excitation spot from the interface will affect our measure ˜ δ$_{PMT}$ . Our absolute positional accuracy is roughly 5 µ m and, by measuring away from the interface on either side for a given experimental alignment, we estimate the introduced error to be roughly 10% of the measured fluctuation for unstable flows at higher flow rates (0.2 kV and above driving voltages) which is also the estimated error found by comparing differing experimental runs (which requires realignment). At lower flow velocities the system noise begins to dominate, and for no-flow conditions run-to-run differences in measured fluctuations is ca. 30%. 

Polymer-free solutions display laminar flow for all driving voltages (Fig. 5.a). In contrast to this laminar flow, fluctuations grow in size as the driving voltage increases for a concen- tration of 120 ppm of the HMW sample (Fig. 5.a). As seen in Fig. 5.b similar behavior occurrs at other concentrations of the HMW sample, both below (60, 90 ppm), and above (480 ppm) the overlap concentration. 

Scaling properties of viscoelastic instabilities are not well understood, but the basic ele- ments of curvature and elastic normal stress (stretching) along streamlines are well estab- lished as underlying instability [25] as these mechanisms lead to “hoop-stress” [1] allowing stretched polymers to cross streamlines, disturbing laminar flow. Fluctuations should grow with concentration (as the movement of individual polymer coils underlies the fluctuations). Changing the concentration and investigating fluctuations at a constant (0.2 kV) voltage indicates that fluctuations rapidly grow from the noise floor and then peak and plateau with a gentle decrease as we increase concentration, with fairly uniform measured fluctuations from ≈ 1 $_{10}$X to ≈ 1X the overlap concentration, Fig. 6. 

Our fluctuation measure var ( V 〈 V 〉 $^{)}$takes its maximum possible value of 1 for square-wave signals; the precise maximal value for a given signal is ≤ 1 and depends on the nature of the waveform. Smooth growth in our measure occurs until the maximum, for a given waveform with linear amplitude growth. When the maximal value is reached a sudden apparent threshold will occur and our measure will saturate. The maximal threshold observed here is followed by a mild decrease rather than a strict capping and saturation; this is suggestive of a qualitative change in behavior rather than an artifact of our measure. 

Modeling the polymer solution as a colloidal solution [26] one expects hydrodynamic in- teraction mediated molecular crowding effects above a volume fraction φ of approximately 0.1 [27] where such effects are empirically found [51]. To estimate the concentration when hydrodynamic interactions between polymers play a significant role we take c$_{thresh}$ c$_{∗}$ = φ$_{thresh}$ φ$_{∗}$ , 

