Shear-banding in wormlike micelles: Beware of elastic instabilities

We report on the shear banding flow of semidilute and concentrated wormlike micelle systems which have been probed in the recent past using rheology, velocimetry, and small angle neutron scattering techniques. We show that the samples under consideration exhibit unstable shear banding flow due to the development of flow instabilities reminiscent of elastic or inertio-elastic instabilities and turbulence. Evidence for such unstable shear banding flows can be obtained either from direct visualizations, flow visualizations or two-dimensional velocimetry. The presence or not of instabilities on top of shear banding flows cannot be deduced from global rheology and/or 1D velocimetry alone. These flow instabilities are associated with more or less complex flow kinematics and appear to be ubiquitous in the flow of wormlike micelles. Using a pedagogical example, we show that their presence can strongly impact microstructural characterizations of the banded state. (C) 2016 The Society of Rheology.