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 solidication 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
...**