The Role of Fluid Elasticity and Surfactant Chemistry on Kinetics of Shear Banding Flow Formation in Wormlike Micelles*

Shear banding, i.e., discontinuities in velocity gradients, has been best documented in self-assembled wormlike micellar solutions. We report experiments on the kinetics of shear banding flow formation on two wormlike micellar solutions based on CTAB/NaSal and CPCl/NaSal that exhibit similar rheological properties. We examine the effect of fluid elasticity and surfactant chemistry on the spatiotemporal evolution of flow profiles in these two systems in Taylor-Couette geometries (i.e., flow between two concentric cylinders). The fluid elasticity is changed over a range of 1.3×10⁵ to 1.3×10⁷ through variation of the gap size of the measuring flow geometry. Our results indicate that, unlike CTAB/NaSal system, the wormlike micellar fluid based on CPCl/NaSal does not show any signs of transient flow reversal despite having similar rheological properties suggesting a vital role of surfactant chemistry on shear banding dynamics. In addition, beyond a critical elasticity number, the transient flow reversal appears in CTAB/NaSal system, and the strength of the flow reversal depends on the fluid elasticity. Concurrently, we adopt advanced rheo-NMR techniques to examine diffusion coefficients in 50μm voxels across the gap of the Taylor-Couette cell. This microscale analysis allows us to compare the diffusive dynamics of shear bands in a spatially resolved manner, shedding light on their distinct properties and behavior, such as the micelles' physical structure, concentration, and orientation.