A new phase transition is observed experimentally in a dry granular gas subject to vertical vibration between two horizontal plates. Molecular dynamics simulations of this system allow us to investigate the observed phase separation in detail. We find a high-density, low temperature liquid, coexisting with a low-density, high temperature gas moving coherently. The importance of the coherent motion for phase separation is investigated using frequency modulation.

We describe experiments and simulations carried out to investigate spinodal decomposition in a vibrated, dry granular system. The dynamics is found to be similar to that of systems evolving under curvature-driven diffusion, which suggests the presence of an effective surface tension. By studying quasi-2D droplets in the steady state, we find behavior consistent with Laplace's equation, demonstrating the existence of an actual surface tension. Detailed measurements of the pressure tensor in the interfacial region show that the surface tension results predominantly from an anisotropy in the kinetic energy part of the pressure tensor, in contrast to thermodynamic systems where it arises from either the attractive interaction between particles or entropic considerations.

Experiments and computer simulations are carried out to investigate phase separation in a granular gas under vibration. The densities of the dilute and the dense phase are found to follow a lever rule and obey an equation of state. Here we show that the Maxwell equal-areas construction predicts the coexisting pressure and binodal densities remarkably well, even though the system is far from thermal equilibrium. This construction can be linked to the minimization of mechanical work associated with density fluctuations without invoking any concept related to equilibrium-like free energies.

Dry, freely evolving granular materials in a dilute gaseous state coalesce into dense clusters only due to dissipative interactions. This clustering transition is important for a number of problems ranging from geophysics to cosmology. Here we show that the evolution of a dilute, freely cooling granular gas is determined in a universal way by the ratio of inertial flow and thermal velocities, that is, the Mach number. Theoretical calculations and direct numerical simulations of the granular Navier-Stokes equations show that irrespective of the coefficient of restitution, density or initial velocity distribution, the density fluctuations follow a universal quadratic dependence on the system's Mach number. We find that the clustering exhibits a scale-free dynamics but the clustered state becomes observable when the Mach number is approximately of O(1). Our results provide a method to determine the age of a granular gas and predict the macroscopic appearance of clusters.Geophysical processes [1], the solar corona [2], the asteroid belt between Mars and Jupiter, planetary rings [3,4], protoplanetary disks [5], and the formation of cosmological structures [6] are systems where granular clustering processes are at play. Even a small degree of dissipation in the kinetics of granular particles produces spatial correlations and structures in a dilute, homogeneous gas [7]. Hydrodynamic treatments suggest that a shear instability initiates this transition [7]. However, what exactly initiates this process is not known. The equations of granular hydrodynamics [8] predict a linear instability of the transverse mode [9] when the wavevectorand ε is the coefficient of restitution. This instability leads to the formation of vortices. In regions where the particle velocities are correlated the temperature drops, which, in turn, creates a region of low pressure. These are the seeds for a second instability if the system size is larger than k * −1 [13,15,17].Experimental studies [18][19][20][21][22] of granular gases are rather scarce because of formidable challenges in preparing a system free of external forces. Microgravity experiments on parabolic flights or drop towers potentially offer good conditions, but it is very difficult to remove the influence of confining potentials or surrounding walls on the granular system. The necessity of studying large system sizes and of characterizing fluctuations in regions of sharp gradients in temperature and density, developing into supersonic flow, without ambiguities motivates us to tackle the continuous hydrodynamic equations beyond perturbative schemes. Hydrodynamic fields can be rigorously defined in a manner similar to molecular fluids by means of a coarse-graining of the microscopic kinetic equations. First, one considers the one-particle distribution function f ( r, w, t), which obeys the Boltzmann equation and represents the number of particles within a volume d r centered at r and with velocity w within the interval d w. Then, the transport equations for inelastic systems are deri...