1. Institut de Physique de la Matière Complexe, Faculté des Sciences de Base, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
2. Laboratoire de Physique Théorique, Unité Mixte de Recherche UMR 8627, Bâtiment 210, Université de Paris-Sud, 91405 Orsay Cedex, France
3. Università degli Studi di Roma La Sapienza, Dipartimento di Fisica, and INFM, Center for Statistical Mechanics and Complexity, Piazzale A. Moro 5, 00185 Rome, Italy
4. Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan
5. Center for Theoretical Physics, Department of Physics, CSCS, University of Michigan, Ann Arbor, Michigan 48109-1120, USA

Correspondence to: G. D'Anna¹ Email: gianfranco.danna@epfl.ch

Abstract

Observation of the rotational brownian motion^{1, 2} of a very fine wire immersed in a gas led to one of the most important ideas of equilibrium statistical mechanics. Namely, the many-particle problem of a large number of molecules colliding with the wire can be represented by just two macroscopic parameters: viscosity and temperature. Interest has arisen in the question of whether this idea (mathematically developed in the Langevin model and the fluctuation-dissipation theorem^{3, 4}) can also be used to describe systems that are far from equilibrium. Here we report an experimental investigation of an archetypal non-equilibrium system, involving a sensitive torsion oscillator immersed in a granular system^{5, 6} of millimetre-size grains that are fluidized by strong external vibrations. The vibro-fluidized granular medium is a driven environment, with continuous injection and dissipation of energy, and the immersed oscillator can be seen as analogous to an elastically bound brownian particle. By measuring the noise and the susceptibility, we show that the experiment can be treated (to a first approximation) with the equilibrium formalism. This gives experimental access to a granular viscosity and an effective temperature; however, these quantities are anisotropic and inhomogeneous. Surprisingly, the vibro-fluidized granular matter behaves as a 'thermal' bath satisfying a fluctuation-dissipation relation.

1. Institut de Physique de la Matière Complexe, Faculté des Sciences de Base, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
2. Laboratoire de Physique Théorique, Unité Mixte de Recherche UMR 8627, Bâtiment 210, Université de Paris-Sud, 91405 Orsay Cedex, France
3. Università degli Studi di Roma La Sapienza, Dipartimento di Fisica, and INFM, Center for Statistical Mechanics and Complexity, Piazzale A. Moro 5, 00185 Rome, Italy
4. Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), Wako-shi, Saitama 351-0198, Japan
5. Center for Theoretical Physics, Department of Physics, CSCS, University of Michigan, Ann Arbor, Michigan 48109-1120, USA

Correspondence to: G. D'Anna¹ Email: gianfranco.danna@epfl.ch