FIG. 5: Mixing evolution. From the top to the bottom 64 ppm sample passes from the 2 nd (a) to the 4 th (b) to the 8 th (c) constriction. The effects of diffusion are apparent, with a reduction of contrast between the 2 nd (a) and the 4 th constriction (b), and finally to a visually homogeneous mixture by the 8 th constriction (c). In addition it can be seen that strong cross channel fluctuations exist, but the pattern is highly similar and a comb-like pattern persists between the 2 nd (a) and the 4 th (b) constriction, with addition of “teeth” but without significant pattern changing deformation. This persistence is characteristic of chaotic flows undergoing symmetric oscillatory forcing. The diagrams are 60 µ m by 60 s and have not been scaled, allowing comparison. present in most prior work due to no-slip conditions. In a study of electrokinetic driven 2D sheet flow [35], which also excludes no-slip induced shear, viscoelasticity was found to mildly suppress mixing for flows below the instability threshold, a finding that is consistent with other studies that find either mild increase [36] or decrease [37] in this regime where polymers are only weakly distorted. Additionally, in Ref. [35] they show that above the instability threshold stretching fields are reduced in viscoelastic fluids. By comparing a Newtonian and a highly shear-thinning/viscoelastic fluid they find enhanced stretching and mixing for the shear-thinning/viscoelastic fluid, indicating that mixing trends are correlated to stretching field magnitude. Our finding of reduced mixing, combined with the reduced stretching fields observed in Ref. [35], shows the correlation between stretching fields and mixing holds for viscoelastic fluids above the instability threshold. Our results which demonstrate reduced mixing in unstable viscoelastic flows are consistent with the understanding that viscoelastic fluids driven well above the viscoelastic instability threshold progress towards 10