## I. INTRODUCTION 

This paper explores how mixing is affected by adding high molecular weight polymers to electro-osmotically pumped microflows. Adding low concentrations of polymer to fluids moderately increases viscosity and dramatically increases elasticity, allowing microflows to undergo viscoelastic instability. Based on the relatively small increase in viscosity, the striking qualitative change in stability, and early experiments that demonstrate up to orders of magnitude improvement in mixing there is an expectation of dramatic increase in mixing by incorporation of polymers. By excluding shear, present in all mechanically pumped cavity flows, we find that contrary to such expectations absolute mixing is reduced by adding polymer to induce viscoelastic instability. Our findings demonstrate that viscoelastic instabilities do not, in themselves, promote mixing and suggest that electro-osmotic flows are an ideal platform for isolating and studying behavior of viscoelastic instabilities and elastic turbulence. 

Viscoelastic flows are of practical interest for lab on a chip applications as many biological materials of interest for biomedical testing, such as saliva [1] and blood [2], are viscoelastic in nature as are many other suspensions [3]. Several devices sort particles using laminar flow interactions [4], suggesting that viscoelastic induced instabilities can have potential unde- sirable effects as more concentrated and complex fluids are investigated; it has already been noted that viscoelasticity in saliva testing negatively effects performance [1]. In addition to the detrimental effects of instabilities there is suggestion that enhanced mixing may occur by intentionally spiking samples with polymers [5], thereby causing instability and subsequently increased mixing. As chemical reaction rate and completion depends on good mixing and as many clinical, environmental, pharmaceutical, and other diagnostic applications require rapid and complete reactions, in particular in the presence of small concentrations of sample, efficient mixing is an important aspect to device performance. 

In microfluidic devices the scales involved (typically O(100) µ m) are such that diffusive processes are slow, yet inertial fluid instabilities are suppressed due to the small scales leading to small Reynolds numbers. This intermediate “slow scale” regime makes mixing a key issue in microfluidics [6] and various methods have been investigated in order to enhance mixing, including using bubbles [7], electrohydrodynamic instability [8], flow focusing [9], and bas- relief patterning [10]. These approaches generally rely on creating inhomogeneities to the 

