sures indicate that the flow is chaotic [8]. More detailed evidence is required in order to determine if the system investigated here displays true elastic turbulence [21], for example measurement of flow velocity fluctuations and their spectral scaling properties. The PMT signal is dependent on dye concentration, possible thinning effects [24], and flow velocities. To reduce such effects the PMT signal is normalized by the average voltage. To study the development of the instability versus driving voltage, the PMT signal fluctuations, ˜ δ$_{PMT}$ , defined as var ( V 〈 V 〉 $^{)}$, where var(V) is the variance of the PMT voltage signal from the average 〈 V 〉 , are measured for various driving potentials. The precise location of the FIG. 4: Photomicrographs of (a) no polymer and (b, c) 120 ppm HMW polymer flows at 0.2 kV. Image (c) is taken one second after (b). Dashed lines indicate field of view and microchannel constriction outline. The interface between dyed and undyed liquid streams is (a) stable for polymer free samples and becomes (b, c) unstable with the inclusion of polymer. The scale bar is 100 µ m. 8