Observations of the influence of Taylor-Couette geometry on the onset of shear-banding in surfactant wormlike micelles

One-dimensional velocity profiles have been acquired using nuclear magnetic resonance (NMR) techniques to observe the onset of shear banding in the cetylpyridinium chloride (CPC1)/sodium salicylate (NaSal) in brine wormlike micelle system during steady shear rate controlled experiments. In particular, velocity profiles acquired with NMR were used to calculate the local shear rate next to the fixed wall of concentric cylinder (Taylor-Couette or TC) shear geometry. The change in this local shear rate, as a function of the applied shear rate, was then used to infer when the fluid began to shear-band. Using this technique, measurements were made on a series of TC geometries to explore the influence of the inherent shear rate inhomogeneity between the curved surfaces of the shear geometry. These experiments showed that the onset of shear-banding in the CPCl/NaSal/brine system was dependent on the TC geometry dimensions. Specifically, greater differences in curvature between the two shear surfaces caused shear-banding to begin at a lower applied shear rate. To confirm these findings, we developed a purpose-built Rheo-NMR apparatus, which allows for the acquisition of shear stress in the NMR magnet while recording local velocities simultaneously. Results from these simultaneous measurements showed that the observed applied shear rate at which shear-banding began was the same for both the bulk rheometry (i.e., flow curves) and NMR velocimetry data. (C) 2016 The Society of Rheology.