working fluid or the microchannels. Viscoelastic fluids can exhibit flow instabilities [11] when polymer coils stretch and bend along stream lines which creates hoop-stress, allowing polymers to cross steam lines and thereby disrupting laminar flow [12]. These viscoelastic instabilities progress to elastic turbulence [13], a recently established turbulence that arises from elastic effects that can be excited at arbitrarily low Reynolds numbers. Use of viscoelastic fluids to promote mixing has been demonstrated in both mi- croscaled Dean flows [14], where chaotic flow was established and where earlier study of a macroscaled version of the serpentine flow geometry demonstrated true elastic turbulence [5]. The establishment of viscoelastic turbulence [13] and efficient mixing [5] along with favorable scaling properties of viscoelastic instabilities, which become easier to excite with reducing dimensions, suggests that the use of viscoelastic fluids may be a promising route to promote mixing in microfluidic devices. Furthermore, it is well know that solution viscosity increases with polymer concentration and at the overlap concentration, where neighboring coils impinge on each other, viscosity is only double that of the polymer-free solution [14]. In contrast to this relatively mild quantitative change in viscosity (and thus decrease in diffu- sion) elasticity effects have dramatic qualitative impact with polymer concentrations as low as ≈ 10 ppm leading to both large drag reduction in high Reynolds number hydrodynamic turbulent flows [15] and sizable instability and elastic turbulence in low Reynolds number flows [5]. Several studies (see, for example, [14, 16, 17]) have found that viscoelastic instabilities can be excited in microdevices, however additional deformations of the flows - most notably shear - exists in prior work and highly viscous solvents are used, which is atypical for many lab on a chip applications. Here we use electro-osmotic flow (EOF) in order to exclude shear and solely induce elongational viscoelastic instabilities by driving low viscosity fluids through sudden 2:1 constrictions in linear microchannels; we compare mixing in unsteady viscoelastic flows relative to mixing in polymer-free solutions in order to measure mixing effects. Mechanically driven cavity flows always have a significant shear component to the flow due to no-slip conditions. In contrast EOF has effective slip boundary conditions and shear is confined to the Debye layer close to the channel boundary [4]; flows are shear-free external to the Debye layer as the electric fields that the flow follows are purely elongational [18]. Further, for large polymers, entropic repulsion from the walls reduces interactions of the 3