Shear-banding in surfactant wormlike micelles: elastic instabilities and wall slip

We report on the flow dynamics of a wormlike micellar system (CPCl/NaSal/brine) undergoing a shear-banding transition using a combination of global rheology, 1D ultrasonic velocimetry and 2D optical visualisation. The different measurements being performed in a single Taylor-Couette geometry, we find a strong correlation between the induced turbid band observed optically and the high shear rate band. This correspondence reveals that fluctuations observed in the 1D velocity profiles are related to elastic instabilities triggered in the high shear rate band: 3D coherent (laminar) flow and 3D turbulent flow successively develop as the applied shear rate is increased. The specific characteristics of the resulting complex dynamics are found to depend on subtle changes in the sample, due to temporary light exposure. The CPCl molecules exhibit a photochemistry mainly influenced by the photo-induced cleavage of the pyridine ring that yields an unstable aldehyde enamine, which further decays by thermally activated processes. The products of the reaction possibly build up a lubrication layer responsible for pathological flow dynamics. Overall, our results bridge the gap between previous independent optical and local velocity measurements and explain most of the observed fluctuations in terms of a sequence of elastic instabilities which turns out to be widespread among semidilute wormlike micellar systems.