Enhanced Microfluidic Mixing via a Tricritical Spiral Vortex Instability

TL;DR

This study investigates how a spiral vortex instability in microfluidic cross-slot devices enhances mixing, revealing a tricritical point and different transition orders that inform device design for improved fluid mixing.

Contribution

It introduces the discovery of a tricritical spiral vortex instability in microfluidic flows, combining experimental and numerical analysis to understand flow bifurcations and mixing enhancement.

Findings

Flow undergoes symmetry-breaking bifurcation above a critical Re

Spiral vortex acts as an effective mixing region

A tricritical point occurs at alpha ~ 0.55

Abstract

Experimental measurements and numerical simulations are made on fluid flow through cross-slot devices with a range of aspect (depth:width) ratios, 0.4 < alpha < 3.87. For low Reynolds numbers Re, the flow is symmetric and a sharp boundary exists between fluid streams entering the cross-slot from opposite directions. Above an alpha-dependent critical value Re_c, the flow undergoes a symmetry-breaking bifurcation (though remains steady and laminar) and a spiral vortex structure develops about the central axis of the outflow channel. An order parameter characterizing the instability grows according to a sixth-order Landau potential, and shows a progression from second order to first order transitions as alpha increases. A tricritical point occurs for alpha ~ 0.55. The spiral vortex acts as a mixing region in the flow field and this phenomenon can be used to drive enhanced mixing in microfluidic devices.