Three-dimensional phenomena in microbubble acoustic streaming

TL;DR

This paper reveals that microbubble acoustic streaming in microfluidic devices exhibits complex three-dimensional behaviors, challenging the common assumption of quasi-two-dimensional flow, with implications for particle manipulation applications.

Contribution

The study demonstrates that acoustic streaming around microbubbles is inherently three-dimensional, using advanced particle tracking to uncover out-of-plane dynamics overlooked in previous two-dimensional models.

Findings

Micro-particle trajectories show strong out-of-plane motion near microbubbles.

Streamlines are projections of a pseudo-toroidal streamsurface, not planar.

Assumption of 2D flow in microfluidics is generally invalid.

Abstract

Ultrasound-driven oscillating micro-bubbles have been used as active actuators in microfluidic devices to perform manifold tasks such as mixing, sorting and manipulation of microparticles. A common configuration consists on side-bubbles, created by trapping air pockets in blind channels perpendicular to the main channel direction. This configuration consists of acoustically excited bubbles with a semi-cylindrical shape that generate significant streaming flow. Due to the geometry of the channels, such flows have been generally considered as quasi two-dimensional. Similar assumptions are often made in many other microfluidic systems based on \emph{flat} micro-channels. However, in this paper we show that microparticle trajectories actually present a much richer behavior, with particularly strong out-of-plane dynamics in regions close to the microbubble interface. Using Astigmatism Particle Tracking Velocimetry, we reveal that the apparent planar streamlines are actually projections of a \emph{streamsurface} with a pseudo-toroidal shape. We therefore show that acoustic streaming cannot generally be assumed as a two-dimensional phenomenon in confined systems. The results have crucial consequences for most of the applications involving acoustic streaming as particle trapping, sorting and mixing.