MIXING AND DISORDER

Ordinary diffusion equation as described by a Brownian walk applies to a large number of physical processes. However, qualitative deviations are obtained in a number of situations involving correlations or larg e distributions of steps or waiting times in the walk. Anomalous diffu sion laws result which can be expressed in terms of fractal Brownian w alks. We examine from an experimental point of view some manifestation s of such systems: dispersion in porous media, chaotic advection, turb ulent flows.