Zhang, B. W., Hong, Z. Y., & Drinkwater, B. W. (2022). Transfer of Orbital Angular Momentum to Freely Levitated High-Density Objects in Airborne Acoustic Vortices. Physical Review Applied, 18(2), 024029.
Vortex-field acoustic levitation is an application of acoustic vortices that allows the manipulation of a wide range of materials in various media. However, to date, its levitation capability in air is limited to relatively low-density objects. Here, by optimizing an array-tube setup, we levitate iridium (22.56g/cm3) in a first-order acoustic vortex in air. This technique makes it possible to observe the orbital angular momentum transfer carried by acoustic vortices to freely levitated high-density objects, of which the highest rotation speed reaches up to 6500 revolutions per second and increases with a decrease in the object size and an increase in the source amplitude. It is revealed that the thermal-viscous boundary layer dissipation is the dominant effect and leads directly to the rapid rotation of high-density objects. This work can expand the application scope of acoustic vortices, especially in the fields of containerless processing of various materials and rapid rotation of objects, and help to understand the mechanisms of orbital angular momentum transfer to matter.