Nonlinear flow of wormlike micellar gels: regular and chaotic time-dependence of stress, normal force and nematic ordering

Abstract

We present our recent experiments on the nonlinear dynamics in the flow behaviour of shear-thinning wormlike micellar gels. In particular, we have shown experimentally that above a critical shear rate, the initial transient stress response comprises of overshoots and undershoots and this is followed by periodic, quasiperiodic, intermittent and chaotic behaviour. The normal force dynamics is similar to that of the stress. This can be classified as Type-II intermittency route to chaos. In our system, shear-thinning wormlike micellar solution of cetyltrimethylammonium tosylate, the strength of flow-concentration coupling is tuned by the addition of salt sodium chloride. The existence of a "butterfly" intensity pattern in small angle light scattering (SALS) performed simultaneously with the rheological measurements confirms the coupling of flow to concentration fluctuations in the system under study. Dynamic light scattering measurements to extract the relaxation time scales of concentration fluctuations are in excellent agreement with the in situ SALS measurements performed under shear. The scattered depolarised intensity in SALS, sensitive to orientational order fluctuations, shows the same time-dependence (like intermittency) as that of shear stress at various wave vectors.