2D - Magnetic Granular Gas

Basic constituents of matter experience continuous competition between thermal agitation and interactions, leading to global structuring. We present here a macroscopic example of such structuring using a two-dimensional granular gas with tunable repulsive interactions between particles. Soft-ferromagnetic particles are placed on a vibrating rough plate and vertically confined, so that they perform a horizontal Brownian motion in a cell. When bathed in an external vertical magnetic field, the particles become magnetized and thus interact according to a dipolar repulsive law. Therefore, such a granular gas display properties that depend on the tunable particle interactions intensity. For a moderate increase of the magnetic field compared to the mechanical agitation, we observe a decrease of dissipative inter-particles collisions. Granular gases were extensively studied as an example of macroscopic out-equilibrium system. Here we notice that features related to the non-equilibrium properties like the non-gaussian velocity distributions and clustering, are reduced and the properties approach those expected for a perfect gas at thermal equilibrium. Therefore by tuning the magnetic field in this system, we can in someway adjust the degree of distance of equilibrium, which could be useful regarding recent developments of out-equilibrium statistical physics.

At high magnetic field compared to the mechanical agitation and low particle area fraction, a hexagonal crystal-like structure sets up due to magnetic repulsion between particles. In contrast, for higher particle area fraction and high magnetic field, the particles self-organize into a labyrinthine, amorphous-like structure which is mostly constituted by small chains of particles. We characterize these different phases using relevant statistical tools. Our aim is to provide a better understanding of the ordered/disordered phase transitions induced by the competition between agitation and interactions in many-particle systems. This model system should be useful as a new approach in the study of the solidi cation dynamics as well as the physics of amorphous systems.

Article about the out-equilibrium properties at moderate density : 
S. Merminod, M Berhanu and E. Falcon,  « Transition from a dissipative to a quasi-elastic system of particles with tunable repulsive interactions » Europhysics Letters 106, Editor Choice, (2014)

Work in progress ...