Extensional Instability in Electro-Osmotic Microflows of Polymer Solutions

TL;DR

This study demonstrates that electro-osmotic microflows of dilute polymer solutions can exhibit extensional instabilities due to polymer elasticity, affecting flow stability and mixing in microfluidic systems.

Contribution

It reveals the conditions under which electro-osmotic flows of polymer solutions become unstable, highlighting the role of polymer concentration and molecular crowding.

Findings

Flow velocities induce instabilities in high molecular weight polymers.

Fluctuations depend on polymer concentration, plateauing below overlap concentration.

Instabilities suggest enhanced mixing and modeling challenges in complex fluids.

Abstract

Fluid transport in microfluidic systems typically is laminar due to the low Reynolds number characteristic of the flow. The inclusion of suspended polymers imparts elasticity to fluids, allowing instabilities to be excited when substantial polymer stretching occurs. For high molecular weight polymer chains we find that flow velocities achievable by standard electro-osmotic pumping are sufficient to excite extensional instabilities in dilute polymer solutions. We observe a dependence in measured fluctuations on polymer concentration which plateaus at a threshold corresponding to the onset of significant molecular crowding in macromolecular solutions; plateauing occurs well below the overlap concentration. Our results show that electro-osmotic flows of complex fluids are disturbed from the steady regime, suggesting potential for enhanced mixing and requiring care in modeling the flow of complex liquids such as biopolymer suspensions.