Acoustic radiation force on small spheres due to transient acoustic fields

TL;DR

This paper extends the theory of acoustic radiation force to finite-duration pulses, revealing how pulse width affects trapping regions for small spheres with different acoustic contrasts, with implications for acoustic tweezers.

Contribution

It introduces a generalized model for acoustic radiation force due to transient pulses and extends Gor'kov's formula to unsteady fields, enhancing understanding of particle trapping.

Findings

Shorter pulses narrow trapping regions for negative contrast particles.

Optimal pulse width maximizes trapping efficiency for positive contrast particles.

The concept of acoustic contrast remains relevant in unsteady acoustic fields.

Abstract

Acoustic radiation force is a net force experienced by an object under the action of an acoustic wave. Most theoretical models require the acoustic wave to be periodic, if not purely monofrequency, and are therefore irrelevant for the study of acoustic radiation force due to acoustic pulses. Here, we introduce the concept of finite-duration pulses, which is the most general condition to derive the acoustic radiation force. In the case of small spheres, we extend the Gor'kov to formula to unsteady acoustic fields such as traveling pulses and interfering wave packets. In the latter case, our study suggests that the concept of acoustic contrast is also relevant to express the acoustic radiation force. For negative acoustic contrast particles, the acoustic trapping region narrows with shorter pulses, whereas positive contrast particles (such as biological cells) can fall in secondary traps when the pulse width deviates from an optimal value. This theoretical insight may help to improve the selectivity of pulsed acoustic tweezers.