Séminaires du mardi 13 mars

Séminaire du LPTMS: Alexandre Nicolas


Bottleneck flows of pedestrians and grains

Alexandre Nicolas (LPTMS, Université Paris-Sud)

Simple models from Statistical Physics can be surprisingly useful to study practical problems that lie outside the traditional realm of Physics. I will illustrate this statement with two distinct (yet related!) examples: granular flows through a constriction and competitive passages of pedestrians through a door. To destroy clogs in constricted granular flows, vibrations are often applied to the hopper or silo, but clogs do not yield instantly. Instead, they exhibit an anomalously broad distribution of lifetimes. This elusive feature was recently captured within a model in which we likened the destruction of clogs to thermally activated escapes from a set of energy traps; the model also reproduces other salient experimental observations [1]. Turning to the second example, competitive pedestrian flows through a narrow door look very disordered, and yet they were found to present robust statistical features, such as anticorrelated time gaps between escapes and exponentially distributed bursts of escapes. On the basis of simple models, we have shown that in fact these features emerge generically in constricted flows [2].
  • [1] Alexandre Nicolas, Ángel Garcimartín and Iker Zuriguel, "A trap model for clogging and unclogging in granular hopper flows" (2017), preprint arxiv:1711.04455
  • [2] Alexandre Nicolas and  Ioannis Touloupas, Origin of the correlations between exit times in pedestrian flows through a bottleneck, J. Stat. Mech.: Theor. Exp. 2018(1), 013402.

Séminaire du LPTMS: Izaak Neri


Thermodynamic bounds on the statistics of first-passage times and extreme values of stochastic processes

Izaak Neri (Max Planck Institute for the Physics of Complex Systems, Dresden, Allemagne)

  Stochastic thermodynamics generalizes concepts from thermodynamics, and makes them useful to study mesoscopic systems driven far from thermal equilibrium, such as, optically driven colloidal particles, noisy processes in cell biology or microelectronic devices. In mesoscopic systems thermodynamic observables -- such as, entropy production, heat and mesoscopic currents -- are fluctuating quantities, and stochastic thermodynamics characterizes universal properties of these fluctuating quantities. Established results are the fluctuation relations and the thermodynamic uncertainty relations, which express universal properties of fluctuations of stochastic currents at a fixed time. In this talk I will present thermodynamic bounds for the statistics of first-passage times and extreme values of stochastic currents, which are fluctuating properties of trajectories of stochastic currents. Some interesting results are: a bound for the mean first-passage time of current variables in terms of the dissipation rate, a fluctuation theorem for first-passage times of entropy production, and a universal bound on the supremum statistics of the heat absorbed by a nonequilibrium system. These results will be illustrated on examples of physical processes, such as, the dynamics a molecular motor and charge transport in microelectronic devices.