| Wet granular materials are composed of grains, covered by a microscopically thin liquid film. When two particles come into contact, they form a capillary bridge, which ruptures upon receding such that a fixed amount of energy is dissipated. A freely cooling system is is shown to undergo a nonquilibrium dynamic phase transition, from a state with mainly single particles and fast cooling to a state with growing aggregates, where bridge rupture becomes a rare event and cooling is slow. The aggregation of particles into clusters is a self-similar growth process with a cluster size distribution that obeys scaling. Simple scaling arguments are used to derive the temperature decay in the early and late stages of cooling, and the findings are verified by event-driven simulations. |
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