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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.

Wet granular materials are characterized by a defined bond energy in their particle interaction such that breaking a bond implies an irreversible loss of a fixed amount of energy. Associated with the bond energy is a nonequilibrium transition, setting in as the granular temperature falls below the bond energy. The subsequent aggregation of particles into clusters is shown to be a self-similar growth process with a cluster size distribution that obeys scaling. In the early phase of aggregation the clusters are fractals with D f = 2, for later times we observe gelation. We use simple scaling arguments to derive the temperature decay in the early and late stages of cooling and verify our results with event-driven simulations.PACS numbers: 61.43.Hv Granular systems have encountered strongly increasing interest within recent years [1, 2] as they are both close to industrial applications and relevant as model systems for collective systems far from thermal equilibrium. Most research has focused on dry granulates resulting in rich non-equilibrium phenomena [3]. The dramatic change of mechanical properties due to some wetting liquid additive is apparent, when we compare the fluid-like state of dry sand flowing through an hourglass with the shapeable plastic state of wet sand, which is suitable for molding a sandcastle. This change in bulk properties can be attributed to the differences in the underlying particle interactions [4,5]. Whereas in collisions of dry particles, a certain fraction of the initial kinetic energy is dissipated into the atomic degrees of freedom of the particles, in the wet case, the interaction is mainly due to the interfacial forces exerted by liquid capillary bridges which form between adjacent particles and dissipate energy upon rupture.More recently, the dynamics of wet granular media has been addressed in several studies [5,6,7,8,9, 10], focusing on nonequilibrium phase transitions [10], agglomeration [6,7], shear flow [8] and cooling in one dimension [9]. A particularly important aspect of free cooling in cohesive gases is the structure of the emerging clusters. It pertains to the formation of dust filaments and the microscopic mechanisms of cloud formation as well as to the size distribution and impact probability of planetesimals in accretion discs. Structure formation in wet granulates during free cooling has hardly been studied yet and is the central theme of our paper. We suggest a very simple model for the interaction of two wet grains, which only takes into account the essential features of a capillary bridge: hysteresis and dissipation with a well defined energy loss. Cooling is controlled by the proba- * Electronic address: ulrich@theorie.physik.uni-goettingen.de bility for a bridge to rupture and hence logarithmically slow in the long time limit, when a percolating structure has been formed. For smaller times the structure is characterized by coexisting fractal clusters of all sizes, whose size distribution is shown to scale.Model-Consider a gas of N hard spheres of diameter d ...

We investigate a gas of wet granular particles covered by a thin liquid film. The dynamic evolution is governed by two-particle interactions, which are mainly due to interfacial forces in contrast to dry granular gases. When two wet grains collide, a capillary bridge is formed and stays intact up to a certain distance of withdrawal when the bridge ruptures, dissipating a fixed amount of energy. A freely cooling system is shown to undergo a nonequilibrium dynamic phase transition from a state with mainly single particles and fast cooling to a state with growing aggregates such that bridge rupture becomes a rare event and cooling is slow. In the early stage of cluster growth, aggregation is a self-similar process with a fractal dimension of the aggregates approximately equal to Df approximately 2 . At later times, a percolating cluster is observed which ultimately absorbs all the particles. The final cluster is compact on large length scales, but fractal with Df approximately 2 on small length scales.