Acoustic manipulation of multi-body structures and dynamics

TL;DR

This paper explores how sound waves can manipulate multiple objects simultaneously, revealing complex interactions and dynamics that extend beyond single-object effects, with implications for contactless control in various applications.

Contribution

It introduces the mechanisms of sound-mediated interactions among multiple particles, emphasizing the roles of scattering, Bjerknes forces, and shape effects, and discusses recent advances in understanding complex behaviors.

Findings

Sound-mediated interactions involve secondary scattering, Bjerknes forces, and micro-streaming.

Particle shape significantly influences acoustic interactions.

Strong sound energy induces instabilities and active fluctuations.

Abstract

Sound can exert forces on objects of any material and shape. This has made the contactless manipulation of objects by intense ultrasound a fascinating area of research with wide-ranging applications. While much is understood for acoustic forcing of individual objects, sound-mediated interactions among multiple objects at close range gives rise to a rich set of structures and dynamics that are less explored and have been emerging as a frontier for research. We introduce the basic mechanisms giving rise to sound-mediated interactions among rigid as well as deformable particles, focusing on the regime where the particles' size and spacing are much smaller than the sound wavelength. The interplay of secondary acoustic scattering, Bjerknes forces, and micro-streaming is discussed and the role of particle shape is highlighted. Furthermore, we present recent advances in characterizing non-conservative and non-pairwise additive contributions to the particle interactions, along with instabilities and active fluctuations. These excitations emerge at sufficiently strong sound energy density and can act as an effective temperature in otherwise athermal systems.