DIRECT EVIDENCE OF THE SHEAR-INDUCED STRUCTURE OF WORMLIKE MICELLES -GEMINI SURFACTANT-12-2-12

The direct observation of a shear-induced structure indilute/semidilut e giant micellar solutions in water is reported. At rest, the micelles are randomly oriented and the zero-shear viscosity is concentration d ependent and varies between 10(-3) and 10(-2) Pa s. Under shear, the s olutions have no measurable anisotropy as long as the applied shear ra te is less than a critical value determined from optical birefringence and electric conductivity measurements. Above this ''critical shear r ate'', the viscosity increases,and the solution becomes strongly aniso tropic. However, it was found that the ''critical shear rate'' depends strongly on the gap distance of the Couette cell, thus excluding the possibility of a phase transition at the critical shear rate. Cryo-tra nsmission electron microscopy is used to show shear-induced aggregatio n of wormlike micelles. It is concluded that shear induces a phase sep aration between a surfactant rich and a surfactant poor phase at shear rates much lower than the critical value. Initially, the surfactant r ich phase forms a network-like superstructure with domain sizes increa sing with shear. A strong anisotropy due to the deformation of the net work is observed when the domain size reaches the order of the gap siz e of the Couette. This phenomenon was observed for a large range of co ncentrations from far below up to several times above the overlap conc entration phi.