The Wormlike Micellar Solution made of a Zwitterionic Surfactant (TDPS), an Anionic Surfactant (SDS), and Brine in the Semidilute Regime

Structural and dynamical properties of a micellar solution are studied mainly through examining its rheological behavior in the semidilute regime. The micellar solution is made of a zwitterionic surfactant N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, sodium dodecyl sulfate, and salty water. In particular, we are interested in how the system is affected when the ionic strength of the media is modified by adding salt. Until recently, it was known that this solution forms wormlike micelles. In a range of chemical composition, the solution behaves as a viscoelastic Maxwellian fluid at low frequencies. We present measurements of the elastic (storage) modulus and the viscous (loss) modulus varying the surfactant ratio (R = [SDS]/[TDPS]), and how the Maxwellian relaxation time abruptly increases when the NaCl concentration is also varied. Reptation and breaking/recombination times were estimated. The effect of temperature in the viscoelastic solution is also studied. Shear stress versus shear rate flow curves were measured under shear and stress control, for different micellar solutions with different composition, brine concentration, and temperature, showing a nonlinear behavior. Flow curves present two branches, one corresponding to high viscous fluid and another to low viscous fluid, separated by a stress plateau. We were able to develop a master dynamic phase diagram, which summarizes the nonlinear behavior by appropriately reducing the rheological variables. In the stress plateau, the micellar solution presents gradient shear banding, which was observed with the scattered light of a sheet of light perpendicular to the fluid flow velocity in the gap of a transparent Couette rheometer.