The acoustic force density acting on inhomogeneous fluids in acoustic fields

TL;DR

This paper develops a theory describing how acoustic forces influence inhomogeneous fluids in microfluidic systems, predicting stable configurations driven by density and compressibility gradients, validated through experiments.

Contribution

It introduces a new theoretical framework for acoustic force density on inhomogeneous fluids, considering slow time scales and inhomogeneity gradients, with experimental validation.

Findings

Inhomogeneities relocate into stable configurations under acoustic forces.

Theoretical predictions match confocal imaging experiments.

Different from gravity-driven layering, stable configurations depend on acoustic field gradients.

Abstract

We present a theory for the acoustic force density acting on inhomogeneous fluids in acoustic fields on time scales that are slow compared to the acoustic oscillation period. The acoustic force density depends on gradients in the density and compressibility of the fluid. For microfluidic systems, the theory predicts a relocation of the inhomogeneities into stable field-dependent configurations, which are qualitatively different from the horizontally layered configurations due to gravity. Experimental validation is obtained by confocal imaging of aqueous solutions in a glass-silicon microchip.