Theoretical analysis of a torque exerted by the vortex ultrasonic beam on the absorber located in liquid

Waves of any nature carry energy and momentum. In certain configurations of wave beams, angular momentum transfer is also possible: the corresponding beams are called “vortex beams”. In this paper, we study the ability of a vortex acoustic beam of the megahertz range to rotate a large-sized absorber (of several dozen wavelengths in diameter) in a liquid. The vortex beam was formed in water by passing ultrasonic radiation from a focusing piezoceramic source through a non-uniform in thickness 12-sector phase plate made from Plexiglas. The beam propagated vertically upward and was directed to an acoustic absorber in the form of a cylinder 10 cm in diameter and 3 cm in height made of silicone rubber. The absorber was freely suspended on a thin elastic thread. It is demonstrated that when the absorber is irradiated, it is rotated. By scanning the acoustic field in the transverse plane by a miniature hydrophone, two-dimensional distributions of the amplitude and phase of the wave were found, and on the basis of these calculations the moment of the radiation beam force applied to the absorber was calculated. The results of the calculations are in agreement with the observations.