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In this work, we use flow visualization and rheometry techniques to study thedynamics and evolution of secondary flows in a model wormlike micellar solutionsheared between concentric cylinders, i.e., in a Taylor-Couette (TC) cell. Thewormlike micellar solution studied in this work contains cetyltrimethylammoniumbromide (CTAB) and sodium salicylate (NaSal). This system can be shear bandingand highly elastic, non-shear banding and moderately elastic, or nearlyNewtonian as the temperature is varied over a narrow range. The effect ofelasticity on transitions and instabilities is probed by changing thetemperature 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 Reynoldsnumber. For shear banding wormlike micelle solutions where El>> 1, a primarytransition from the base Couette flow to stationary vortices that are evenlyspaced in the axial direction of the shear cell and are characterized by anasymptotic wave-length is observed. The dimensionless wave- length at the onsetof this shear banding transition for CTAB/NaSal system turns out to be muchlarger than those reported for other shear banding wormlike micelle systems.For the same fluid at a temperature where it shear-thins but does not displayshear banding, El~1, and for slow ramp speeds, the primary transition is todistinct structures that are not stationary but rather travel in the axialdirection. At low elasticity (El<< 1), where the fluid behaves as a nearlyNewtonian fluid, several transitions from purely azimuthal Couette flow tomodified Taylor vortex flows and finally chaotic regimes are documented. Thebehavior in the shear-banding and non-shear-banding regimes are discussed andcompared with results in related systems.

Inertio-elastic instability in Taylor-Couette {\deg}ow of a model wormlike micellar system