Linear and nonlinear rheology of wormlike micelles

Several surfactant molecules self-assemble in solution to form long, cylind rical, flexible wormlike micelles. These micelles can be entangled with eac h other leading to viscoelastic phases. The rheological properties of such phases are very interesting and have been the subject of a large number of experimental and theoretical studies in recent years. We shall report our r ecent work on the macrorheology, microrheology and nonlinear flow behaviour of dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium T osilate). This system forms elongated micelles and exhibits strong viscoela sticity at low concentrations (similar to 0.9 wt%) without the addition of electrolytes. Microrheology measurements of G(omega) have been done using d iffusing wave spectroscopy which will be compared with the conventional fre quency sweep measurements done using a cone and plate rheometer. The second part of the paper deals with the nonlinear rheology where the measured she ar stress sigma is a nonmonotonic function of the shear rate (gamma) over d ot. In stress-controlled experiments, the shear stress shows a plateau for (gamma) over dot larger than some critical strain rate, similar to the earl ier reports on CPyCl/NaSal system. Cates ei nl have proposed that the plate au is a signature of mechanical instability in the form of shear bands. We have carried out extensive experiments under controlled strain rate conditi ons, to study the time-dependence of shear stress. The measured time series of shear stress has been analysed in terms of correlation integral and Lya punov exponent to show unambiguously that the behaviour is typical of low d imensional dynamical systems.