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Transient evolution of flow profiles in a shear banding wormlike micellar solution: experimental results and a comparison with the VCM model.

In this paper we investigate the flow of a shear banding wormlike micellar fluid based on cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). The flow is studied in a custom-built Taylor-Couette (TC) cell via a combination of particle tracking velocimetry and in situ rheology. The spatiotemporal evolution of the velocity profile across the rheometer gap is tracked after an imposed step in the shear rate. In a range of shear rates the mixture shows shear banding behavior, that is distinct and differing shear rate profiles across the gap. As the shear bands form temporally, an elastic recoil including negative velocity (that is in the opposite direction to that of the imposed motion) is observed in a subset of the gap. While elastic recoil has been reported in experiments on monodisperse polymers [S. Ravindranath, et al., Macromolecules, 2008, 41, 2663-2670], on a wormlike micellar solution in a cone-plate rheometer [P. E. Boukany and S. Q. Wang, Macromolecules, 2008, 41(4), 1455-1464], and in theoretical studies [L. Zhou, et al., J. Non-Newtonian Fluid Mech., 2014, 211, 70-83; J. M. Adams, et al., J. Rheol., 2011, 55, 1007-1032] of wormlike micellar flows, it has not been previously reported in experiments on shear banding wormlike micelles in Taylor-Couette flows. Additionally, the mixture shows significant wall slip at the outer (stationary) Couette cylinder at high shear rates. Experimental results are compared to simulations of models of wormlike micelles, particularly the VCM model [L. Zhou, et al., J. Non-Newtonian Fluid Mech., 2014, 211, 70-83]. There are differences between the experimental results for this fluid and those reported previously. The difference arises from the size of the elasticity number which for the fluid reported in the paper is four orders of magnitude larger than that of other preparations.