SEMINARIO DI PROBABILITA' E FISICA STATISTICA

Giovedi' 7 Maggio, ore 14.30 , in Aula B (Dipartimento di Matematica. Univ.La Sapienza, Roma)

Speaker: Andrea Puglisi (Istituto dei Sistemi Complessi, Sapienza)

Title: Dynamics of an intruder in a granular fluid: from dilute to dense experiments.

A single experimental setup is the occasion for a tour into many issues in non-equilibrium statistical mechanics. In the experiment, a rotating intruder performs a Brownian-like dynamics under the influence of collisions with a shaken granular media. Dissipations in the form of inelastic collisions and dry tangential friction make the system inherently out of equilibrium. When the granular medium is diluted, the results agree with a Boltzmann-Lorentz description which, in the large mass limit, is well approximated by a Fokker-Planck equation for a Ornstein-Uhlenbeck process with Coulomb friction. When the intruder is shaped so as to break the symmetry under rotation-inversion, an average drift ("ratchet" or "motor" effect) is observed, with properties depending on the dominant dissipation: friction or inelastic collisions. When the density of the surrounding medium increases, non-Markovian effects appear. The first consequence is a violation of the Einstein relation which - near equilibrium - describes the linear response to a small force provided by an external motor. The analysis of a generalized flctuation-dissipation relation explains the nature of the violation: a joint effect of dissipation and coupling with the dense flid. When the density is increased further and the jamming transition is approached, anomalous diffusion appears in the form of transient cage effects (subdiffusion) and - more surprisingly - superfdiffusion at large times.