Effects of elasticity and flow ramp up on kinetics of shear banding flow formation in wormlike micellar fluids

We report experiments on spatiotemporal evolution of the velocity profiles in shear-banding wormlike micelles upon inception of the flow in a Taylor-Couette (TC) cell. Both moderately entangled and highly entangled solutions are considered over a broad range of fluid elasticityE. Fluid elasticity,E = Wi/Re, characterizes the relative importance of the elastic to inertial effects. For both moderately and highly entangled solutions, upon inception of the shear, and during the stress decay period, fluid moves in the opposite direction to that of the imposed motion in a subset of the gap beyond critical thresholds of elasticity and flow ramp up rate, which depend on the fluid entanglement density. Surprisingly, beyond a second critical threshold of the fluid elasticity, the transient backflow disappears in moderately entangled solutions, highlighting the importance of the micellar entanglement on transient evolution of the flow in shear banding systems. More interestingly, we report the formation of multibanded quasisteady velocity profiles under certain conditions of fluid elasticity and flow ramp up rate. The multibanded profiles are characterized by a low shear band near the inner cylinder, a high shear band in the middle of the TC gap, and another low shear band near the outer cylinder. Finally, we show that the apparent wall slip at the inner cylinder of the TC cell is more pronounced for highly entangled solutions and decreases as the fluid elasticity increases. Experimental observations are compared with the existing simulations of the Vasquez-Cook-McKinley model, and several suggestions are made for future simulations.