Acoustic radiation torque exerted on a subwavelength spheroidal particle by a traveling and standing plane wave

TL;DR

This paper derives exact expressions for the acoustic radiation torque on a subwavelength spheroidal particle caused by traveling and standing ultrasonic waves, revealing how torque varies with particle shape and orientation.

Contribution

It provides the first exact analytical derivation of radiation torque on a nonspherical particle in ultrasonic waves using spheroidal coordinates and asymptotic analysis.

Findings

Torque equals that of a traveling wave at pressure nodes in standing waves

Torque depends on particle aspect ratio and orientation

Results agree with previous numerical simulations

Abstract

The nonlinear interaction of ultrasonic waves with a nonspherical particle may give rise to the acoustic radiation torque on the particle. This phenomenon is investigated here considering a rigid prolate spheroidal particle of subwavelength dimensions that is much smaller than the wavelength. Using the partial wave expansion in spheroidal coordinates, the radiation torque of a traveling and standing plane wave with arbitrary orientation is exactly derived in the dipole approximation. We obtain asymptotic expressions of the torque as the particle geometry approaches a sphere and a straight line. As the particle is trapped in a pressure node of a standing plane wave, its radiation torque equals that of a traveling plane wave. We also find how the torque changes with the particle aspect ratio. Our findings are in excellent agreement with previous numerical computations. Also, by analyzing the torque potential energy, we determine the stable and unstable spatial configuration available for a particle.