III. INSTABILITIES IN EXTENSIONAL MICROFLOWS OF POLYMERIC SO- LUTIONS Creating a visually observable fluid interface by flowing dyed fluid from reservoir 1 and undyed fluid from reservoir 2 to reservoir 4, Fig. 1.a, allows us to investigate fluid flow properties. The channel geometry is designed with 2:1 constrictions in order to induce extensional flow and the fluid interface is monitored in the center of this region (at the white spot in Fig. 1.b). The Reynolds numbers here are < 0 . 01, below the nominal creeping flow limit of Re < 1. Without polymer creeping laminar flow persists to the highest applied voltages (Fig. 2.b), with the PMT traces appearing similar to the no-flow case (see Fig. 2.a). FIG. 2: Normalized photomultiplier tube time traces. A PMT monitors the interface of the dyed/undyed streams of methanol-water solutions with (a) no driving voltage (no flow), (b) driven at 0.4 kV, and (c) driven at 0.2 kV with 120 ppm of HMW polymer added. With no polymer the PMT trace of the flow is smooth (b), albeit with noise from inherent laser diode instability as also seen for no flow (a), indicative of a stable interface and the laminar flow typical in microchannels. With polymer added rapid interface motion and instabilities are observed at moderate driving voltages, as evident by the dramatic fluctuations in the PMT signal (c). In contrast, very large fluctuations arise, at sufficiently large driving electric fields, when polymer is added to the solution (Fig. 2.c). This arises from an instability of the flow causing the local dye concentration to vary under the green excitation spot. The presence of polymer results in large movement of the dye/undyed interface relative to the illumination spot, generating order-unity fluctuations in Fig. 2.c. We note that extensional flow also exists in the cross-channel region, due to acceleration, which excites instabilities in polymeric fluids 6