Characterization of Acoustic Streaming in Gradients of Density and Compressibility

TL;DR

This study investigates how acoustic streaming in a resonator is affected by spatial inhomogeneities in density and compressibility, showing suppression of streaming rolls over time due to solute diffusion and initial distribution.

Contribution

It provides a detailed experimental characterization of acoustic streaming suppression in inhomogeneous fluids using particle tracking and concentration measurements.

Findings

Streaming rolls are suppressed in 70-200 seconds.

Suppression depends on diffusivity and initial solute distribution.

Inhomogeneities as low as 0.1% affect streaming behavior.

Abstract

Suppression of boundary-driven Rayleigh streaming has recently been demonstrated for fluids of spatial inhomogeneity in density and compressibility owing to the competition between the boundary-layer-induced streaming stress and the inhomogeneity-induced acoustic body force. Here we characterize acoustic streaming by general defocusing particle tracking inside a half-wavelength acoustic resonator filled with two miscible aqueous solutions of different density and speed of sound controlled by the mass fraction of solute molecules. We follow the temporal evolution of the system as the solute molecules become homogenized by diffusion and advection. Acoustic streaming rolls is suppressed in the bulk of the microchannel for 70-200 seconds dependent on the choice of inhomogeneous solutions. From confocal measurements of the concentration field of fluorescently labelled Ficoll solute molecules, we conclude that the temporal evolution of the acoustic streaming depends on the diffusivity and the initial distribution of these molecules. Suppression and deformation of the streaming rolls are observed for inhomogeneities in the solute mass fraction down to 0.1 %.