We experimentally investigate the crystallization of a uniformly heated quasi-2D granular fluid as a function of the filling fraction. Our experimental results for the Lindemann melting criterion, the radial distribution function, the bond order parameter, and the statistics of topological changes at the particle level are the same as those found in simulations of equilibrium hard disks. This direct mapping suggests that the study of equilibrium systems can be effectively applied to study nonequilibrium steady states such as those found in our driven and dissipative granular system.

We report an experimental investigation of the caging motion in a uniformly heated granular fluid for a wide range of filling fractions, varphi. At low varphi the classic diffusive behavior of a fluid is observed. However, as varphi is increased, temporary cages develop and particles become increasingly trapped by their neighbors. We statistically analyze particle trajectories and observe a number of robust features typically associated with dense molecular liquids and colloids. Even though our monodisperse and quasi-2D system is known to not exhibit a glass transition, we still observe many of the precursors usually associated with glassy dynamics. We speculate that this is due to a process of structural arrest provided, in our case, by the presence of crystallization.

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We present experimental results on the velocity statistics of a uniformly heated granular fluid, in a quasi-2D configuration. We find the base state, as measured by the single particle velocity distribution P (c), to be universal over a wide range of filling fractions and only weakly dependent on all other system parameters. There is a consistent overpopulation in the distribution's tails, which scale as P ∝ exp(−A × c −3/2 ). More generally, P (c) deviates from a Maxwell-Boltzmann by a second order Sonine polynomial with a single adjustable parameter, in agreement with recent theoretical analysis of inelastic hard spheres driven by a stochastic thermostat. To our knowledge, this is the first time that Sonine deviations have been measured in an experimental system.

We develop the concept of frequency dependent effective mass,M (ω), of jammed granular materials which occupy a rigid cavity to a filling fraction of 48%, the remaining volume being air of normal room condition or controlled humidity. The dominant features ofM (ω) provide signatures of the dissipation of acoustic modes, elasticity and aging effects in the granular medium. We perform humidity controlled experiments and interpret the data in terms of a continuum model and a "trap" model of thermally activated capillary bridges at the contact points. The results suggest that attenuation in the granular materials studied here can be influenced significantly by the kinetics of capillary condensation between the asperities at the contacts. 81.05.Rm A distinct feature of jammed or loosely packed granular materials made of a variety of different materials such as sand, steel, polymer or glass is the ability to dissipate acoustic energy through the network of interparticle contacts or viscous dissipation through the surrounding medium. Indeed, loose grains damp acoustic modes very efficiently [1][2][3][4][5] and they are routinely used as an effective method to optimize the damping of unwanted structure-borne acoustic signals [1]. Despite its fundamental importance and practical applications, the microscopic origins of the mechanisms of dissipation in jammed granular materials are still unknown.In this Letter, we pursue the concept of a frequency dependent effective mass,M (ω), of a loose granular aggregate contained within a rigid cavity [5]. The effective massM (ω) is complex valued; its real part reflects the inertial and elastic properties while its imaginary part reflects the dissipative properties of the granular medium. We demonstrate how the features ofM (ω) allow the study of some of the mechanisms of damping of acoustic modes, aging and elasticity in granular matter.Generally speaking, the real part ofM (ω) exhibits a sharp resonance which we interpret in terms of an effective sound speed. The imaginary part ofM (ω) shows a broad resonance peak which quantifies the attenuation of acoustic waves in the system. We observe significant changes in the stiffness and attenuation as a function of humidity. By monitoring the effective mass in time, we find a logarithmic aging effect in the resonance frequency as well as an increase of the damping upon humidification. These effects can be modeled as capillary condensation occurring between the asperities at the contact surfaces between the grains during humidity-dry cycles. We interpret this phenomenon in the context of a "trap model" of thermally activated liquid bridges. Our results suggest that, in the granular materials investigated in the present study, dissipation of acoustic energy is dominated by the asperities at the interparticle contact surfaces. In addition, humidity drastically affects the attenuation of the material through the capillary condensation of liquid bridges.Experiments.-A cylindrical cavity (of diameter D=25.4 mm) excavated in a rigid aluminu...