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

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. In this paper, asymptotic expressions of the torque as the particle geometry approaches a sphere and a straight line are obtained. As the particle is trapped in a pressure node of a standing plane wave, its radiation torque equals that of a traveling plane wave. This paper also finds how the torque changes with the particle aspect ratio. The findings in this paper are in excellent agreement with previous numerical computations. Also, by analyzing the torque potential energy, the stable and unstable spatial configurations available for the particle are determined.