Flow of viscoelastic fluids around a sharp microfluidic bend: Role of wormlike micellar structure

We examine the flow and instabilities of three viscoelastic fluids-a semidilute aqueous solution of polyethylene oxide (PEO) and two wormlike micellar solutions of cetylpyridinium chloride and sodium salicylate-around a microfluidic 90. bend, in which shear deformation and streamline curvature dominate. Similar to results reported by Gulati et al. [S. Gulati et al., Phys. Rev. E 78, 036314 (2008); J. Rheol. 54, 375 (2010)] for PEO solutions, we report a criticalWeissenberg number (Wi) for the onset of lip vortex formation upstream of the corner. However, the decreased aspect ratio (channel depth to width) results in a slightly higher critical Wi and a vortex that grows more slowly. We consider wormlike micellar solutions of two salt to surfactant concentration ratios R = 0.55 and R = 0.79. At R = 0.55, the wormlike micelles are linear and exhibit strong viscoelastic behavior, but at R = 0.79, the wormlike micelles become branched and exhibit shear- banding behavior. Microfluidic experiments on the R = 0.55 solution reveal two flow transitions. The first transition, at Wi = 6, is characterized by the formation of a stationary lip vortex upstream of the bend; at the second transition, at Wi = 20, the vortex fluctuates in time and changes size. The R = 0.79 solution also exhibits two transitions. The first transition at Wi = 4 is characterized by the appearance of two intermittent vortices, one at the lip and one at the far outside corner. Increasing the flow rate to Wi > 160 results in a transition to a second unstable regime, where there is only a lip vortex that fluctuates in size. The difference in flow transitions in PEO and wormlike micellar solutions presumably arises from the additional contribution of wormlike micellar breakage and reformation under shear. The flow transitions in wormlike micellar solutions are also significantly affected by chain branching.