FIG. 1: (a) Photograph of the microfluidic device, and (b) detail of the microchannels with mixing units. We electro-osmotically drive fluids from the top and bottom wells to the left side well. Fluid in the top well contains fluorescent dye, and mixing is optically observed by exciting the dye with an external 532 nm laser and collecting video data. Each 100:200 µ m constriction/expansion unit creates extensional flow which induces instabilities in viscoelastic fluids with high enough Deborah number. Due to the slip conditions of electro-osmotic flow and symmetry non-viscoelastic instability mixing mechanisms are minimized. We find flow rates are not affected by the presence of polymer in the dilute regime and drive all flows with a voltage of 200 V, resulting in equal flow rates and comparable flows. Debye shear regime with polymers [19]. Our pumping scheme therefore excludes the shear component that is ubiquitous to mechanically pumped flows. In addition to deformations caused by shearing there are possible Lagrangian chaos effects [20], where flow fields result in chaotic trajectories which can lead to rapid mixing. We stack identical constriction units equally spaced down a microchannel, imposing symmetry as to reduce possible Lagrangian chaos effects [21]. II. EXPERIMENTAL SETUP See Ref. [22] were we demonstrate viscoelastic instabilitites under steady electro-osmotic pumping and discuss our device in more detail. Briefly, the experimental details are as follows: a microfluidic toolkit ( µ TK) from MicraLyne Inc. [44] is used to impose EOF flow 4