Willis coupling-induced acoustic radiation force and torque reversal

TL;DR

This paper demonstrates how designing metamaterials with Willis coupling can control and reverse acoustic radiation force and torque on small particles, enabling shape-dependent manipulation and sorting.

Contribution

It introduces a model incorporating Willis coupling into particle scattering, revealing how shape asymmetry affects acoustic forces and enables force reversal.

Findings

Willis coupling enables reversal of radiation force and torque.

Shape asymmetry shifts stable trapping locations.

Metamaterial design allows tunable acoustic manipulation.

Abstract

Acoustic meta-atoms serve as the building blocks of metamaterials, with linear properties designed to achieve functions such as beam steering, cloaking and focusing. They have also been used to shape the characteristics of incident acoustic fields, which led to the manipulation of acoustic radiation force and torque for development of acoustic tweezers with improved spatial resolution. However, acoustic radiation force and torque also depend on the shape of the object, which strongly affects its scattering properties. We show that by designing linear properties of an object using metamaterial concepts, the nonlinear acoustic effects of radiation force and torque can be controlled. Trapped objects are typically small compared to the wavelength, and are described as particles, inducing monopole and dipole scattering. We extend such models to a polarizability tensor including Willis coupling terms, as a measure of asymmetry, capturing the significance of geometrical features. We apply our model to a three-dimensional, sub-wavelength meta-atom with maximal Willis coupling, demonstrating that the force and the torque can be reversed relative to an equivalent symmetrical particle. By considering shape asymmetry in the acoustic radiation force and torque, Gorkov's fundamental theory of acoustophoresis is thereby extended. Asymmetrical shapes influence the acoustic fields by shifting the stable trapping location, highlighting a potential for tunable, shape-dependent particle sorting.