Acoustic radiation force acting on a heavy particle in a standing wave can be dominated by the acoustic microstreaming

Abstract

When an acoustic wave scatters on a particle the acoustic radiation force and the microstreaming appear as non-linear time-averaged effects. Although they appear simultaneously, the microstreaming, which is driven by viscous losses, is often neglected in the theoretical modeling of the acoustic radiation force. Here, we investigate the contribution of the acoustic microstreaming to the acoustic radiation force acting on a small elastic spherical particle placed into an ultrasonic standing wave [T. Baasch, A. Pavlic, and J. Dual, Phys. Rev. E 100(6), 061102 (2019)]. The compressible Navier-Stokes equations are solved up to second-order in terms of the small Mach number using a finite element method. Our study shows that above a certain viscosity, when the viscous boundary layer thickness to particle radius ratio is sufficiently large, the contribution of the microstreaming dominates the acoustic radiation force and defines the stable position of the particle, provided that the particle is sufficiently dense. In such cases (e.g., combination of a copper particle of 1 μm radius in a mineral oil), our theory predicts migration of the particle to the pressure antinode.