content_image/328070.jpg
<img src='content_image/328070.jpg'>

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 

