(298h) Spatio-Temporal Evolution of Structure in a Wormlike Micellar System

Authors: 
Mohammadigoushki, H., UC Berkeley
Muller, S. J., University of California at Berkeley

Wormlike micellar fluids are important in a variety of industrial applications such as food processing, oil and gas. In this work, we use flow visualization to study the dynamics and evolution of structure in a model wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette (TC) cell. A large, custom-built, computer controlled TC cell allows us to rotate both cylinders independently and to visualize the flow field in the θ-z as well as r-z plane using a CCD camera. The wormlike micellar solution is made of CTAB/ NaSal. This system is highly elastic, and can be shear banding or non-shear banding in a narrow range of temperature. We probe the effect of elasticity on instability by varying the elasticity number over a wide range (El <1, El ≈1, and El >1). Elasticity is defined as the ratio between Weissenberg number and Reynolds number at the onset of instability. For non-shear banding fluids, El ≈ 1, and slow ramp speeds, distinct bands in the r-z plane are observed that are not stationary in the z-direction in contrast to previous reports (on a different wormlike micellar fluid) where stationary vortices were observed. In fact, bands originate somewhere close to the middle of the cylinder and merge if they are close enough, otherwise, bands move towards either the top or the bottom of the cylinder depending on their initial locations. To the best of our knowledge, this transition is reported for the first time in wormlike micellar fluids in the limit of El ≈ 1. For the shear banding case and El > 1, we observe the formation of bands that are not evenly spaced in the z-direction. This is also a new regime for shear banding micellar fluids. At low elasticities (El <1), we report a transition from purely azimuthal to modified Taylor vortex flow.  The behavior in the shear-banding and non-shear-banding regimes are discussed and compared with results in related systems.