Acoustic radiation- and streaming-induced microparticle velocities determined by micro-PIV in an ultrasound symmetry plane

TL;DR

This study combines micro-PIV measurements and theoretical modeling to analyze microparticle velocities in an ultrasound field, distinguishing effects of acoustic radiation and streaming across particle sizes, with predictions validated experimentally.

Contribution

It introduces a comprehensive model including wall interactions and thermoviscous effects, validated by experiments, to predict particle velocities in acoustophoresis.

Findings

Small particles are dominated by streaming flow drag.

Large particles are dominated by acoustic radiation force.

Velocity ratio scales with particle size squared, frequency, and contrast factor.

Abstract

We present micro-PIV measurements of suspended microparticles of diameters from 0.6 um to 10 um undergoing acoustophoresis in an ultrasound symmetry plane in a microchannel. The motion of the smallest particles are dominated by the Stokes drag from the induced acoustic streaming flow, while the motion of the largest particles are dominated by the acoustic radiation force. For all particle sizes we predict theoretically how much of the particle velocity is due to radiation and streaming, respectively. These predictions include corrections for particle-wall interactions and ultrasonic thermoviscous effects, and they match our measurements within the experimental uncertainty. Finally, we predict theoretically and confirm experimentally that the ratio between the acoustic radiation- and streaming-induced particle velocities is proportional to the square of the particle size, the actuation frequency and the acoustic contrast factor, while it is inversely proportional to the kinematic viscosity.