Rheology and microstructural transitions in the lamellar phase of a cationic surfactant

The rheological properties of mixtures of cationic surfactant (dialkyldimet hylammonium chloride) and water are studied for a range of surfactant conce ntrations. In particular, we investigate rheological signatures of the conf ormational transformation of the system from lamellar sheets to lamellar ve sicles under the action of shear. For a range of rheological test regimes, including transient, steady, and oscillatory shear, this transition is sign aled by a critical stress or yield stress involving step changes in sample strain. viscosity, or dynamic moduli. However, only a portion of the sample undergoes the transition, resulting in lamellar sheets coexisting with ves icles. We further show that the transition is reversible in that ramping th e stress down from above the critical stress value results in the disappear ance of all of the vesicles. The final viscosity of the mixture can he high er or lower than that for the initial untreated material-possibly reflectin g an imbalance in the net generation or annealing of defects in the lamella r structure by the shear treatment. Oscillatory rheometry indicates that a minimum strain is also required to obtain a phase transition. For transient shear, this corresponds to a minimum residence time in the shear field. In general, the critical stresses increase with the surfactant concentration, though they are relatively constant at 15-20 Pa for concentrations between 20 and 40 wt %.