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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. 

<img src='content_image/364189.jpg'>

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- 

