with φ$_{thresh}$ empirically known [27] to be ≈ 0.1 and φ$_{∗}$ corresponding to the overlap con- centration which, depending on the particular packing taken near overlap, ranges from roughly 0.5-0.7; this leads to an estimate of c$_{thresh}$ ≈ 40-60 ppm for our HMW polymer; this estimated threshold is close in value to the observe peak in Fig. 6. The similarity in thresholds suggests that crowding may be the mechanism inhibiting growth in measures fluctuations; the tendency for polymer coils to deform makes the hard colloid model, which assumes polymer coils act as hard spheres, only approximate. It is known that molecular crowding strongly affects and reduces molecular transport [28] by hindering movement of macromoleclues; hindered movement of polymers across streamlines will limit fluctuations, as we observe here. FIG. 5: Fluctuation amplitude in normalized fluorescence signals. Fluctuations in the motion of the dyed/undyed fluid interface are measured with normalized PMT voltage for (a) 120 ppm of HMW polymer; the error is roughly on the order of 10% at higher flow rates for unstable flows. Control (polymer-free) solution fluctuations are nominally zero (compare with Fig. 2.b). The instability threshold is predicted to be ≈ 0.02 kV. Similar behavior is displayed (b) for 60, 90, and 480 ppm samples of HMW polymer. Compelling evidence that hydrodynamic interactions are important also comes from brow- nian dynamic simulations of λ -DNA, a biopolymer similar in size and nature to our HMW sample [8], in shear and elongational flows [29] which demonstrate that for c c$_{∗}$ above 0.1 in- cluding hydrodynamic interactions significantly affects calculated properties. The excellent agreement of the ≈ 0.1 c$_{∗}$ threshold found in the simulation of (bio)polymers where hydro- dynamic interactions become important and the peaking of fluctuations here at ≈ 0.1 c$_{∗}$ supports the hypothesis that hydrodynamic interactions are limiting growth in fluctuations. 10