Inertio-elastic instability in Taylor-Couette flow of a model wormlike micellar system

In this work, we use flow visualization and rheometry techniques to study the dynamics and evolution of secondary flows in a model wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette (TC) cell. The wormlike micellar solution studied in this work contains cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). This system can be shear banding and highly elastic, non-shear banding and moderately elastic, or nearly Newtonian as the temperature is varied over a narrow range. The effect of elasticity on transitions and instabilities is probed by changing the temperature over a wide range of elasticity (El ≪ 1, El ≈ 1, and El ≫ 1). Elasticity is defined as the ratio of the Weissenberg number to the Reynolds number. For shear banding wormlike micelle solutions where El ≫ 1, a primary transition from the base Couette flow to stationary vortices that are evenly spaced in the axial direction of the shear cell and are characterized by an asymptotic wave-length is observed. The dimensionless wavelength at the onset of this shear banding transition for CTAB/NaSal system turns out to be much larger than those reported for other shear banding wormlike micelle systems. For the same fluid at a temperature where it shear-thins but does not display shear banding, El ≈ 1, and for slow ramp speeds, the primary transition is to distinct structures that are not stationary but rather travel in the axial direction. At low elasticity (El ≪ 1), where the fluid behaves as a nearly Newtonian fluid, several transitions from purely azimuthal Couette flow to modified Taylor vortex flows and finally chaotic regimes are documented. The behavior in the shear-banding and non-shear-banding regimes are discussed and compared with results in related systems. The possibility of hysteresis in the flow transitions as well as the effects of co-rotation and counter-rotation of the cylinders on transitions and instabilities are also examined for a wide range of elasticity.