Acoustic radiation force and torque on an absorbing compressible particle in an inviscid fluid

TL;DR

This paper derives exact formulas for the acoustic radiation force and torque on small, absorbing, compressible particles in inviscid fluids, highlighting absorption's role in trapping and torque generation with specific beam types.

Contribution

It provides new analytical formulas for radiation force and torque on absorbing particles in inviscid fluids, considering the Rayleigh scattering limit and specific beam configurations.

Findings

Absorption significantly influences particle trapping in acoustical tweezers.

Only first-order Bessel vortex beams can produce radiation torque on small particles.

Numerical results demonstrate force and torque on benzene and olive oil droplets.

Abstract

Exact formulas of the acoustic radiation force and torque exerted by an arbitrary time-harmonic wave on an absorbing compressible particle that is suspended in an inviscid fluid are presented. It is considered that the particle diameter is much smaller than the incident wavelength, i.e. the so-called Rayleigh scattering limit. Moreover, the particle absorption assumed here is due to the attenuation of compressional waves only. Shear waves inside and outside the particle are neglected, since the inner and outer viscous boundary layer of the particle are supposed to be much smaller than the particle radius. The obtained radiation force formulas are used to establish the trapping conditions of a particle by a single-beam acoustical tweezer based on a spherically focused ultrasound beam. In this case, it is shown that the particle absorption has a pivotal role in single-beam trapping at the transducer focal region. Furthermore, it is found that only the first-order Bessel vortex beam can generate the radiation torque on a small particle. In addition, numerical evaluation of the radiation force and torque exerted on a benzene and an olive oil droplet suspended in water are presented and discussed.