FIG. 2: Laminar flow in the first constriction. With no polymer added creeping laminar flow with steady diffusion is observed (a). A space time diagram (a) is constructed by taking a cross channel intensity profile across the middle of the constriction and is 60 µ m by 75 s. Note that 20 µ m is trimmed from the edges (where wet etch rounding may lead to aberrations), and that transitory start-up flow is removed, from all xt-diagrams here. The cross channel intensity profile 0.6 s from the start of the xt diagram is shown in (b). An erf-like profile is observed in (b), as expected in coflowing two-stream mixing. are added, as seen in Fig. 5, however the general pattern persists with diffusion appearing to be the dominant mixing mechanism. Absolute mixing is reduced by reduced diffusion, and increased by instability induced deformation of the liquid. There is therefore a tradeoff involved in adding polymer to the solution which will allow viscoelastic instabilities yet reduces diffusion. By measuring mixing (M1) in the second expansion we find that mixing is reduced as concentration is increased (see Fig. 6), demonstrating that the effect of instabilities in promoting mixing is not dominant for mixing our dye. As our dye is a small molecule ( ≈ 1 . 6 nm hydrodynamic diameter [24]) our mixing experiments is a stringent - but realistic - test. Further, as we trim the edges of the xt-diagrams to reduce image processing artifacts we possibly artificially increase the first moment mixing index M1 calculated for unstable flows, as material with high/low dye concentration can be injected from these regions, increasing the variance seen. Note that in contrast to high Reynolds number turbulent flows an enhanced diffusion rate, indicative of broad-band excitation of instability scales, is not observed [29] supporting 7