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

Acoustic Radiation Force on Small Spheres Due to Transient Acoustic Fields

“…It is customary to distinguish three contributions, namely the acoustic momentum that would have been transmitted to the fluid even in the absence of any particle (such as Eckart [34] and Rayleigh streaming [35]), the acoustic momentum exchanged with the particle by scattering, 044034-3 and the acoustic momentum exchanged with the particle by local steady viscous stress (acoustic microstreaming [36]). Except for very special cases such as heavy particles in viscous fluids [37], the viscoacoustic boundary layer is generally thinner than the particle size such that the microstreaming contribution to the acoustic radiation force is negligible [37]. Then, the acoustic radiation force is well approximated by considering only the inviscid contribution, as long as the thermoviscous effects on the scattering coefficients are well resolved in the vicinity of the particle [12,13].…”

Acoustic radiation force and torque on spheroidal particles in an ideal cylindrical chamber

“…We also restrict our analysis to particles much smaller to the chamber, a, b H, R. Another effect that may appear in an acoustofluidic chamber is the acoustic streaming, which appear near boundaries. Acoustic streaming close to the chamber walls produces causes a drag force on the particle, while near the particle surface, it can alter the radiation force [50][51][52] and produce a viscous torque. 53 As a diffusive process, streaming has a characteristic length known as the viscous boundary layer, δ = (2µ 0 /ρ 0 ω) 1/2 , with µ 0 being the dynamic viscosity of the fluid.…”

A transition from boundary- to bulk-driven acoustic streaming due to nonlinear thermoviscous effects at high acoustic energy densities

“…In such devices, two main forces act on the suspended particles, the acoustic radiation force and the drag force due to acoustic streaming, which is a time-averaged flow caused by the inherent nonlinearities of fluid dynamics. Recent work has clarified many subtle details pertaining to the radiation force on mircoparticles, including thermoviscous effects [1] and microstreaming [2]. Concurrently, similar progress has been made in the theory of acoustic streaming, especially regarding thermoviscous effects.…”