Acoustic interaction forces and torques acting on suspended spheres in an ideal fluid

TL;DR

This paper presents a theoretical analysis of acoustic interaction forces and torques on multiple spheres in an ideal fluid, using partial-wave expansion to account for multiple scattering effects, with practical examples involving olive-oil droplets.

Contribution

It introduces a method to compute acoustic interaction forces and torques on multiple spheres in an inviscid fluid, considering complex scattering effects and nonsymmetric incident waves.

Findings

Interaction forces can significantly differ from those caused by the incident wave alone.

Acoustic interaction torques can arise from nonsymmetric effective incident waves.

The method is demonstrated with droplets comparable to wavelength in size.

Abstract

In this paper, the acoustic interaction forces and torques exerted by an arbitrary time-harmonic wave on a set of N spheres suspended in an inviscid fluid are theoretically analyzed. In so doing, we utilize the partial-wave expansion method to solve the related multiple scattering problem. The acoustic interaction force and torque are computed for a sphere using the farfield radiation force and torque formulas. To exemplify the method, we calculate the interaction forces exerted by an external traveling and standing plane wave on an arrangement of two and three olive-oil droplets in water. The droplets radii are comparable to the wavelength (i.e. Mie scattering regime). The results show that the radiation force may considerably deviates from that exerted solely by the external incident wave. In addition, we find that acoustic interaction torques arise on the droplets when a nonsymmetric effective incident wave interacts with the droplets.