Direct Evidence of the Shear-Induced Structure of Wormlike Micelles: Gemini Surfactant 12−2−12

The direct observation of a shear-induced structure in dilute/semidilute giant micellar solutions in water is reported. At rest, the micelles are randomly oriented and the zero-shear viscosity is concentration dependent and varies between 10 - 3 and 10 - 2 Pa s. Under shear, the solutions have no measurable anisotropy as long as the applied shear rate is less than a critical value determined from optical birefringence and electric conductivity measurements. Above this “critical shear rate”, the viscosity increases, and the solution becomes strongly anisotropic. 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-transmission electron microscopy is used to show shear-induced aggregation of wormlike micelles. It is concluded that shear induces a phase separation between a surfactant rich and a surfactant poor phase at shear rates much lower than the critical value. Initially, the surfactant rich phase forms a network-like superstructure with domain sizes increasing with shear. A strong anisotropy due to the deformation of the network is observed when the domain size reaches the order of the gap size of the Couette. This phenomenon was observed for a large range of concentrations from far below up to several times above the overlap concentration φ*.