Transient evolution of shear banding in wormlike micelle solutions

A series of experiments were performed to further investigate the phenomenon of shear-banding in surfactant solutions. Many surfactant solutions, through their unique amphiphilic chemistry, form long wormlike micelle structures which behave like living polymers. These wormlike micelles have interesting viscoelastic properties and have been the subject of a number of recent studies. These water-based surfactant systems are widely used in many commercial and industrial applications; however, many aspects of their complex flow behavior are still not fully understood. In this study, a Couette cell was designed to allow for high-resolution optical access in a simple shear flow of a surfactant system comprised of cetylpyridinium chloride and sodium salicylate in aqueous sodium chloride. Beyond a critical stress, this system is found to enter a non-linear regime in which there is a plateau in the shear stress with increasing shear rate. Within this plateau, the fluid forms distinct bands of varying shear rate. The goal of this study was to obtain high spatial and temporal resolution particle-image velocimetry and flow-induced birefringence results in both steady and transient-startup flows. As a consequence of the high resolution, steady PIV results suggest the existence of multiple-shear bands. In the transient PIV experiments, we observe a propagating damped elastic wave, as well as fluctuations in the shear-band evolution on timescales of less than one relaxation time. Pointwise FIB gap profiles show a diffuse birefringent region prior to the onset of shear-banding in the velocity profiles. These results provide insight on the flow behavior, as well as a full set of experimental data which will drive development of constitutive models capable of predicting shear-banding