The acoustic radiation force: a gravitation-like field

TL;DR

This paper introduces a new perspective on the acoustic radiation force, describing it as a gravitation-like field that influences particle motion in sound fields, unifying existing theories with a novel interpretation.

Contribution

It presents an expression for the instantaneous acoustic radiation force as a fluctuating gravitation-like field, linking pressure gradients to local acceleration effects.

Findings

The radiation force can be viewed as a gravitation-like field.

The approach recovers Gor'kov's expression upon averaging.

Two main contributions are identified: local and convective.

Abstract

In this letter, we propose an expression for the \emph{instantaneous} acoustic radiation force acting on a compressible sphere when it is immersed in a sound field with a wavelength much larger than the particle size (Rayleigh scattering regime). We show that the leading term of the radiation force can alternatively be expressed as the time average of a fluctuating gravitation-like force. In other words, the effect of the acoustic pressure gradient is to generate a local acceleration field encompassing the sphere, which gives rise to an apparent buoyancy force, making the object move in the incoming field. When averaging over time, we recover the celebrated Gor'kov expression and emphasize that two terms appear, one local and one convective, which identify to the well-known monopolar and dipolar contributions.