Tuning the distance to equipartition by controlling the collision rate in a driven granular gas experiment

Abstract

In a granular gas experiment of magnetized particles confined in a thin layer, the rate of dissipative collisions is tuned by adjusting the amplitude of an external magnetic field. The velocity statistics are analyzed using the dynamic and static structure factors of transverse velocity modes. Using the fluctuating hydrodynamics theory we measure the deviation from kinetic energy equipartition in this out-of-equilibrium system as a function of the dissipative collision rate. When the collision rate is decreased, the distance to equipartition becomes smaller meaning that the dynamical properties of this granular gas approach by analogy those of a molecular gas in thermal equilibrium.

Wave spectroscopy in a driven granular material

“…As we previously showed [17,19], increasing ε—through B—induces a gas-like to solid-like transition in the granular quasi-monolayer (figure 1 d ). More specifically, when ε=0, the combination of homogeneous mechanical forcing and inherently dissipative collisions results in a granular gas state [2,11,43,44,46].…”

“…More specifically, when ε=0, the combination of homogeneous mechanical forcing and inherently dissipative collisions results in a granular gas state [2,11,43,44,46]. From ε∼10, we do not detect collisions anymore and a fluid of interacting dipoles is formed [17,19]. At the highest values of ε, particles self-organize into a hexagonal, crystal-like structure.…”

“…We find that |Ψ6| increases from roughly 0.4 to 0.9 across the full range of ε (figure 1 e ). First, we define the ‘granular gas’ state for 0<ε<10.7, where |Ψ6| is the smallest, collisions occur—although at a decreasing rate as ε increases—and the particle assembly behaves as a dissipative granular gas [19] (electronic supplementary material, movies S1 and S5). Second, for 10.7<ε<21.9, the ‘dipolar fluid’ regime consists of a disordered fluid of strongly coupled dipoles, for which |Ψ6| is still near 0.4, but there are no more collisions (electronic supplementary material, movies S2 and S6).…”

“…By analogy with the hydrodynamics description of molecular systems [16], we characterize a driven granular system in a non-equilibrium stationary state from the analysis of the particle velocity fluctuations in the Fourier space. We previously introduced a model experimental system [17–19] to study the competition between mechanical agitation and remote interactions in a quasi-two-dimensional assembly of magnetized, millimetre-diameter spheres. For a given cell filling and shaking strength, we observed a granular fluid-like phase or a solid-like crystal phase depending on the strength of a tunable dipolar repulsion [17].…”

Particle Dynamics at the Onset of the Granular Gas-Liquid Transition

“…For boundary-driven system (e.g., mechanically shaking), several laws have been proposed to describe their velocity distributions [5][6][7][8] but most of them correspond to non-Gaussian distributions with densitydependent stretched exponential tails [8][9][10][11]. Nonequipartition of kinetic energy between degrees of freedom is also reported [12][13][14]. Recent experimental studies using a spatially homogeneous forcing in volume (by magnetic driving of each particle) leads to several major differences such as velocity distributions with density-independent exponential tails [15].…”