Shear-banded flow and transient rheology of cationic wormlike micellar solutions

Linear oscillatory as well as transient and steady shear measurements for micellar solutions of dodecyltrimethylammonium, bromide (DTAB) and sodium salicylate (NaSal) as a function of the salt-to-surfactant concentration ratio (C-SALT/CDTAB) are presented. Our results indicate that, for molar ratios of salt to surfactant (C-SALT/CDTAB) smaller than 1, the micellar solutions follow closely Maxwell behavior with a single relaxation time, that is, they are in the fast-breaking regime, and exhibit shear banding above a critical shear rate. When this ratio is greater than 1, micellar solutions behave as semidilute polymer solutions with a spectrum of relaxation times and no stress plateau is observed in steady shear. The results are analyzed with the Granek-Cates model (for linear response) and with a simple model that consists of the upper convected Maxwell constitutive equation coupled to a kinetic equation to account for the breaking and reformation of the micelles (nonlinear behavior). The stress plateau and the critical shear rates are determined from an extended irreversible thermodynamic analysis.