Multidimensional velocity fields highlight toroidal forces in acoustically levitated droplets

Acoustic levitation for manipulating fluids has drawn attention from disciplines of material science to analytical chemistry to pharmaceuticals thanks to its ability to minimize contamination and nucleation that containers pose. Many details regarding the ultrasonic control of levitating droplets, however, remain to be determined. New work examining the flow fields of droplets provides a clearer view of the internal dynamics of these droplets.

Sasaki et al. report new findings of multidimensional velocity fields of acoustically levitated droplets. Using stereoscopic particle image velocimetry, the group analyzed the evaporation behaviors and flows of binary ethanol and water droplets. Comparing their results with simple rotational models allowed them to clarify what drives uniaxial rotational flow inside the levitated droplets.

To visualize and quantify the flow, the authors developed a new, three-dimensional measurement system based on a modified approach using the Scheimpflug theorem, a principle that describes how planar subjects can be in focus when not parallel to the image plane, to observe both internal and external flows simultaneously.

“As flow fields became three-dimensional, the transition process of flow structures was revealed,” said author Yuya Sasaki.

Toroidal forces were generated in each droplet and internal and external flow fields demonstrated the same flow structure immediately after they began to levitate.

Within 100 seconds, however, the internal flow structure transitioned to a uniaxial rotational structure. After this time, the group found that the external flow at the top and bottom of the droplets expanded rapidly, changing the concentration of the outer ethanol vapor.

Sasaki next hopes to use dual-wavelength interferometer to explore the driving mechanism of toroidal vortices in volatile droplets and better understand the flow field inside them.

Source: “Transition of flow field of acoustically levitated droplets with evaporation,” by Y. Sasaki, K. Kobayashi, K. Hasegawa, A. Kaneko, and Y. Abe, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5124499 .