Electro-elastic instabilities in cross-shaped microchannels

Abstract

Purely-elastic flow instabilities have been extensively investigated in pressure-driven systems, but little attention has been paid to electrically-driven systems. This work attempts to fill this gap by investigating the electroosmotic flow of viscoelastic fluids in cross-slot and flow-focusing micro-devices, with a special focus on the onset of elastic instabilities. Relying on the visualization of dye patterns, the onset of electro-elastic instabilities for dilute and semi-dilute polyacrylamide solutions was observed at a low bulk Weissenberg number, O(10−2), through a direct transition from a steady symmetric flow to a time-dependent asymmetric flow. Time-series analysis of the dye interface patterns revealed a broad range of excitation frequencies with a power-law decay. The elastic instabilities observed seem not to improve significantly the mixing efficiency, contrarily to what has been previously reported for unstable pressure-driven flows. Numerical simulations of the electroosmotic flows were also carried out with a finite-volume method, showing that the observed electro-elastic instabilities can be predicted numerically. Both the experimental and numerical results suggest that the large stresses developed inside the electric double layer, coupled with the streamline curvature around the geometry corners, play a fundamental role in the onset and in the dynamics of the observed electro-elastic instabilities. However, further research is needed to clarify the mechanism of electro-elastic instabilities, and, in general, to improve our understanding of the complex behavior of polymeric fluids in electroosmotic flows.