Abatement of mixing in shear-free elongationally unstable viscoelastic microflows

TL;DR

This study investigates how adding polymers to viscoelastic fluids in shear-free electro-osmotic microflows affects mixing, revealing that polymer-induced instabilities can actually reduce mixing efficiency despite increased flow disturbances.

Contribution

It demonstrates that in shear-free electro-osmotic flows, polymer addition excites viscoelastic instabilities but leads to decreased mixing, contrasting with prior shear-involved studies.

Findings

Polymer addition increases viscosity and induces viscoelastic instabilities.

Mixing is reduced in polymer-laden flows despite larger flow instabilities.

Electro-osmotic flows serve as a clean platform to study viscoelastic turbulence.

Abstract

The addition of minute amounts of chemically inert polyacrylamide polymer to liquids results in large instabilities under steady electro-osmotic pumping through 2:1 constrictions, demonstrating that laminar flow conditions can be broken in electro-osmotic flow of viscoelastic material. By excluding shear and imposing symmetry we create a platform where only elongational viscoelastic instabilities, and diffusion, affect mixing. In contrast to earlier studies with significant shear that found up to orders of magnitude increase in mixing we find that inclusion of polymers excites large viscoelastic instabilities yet mixing is reduced relative to polymer-free liquids. The absolute decrease in mixing we find is consistent with the understanding that adding polymer increases viscosity while viscoelastic flows progress towards elastic turbulence, a type of mild (Batchelor) turbulence, and indicates that electro-osmotic pumped devices are an ideal platform for studying viscoelastic instabilities without supplementary factors.