at high flow rates; the Deborah number in the cross-channel region is lower than in the designed constriction and these instabilities are eliminated at low flow rates. Photomicrographs (Fig. 4) captured by wide field excitation of the dye using an external laser show the stable interface found in polymer-free flows (Fig. 4.a), sampling this interface with a focused laser spot and a PMT results in a smooth time trace (see Fig. 2.b). Adding polymer leads to fluctuations, of which the largest scale features are apparent in Fig. 4.b and c (compare with Fig. 2.c). The largest spatial scale of viscoelastic instabilities is determined by the system size, as for inertial instabilities [1]. There is no clear lower bound on spatial scale, with polymer size, separation, or diffusion lengths being possible cutoff lengths [21]. No dominant oscillations were observed in PMT signals collected. The autocorrelation of PMT traces of polymer solution flows have quick decay times ( ≈ 0.1 s decorrelation time, taken as the e-folding time), and displayed substructure indicative of multiple frequency components while not exhibiting sharp, large, repeated ringing which denote dominant si- nusoidal frequencies. Autocorrelations of PMT signals for polymer-free fluid decorrelated in ≈ 2 sampling time steps and showed no substructure. Fig. 3 shows the autocorrelation function for Fig. 2 data. Scatter and spectral plots (not shown) also did not show evidence of distinct sinusoidal frequencies [23] in unstable polymer flow. Taken together these mea- FIG. 3: Autocorrelations of photomultiplier tube signals (Fig. 2 data). 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. The autocorrelations of polymer free fluid flow decorrelate within a few sampling time steps and no substructure is apparent, while unstable polymer flows show rapid drop off followed by substructure without prominent peaks, indicating the presence of a multitude of frequencies. 7