Asymmetric steady streaming as a mechanism for acoustic propulsion of rigid bodies

TL;DR

This paper explains how asymmetric steady streaming caused by acoustic waves can propel rigid bodies, using a fluid dynamics model that aligns with experimental observations of particle motion.

Contribution

It introduces a physical mechanism based on inertial forces and shape asymmetry to explain acoustic propulsion of rigid particles, supported by a theoretical derivation.

Findings

Model predicts propulsion speed consistent with experiments

Asymmetry and inertial effects enable autonomous particle motion

Steady streaming induces non-zero average forces on particles

Abstract

Recent experiments showed that standing acoustic waves could be exploited to induce self-propulsion of rigid metallic particles in the direction perpendicular to the acoustic wave. We propose in this paper a physical mechanism for these observations based on the interplay between inertial forces in the fluid and the geometrical asymmetry of the particle shape. We consider an axisymmetric rigid near-sphere oscillating in a quiescent fluid along a direction perpendicular to its symmetry axis. The kinematics of oscillations can be either prescribed or can result dynamically from the presence of an external oscillating velocity field. Steady streaming in the fluid, the inertial rectification of the time-periodic oscillating flow, generates steady stresses on the particle which, in general, do not average to zero, resulting in a finite propulsion speed along the axis of the symmetry of the particle and perpendicular to the oscillation direction. Our derivation of the propulsion speed is obtained at leading order in the Reynolds number and the deviation of the shape from that of a sphere. The results of our model are consistent with the experimental measurements, and more generally explains how time periodic forcing from an acoustic field can be harnessed to generate autonomous motion.