Shear banding structure in viscoelastic micellar solutions

Theoretically, it has been shown that worm-like micellar solutions of surfactant can, for a shear rate (gamma) over dot greater than a critical value (gamma)(c) over dot, undergo a transition giving a plateau evolution (sigma = sigma(c)) Of the shear stress a against shear rate (gamma) over dot. We report here on a experimental study of the linear and nonlinear rheological behaviour of aqueous CTAB solutions with NaNO3 as added salt. With this system, it is possible to observe the evolution of the fundamental characteristics of the flow curve, i.e., the shear rate (gamma)(1c) over dot at which a shear banding structure appears and the second critical shear rate (gamma)(2c) over dot characterizing the end of the shear stress plateau followed by a new increased shear stress. For the first time, experimentally, we obtained evidence for the existence and the evolution of (gamma)(2c) over dot against CTAB and salt concentrations and temperature variations. Experimental results are compared to theoretical predictions correlating sigma(c), (gamma)(1c) over dot and G(0) (the shear modulus) for Maxwellian micellar solutions.