range 3.3 to 3.6 [39]) which leads to weak coupling between scales, in sharp contrast to high Reynolds number turbulence, and the resulting flows are random in time yet spatially smooth and strongly correlated on the integral scale [39]. This results in instabilities dominated by large scale fluctuations, which are set by the system size [39], and therefore there will remain a mismatch between the smallest scale one wants homogeneity on and the instability which is intended to drive mixing. Note that the smooth flow characteristics of Batchelor turbulence are consistent with our observations (see, for example, Fig. 3) and, without some other mechanism (such as shear) to further modify flow, are at odds with enhanced mixing. To create efficient mixing a bridge is needed between the relatively large scale set by viscoelastic instabilities and the smallest scales over which homogeneity is required; diffusion, Lagrangian chaos, or deformations such as shear can bridge these scales. In the “knead and fold” metaphor of mixing [40] viscoelastic instabilities can provide the fold operation that creates large deformations of material to create striations of dis-similiar material, however additional deformations are needed to thin striations allowing diffusion to finally homogenize the fluids. In summary, the addition of high molecular weight polyacrylamide to liquids leads to striking viscoelastic instabilities in extensional electro-osmotic flow thereby upsetting the laminar flow typical of microchannels. The breaking of laminar flow for dilute low viscosity viscoelastic fluids has implications for device design as laminar flow is often assumed in microflows; as lab on a chip applications move towards more complex fluids this assumption must be checked. Our results show that for flows of lightly polymer-doped low viscosity solutions through unoptimized geometries mixing is reduced relative to polymer-free solu- tion, despite the excitation of dramatic large scale viscoelastic instabilities. This reduction is in contrast to earlier studies that found up to orders of magnitude increase in mixing, demonstrating that the role of shear or other deformations of the flow is crucial in promoting mixing. Electro-osmotic flows can be used to isolate viscoelastic instabilities, and combined pressure/EOF driven flows [41] allow controlled application of shear enabling detailed in- vestigation of flow evolution and mixing. In addition to fundamental studies possibility for application exists for the observed highly unstable, yet diffusion limited, flows in applica- IV. CONCLUSION 12