Chaotic dynamics in shear-thickening surfactant solutions

We report the observation of dynamical behaviour in dilute, aqueous solutio ns of a surfactant CTAT ( cetyl trimethylammonium p-toluenesulphonate), bel ow the overlap concentration c*. At these concentrations, CTAT forms cylind rical micelles and shows a pronounced shear-thickening transition above a c oncentration-dependent critical shear rate (gamma )over dot(c) . An analysi s of the time-series of the stress relaxations at controlled shear rates in the shear-thickening regime shows the existence of correlation dimensions greater than two and positive Lyapunov exponents. This indicates the existe nce of deterministic chaos in the dynamics of stress relaxation at these co ncentrations and shear rates. The observed chaotic behaviour may be attribu ted to the stick-slip between the shear-induced structure ( SIS) formed in the sheared surfactant solution and the coexisting dilute phase. At even hi gher shear rates, when the SIS spans the Couette, there is a transition to higher-dimensional dynamics arising out of the breakage and recombination o f the SIS.