Dynamical behavior of a complex fluid near an out-of-equilibrium transition: approaching simple rheological chaos.

We report here an extensive study of sustained oscillations of the viscosity of a complex fluid near an out-of-equilibrium transition. Using well defined protocols, we perform rheological measurements of the onion texture near a layering transition in a Couette flow. This complex fluid exhibits sustained oscillations of the viscosity, on a large time scale (500 s) at controlled stress. These oscillations are directly correlated to an oscillating microstructural change of the texture of the fluid. We observe a great diversity of dynamical behavior and show that there is a coupling with spatial effects in the inverted Delta v direction. This is in agreement with a careful analysis of the temporal series of the viscosity with the dynamical system theory. This analysis indicates that the observed dynamical responses do not strictly correspond to three-dimensional chaotic states, probably because some spatiotemporal effects are present and are likely to play an important role.