Transitions induced by shear in surfactant phases and relaxation of the shear-induced phases

The surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT) in water forms lamellar phases at low salinities and sponge phases at higher salinities. For the lowest salinities, the lamellar phase contains many onion defects and is very viscous and elastic. At higher salinities but still in the lamellar phase, the sample has a low viscosity. The sponge phase is a Newtonian fluid with a very low viscosity. When either the low-viscosity lamellar or the sponge phase are sheared, they exhibit a transition, after a long time, to a phase which has the same rheological properties and the same texture as the very viscous lamellar `onion' phase. The random nature of the transition suggests that the formation of the shear-induced phase is due to activated processes, which we discuss here. In addition, the relaxation of the shear-induced phases is very slow; some of its relaxation mechanisms are described.