  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 wave- length 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.
