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.