  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.
