Rheology, Optical Microscopy, and Electron Microscopy of Cationic Surfactant Gels

A cationic surfactant mixture is studied in aqueous solution by rheological and imaging techniques. We find that the system becomes gel-like above a threshold surfactant concentration, as evidenced by the appearance of an apparent yield stress and a viscosity which is highly non-Newtonian. Hysteresis is observed in the shear stress vs shear rate measurements: the formation of shear-induced microstructures at high shear rates may cause shear thickening. Electron micrographs indicate three distinct structures in the sample: lamellae, large onion-like globules, and small nodules. These structures are discussed in the context of the rheological data. The density of the globular structures in sheared samples is observed to be larger than in unsheared samples.