Acoustic radiation torque on an interface reflecting acoustic vortex beams

Abstract

Torque exerted by acoustic waves to rotate objects can result from asymmetry such as the case of an averaged torque on a Rayleigh disk exerted by an ordinary acoustic field. In the absence of asymmetry, acoustic torque can occur by viscous absorption either in the object or in the adjacent boundary layer of the object when using specific types of acoustic fields such as acoustic vortex beams carrying twisted wave fronts [Zhang and Marston, Phys. Rev. E 84, 065601 (2011)] or two orthogonal standing wave fields [Zhang and Marston, J. Acoust. Soc. Am. 136, 2917 (2014)]. Recent study suggested a torque on a flat water-air interface exerted by an ultrasonic vortex beam obliquely incident and reflecting off the interface [Zou, Lirette, and Zhang, Phys. Rev. Lett. 125, 074301 (2020)]. This torque is not associated with viscous absorption and instead results from variation of orbital angular momentum carried by the vortex beam during the reflection. The torque would otherwise disappear in normal incidence or using ordinary beams. The investigations advance understanding of radiation torque physics and approaches for non-contact manipulations using acoustic waves.