Acoustic radiation torque of an acoustic-vortex spanner exerted on axisymmetric objects

Abstract

Based on the analysis of the wave vector of an acoustic-vortex (AV) spanner, the radiation torque of object rotation is investigated. It is demonstrated that the rotation of an axisymmetric disk centered on the AV spanner is mainly driven by the acoustic radiation force. The radiation torque exerted on a small-radius object is inversely associated with the topological charge in the center AV, and it is enhanced significantly for a larger AV with a higher topological charge. With the sixteen-source experimental setup, radius dependencies of radiation torque for AV spanners with different topological charges are verified by quantitative laser-displacement measurements using disks with different radii. The favorable results demonstrate that the radiation torque is more applicable than the orbital angular momentum in describing the driving capability of an AV spanner and can be used as an effective tool in clinical applications to manipulate objects with a feature size at the wavelength-scale inside body.