Kinetics and mechanism of shear banding in an entangled micellar solution

We have studied shear banding in Couette flow using a combination of particle tracking velocimetry (PTV), small angle light scattering, microscopic visualization, and flow birefringence. Time-resolved local shear rate characterization by PTV has enabled a direct study of the kinetics of shear banding. A first stage, which precedes banding, is tilting of the shear rate during which the local shear rate increases towards the inner and decreases towards the outer gap surface. A shear banding stage then proceeds with a low shear band growing away from the outer gap surface. Shear rate tilting is found to be due to a coupling of local shear thinning with the nonzero stress gradient of the flow geometry. The low shear band starts when the local shear rate at the outer surface touches down to a critical reentanglement shear rate. The effective lifetime of the shear bands is the same as the chain reentanglement time. These factors lead us to suggest that both the progression from tilt to shear banding and the interface between the low and high shear bands are subject to a common local entanglement/disentanglement criterion. Constitutive curves constructed from local shear rates show that there is not a unique stress-the shear rate relation in the shear band coexistence regime, suggesting constitutive instability. The high shear band has fluctuating micellar layers aligned in the flow direction, and is strongly shear thinning. We observe insignificant viscosity differences between adjacent layers, and layer relaxation times are an order of magnitude greater than the effective lifetime of the shear bands. The layers, therefore, do not behave as subshear bands and are not causative for the shear banding. (C) 2005 The Society of Rheology.