Microscopic chaos and diffusion

We investigate the connections between microscopic chaos, defined on a dyna mical level and arising from collisions between molecules, and diffusion, c haracterized by a mean square displacement proportional to the time. We use a number of models involving a single particle moving in two dimensions an d colliding with fixed scatterers. We find that a number of microscopically non-chaotic models exhibit diffusion, and that the standard methods of cha otic time series analysis are ill suited to the problem of distinguishing b etween chaotic and nonchaotic microscopic dynamics. However, we show that p eriodic orbits play an important role in our models, in that their differen t properties in our chaotic and nonchaotic models can be used to distinguis h them at the level of time series analysis, and in systems with absorbing boundaries. Our findings are relevant to experiments aimed at verifying the existence of chaoticity and related dynamical properties on a microscopic level in diffusive systems.