methanol:water mixture, where methanol is used to prevent aging (chain scission) during storage [13]. Neighboring polymer coils impinge on each other at the overlap concentration c$_{∗}$ of ≈ 300 ppm and ≈ 850 ppm for the HMW and LMW samples, respectively. We find concentrations both below and mildly above the overlap concentration of the HMW sample display similar behavior (as seen in Fig. 5.b; see also Fig. 6). 

The glass microchips are loaded into a Microfluidic Toolkit [50] and flow is electro- osmotically driven with voltages of up to 0.4 kV (electric fields as high as ≈ 900 V/cm). The interface between the two input streams is observed by fluorescently tagging one input solution with a small amount ( ≈ 21 µ M) of tetramethylrhodamine (TAMRA) dye and ex- citing with a 532 nm (green) diode laser. Laser-induced fluorescence profiles are collected for various flow rates by focusing the laser to 1/10th of the channel width (10 µ m beam waist) close to the dyed-undyed fluid interface centered in an extensional region (focal spot indicated with a small circle in Fig. 1.b), and collecting light emitted from the dye at 570 nm (yellow) with a photomultiplier tube (PMT) after filtering with a 568 nm bandpass filter (10 nm FWHM). 

Data runs were 100 s in duration, with the first 20 s removed prior to analysis to allow startup and transient behavior to attenuate. Care was taken to thoroughly clean and rinse microchannels before use to ensure clean surfaces to prevent pH shifts or other effects from modifying flows. The cleaning protocol was to flush the channels with: water ( × 2), ≈ 1 M nitric acid, ≈ 1 M NaOH, water ( × 2); first by filling all the reservoirs and then pulling solutions through the channels by applying vacuum to the output reservoir (reservoir 4 in Fig. 1.a). 

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. By timing the movement of a marker of dye over a known distance the electro-osmotic mobility was measured to be (5 . 6 ± 0 . 5) × 10 − 4 cm $^{2}$/ Vs for polymer-free solutions, which agrees with the value of 6 . 00 × 10 − 4 cm $^{2}$/ Vs reported in 50 µ m diameter fused silica capillaries [16]. The HMW solution at 120 ppm has a µ$_{EOF}$ of (5 . 7 ± 0 . 7) × 10 − 4 cm $^{2}$/ Vs, indicating that for dilute solutions of large polymers the electro- osmotic mobility is not significantly modified from the pure solvent case. For the LMW solution at 120 ppm the mobility is reduced to (3 . 1 ± 0 . 2) × 10 − 4 cm $^{2}$/ Vs, possibly due to polymer coils interacting with the Debye layer [17] which is on the order of 100 nm thick [18]. 

