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.
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Prof. Alessandra Faggionato
Department of Mathematics
University La Sapienza
Piazzale A. Moro, 2
00186 Rome - Italy