in glass microchannels etched 20 µ m deep. TAMRA dye, a fast diffusing dye with D ≈ 270 µ m $^{2}$/s [24], fluorescently labels one, otherwise identical, stream of fluid allowing visualization of flows. A dyed and an un-dyed fluid stream are brought together in a cross-slot junction and flow downstream through 200:100 µ m constrictions (see Fig. 1 for images of the device and microchannels). Fluid flow details are captured using a a Casio Exilim EX-F1 camera in video mode (30 fps), by attaching the camera to the µ TK via a custom machined attachment and a Scoptronix Maxview Plus optical adaptor (camera-microscope attachment). Our camera is not scientific grade and comparison of CCD intensity with true (incident) laser power reveals nonlinear characteristics; cubing the CCD values results in a linear response profile allowing the optical intensity as measured to be scaled and related to the dye concentration. From video data space-time (xt) diagrams are created by taking a vertical cut from each video frame across the channel, horizontally centered in the constriction or expansion. Due to chemical etching side walls exhibit curvature, varying the channel depth near the edges, and we remove 20 µ m nearest the channel edges before analysis. Runs are nominally 100 s in duration, but we exclude initial transient flow to prevent start up effects from influencing results. Polymer solutions were made by adding high molecular weight (18 × 10 6 Da) polyacry- lamide from Polysciences Inc. to a 20:80 vol.% methanol:water mixture; methanol is used to prevent aging during storage [25]. The overlap concentration c$_{∗}$ is ≈ 300 ppm, and we work in the dilute regime ( c < c$_{∗}$ ). Flow velocities in EOF are given by v = µ$_{EOF}$E , where µ$_{EOF}$ is the electro-osmotic mobility and E the applied electric field. We find that below the overlap concentration the electro-osmotic mobility is not significantly modified from the pure solvent case, with µ$_{EOF}$ ≈ (5 . 6 ± 0 . 5) × 10 − 4 cm $^{2}$/ Vs here, a result that is predicted by the entropic wall exclusion effect and studied in Ref. [19]. Our flow velocities are on the order of mm/s, and Re << 1, as is typical of standard EOF microflows. Addition of polymer allows elastic effects to induce instabilities for sufficiently high Deb- orah number De above the instability threshold ( > 1/2 for elongational flows [26]), the dimensionless number that characterizes viscoleastic flows. The Deborah number is given III. RESULTS AND DISCUSSION 5