaporation and the corresponding characteristics of the evaporating droplets suspended in the levitator. Furthermore, this study captures boundary-driven acoustic streaming near the droplet surface, known as Rayleigh–Schlichting streaming, whose dimensions and motion vary with the different liquid components and the properties of the corresponding vapor. In summary, the study highlights the presence of Eckart streaming in the ultrasonic levitator and Stefan flow outside an evaporating volatile droplet. Considering these flow patterns is essential for explaining the ‘two-stage’ evaporation and the relevant characteristics within the ultrasonic levitator.