Breakup characteristics of levitated droplets in a resonant acoustic field

To help realize stable droplet manipulation, we present a study on the physical mechanism of interfacial instability and droplet atomization dynamics in acoustic levitation with ultrasonic phased arrays. Acoustic levitation is highly applicable in the fields of analytical chemistry and biology because container-less processing can prevent undesirable wall effects, such as nucleation and contamination resulting from the container walls. Although many studies demonstrated the atomization behavior in single-axis acoustic levitation, the breakup characteristics of levitated droplets in ultrasonic phased array levitation have been less studied. Here, we visualized the atomization behavior of ethanol, ethanol solution, and silicone oil droplets using a high-speed camera. The time evolution of the interfacial velocity of the droplets indicates a threshold for the droplet atomization. To elucidate atomization dynamics, the critical interfacial velocities of the levitated droplet immediately before its atomization are discussed and compared to the theoretical prediction based on the Kelvin-Helmholtz instability. Our experimental findings provide deeper physical insights into the levitation stability of droplets in ultrasonic phased array levitation for futuristic lab-in-a-drop applications.