Understanding steady and dynamic shear banding in a model wormlike micellar solution

Shear banding under steady and dynamic deformation is examined using a combination of rheology and flow-small angle neutron scattering (SANS) in a model wormlike micelle solution comprised of a mixed cationic/anionic surfactant (cetyltrimethylammonium tosylate/sodium dodecyl benzene sulfonate) and sodium tosylate. Flow-vorticity plane rheo-SANS reveals long transients during shear band formation. Flow-gradient plane spatially resolved flow-SANS measurements probe the microstructure during steady and dynamic shear banding. Under large amplitude oscillatory shear (LAOS) deformation, shear banding is dependent on the Deborah and Weissenberg numbers, validating recent theoretical predictions that include shear-induced breakage. Micelle segmental alignment in the flow-gradient plane during LAOS is a nonmonotonic function of cycle time, t/T, and radial position, r/H. The maximum segmental alignment under LAOS often exceeds that of the corresponding shear rate under steady shear, termed “over-orientation,” which can help identify LAOS shear banding. The results of this study present new methods for identifying shear banding under steady and dynamic deformation, while providing an extensive data set for the development and further improvement of constitutive models.