Shear banding instability in wormlike micellar solutions

Using NMR velocimetry and mechanical measurements we study the flow dynamic s, within a cone-and-plate rheometer, of the wormlike micelle system, cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) at 100 mM/BO mM concent ration in distilled water. Depending on precise conditions within the syste m, two classes of behaviour are observed, one in which the boundary between different shear rate phases fluctuates rapidly (on the order of tens of mi lliseconds) and one in which it migrates slowly with a time constant of man y seconds. These modes of behaviour may depend on minor solution impurities , which presumably affect the detailed constitutive properties, but also on the externally applied shear rate within a given system. We argue that the slow migrations are governed by stress relaxation effects while the rapid migrations are flow driven and arise from interfacial instability.