Relating shear banding, structure, and phase behavior in wormlike micellar solutions

Abstract

The rheological behavior of concentrated cationic wormlike micellar surfactant solutions (cetyltrimethylammonium bromide) near the isotropic–nematic (I–N) transition is studied as a function of composition and temperature to determine the relationship between shear banding, fluid microstructure and underlying equilibrium phase behavior. The combination of conventional rheometry, velocimetry, and spatially-resolved flow-small angle neutron scattering allows detailed exploration of the differences between shear banding and non-shear banding solutions. The shear rheology of isotropic WLM solutions are shown to be well-described by the Giesekus constitutive model, which provides a quantitative discrimination between banding and non-banding wormlike micellar fluids through the drag anisotropy coupling parameter. This anisotropy parameter is shown to correlate with the order parameter describing the relative distance to the equilibrium I–N transition. Combining this information with measurements of the critical shear rates for shear banding allows the construction of a non-equilibrium state diagram for the shear banding fluid in terms of the Weissenberg number and the compositional order parameter.