  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.

Soft Condensed Matter