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

TL;DR

This paper investigates how high acoustic energy densities in microfluidic channels cause a transition from boundary-driven to bulk-driven acoustic streaming due to nonlinear thermoviscous effects, combining theory, simulation, and experiments.

Contribution

It reveals a nonlinear transition mechanism in acoustic streaming caused by thermoviscous effects at high energy densities, supported by theoretical, numerical, and experimental evidence.

Findings

Boundary-driven streaming transitions to bulk-driven streaming at high E_ac

Frictional heating induces temperature gradients that generate bulk acoustic forces

Experimental and numerical results show good agreement with theoretical predictions

Abstract

Acoustic streaming is studied in a rectangular microfluidic channel. It is demonstrated theoretically, numerically, and experimentally with good agreement, frictional heating can alter the streaming pattern qualitatively at high acoustic energy densities E_ac above 500 J/m^3. The study shows, how as a function of increasing E_ac at fixed frequency, the traditional boundary-driven four streaming rolls created at a half-wave standing-wave resonance, transition into two large streaming rolls. This nonlinear transition occurs because friction heats up the fluid resulting in a temperature gradient, which spawns an acoustic body force in the bulk that drives thermoacoustic streaming.