Phase transitions far from equilibrium in wet granular matter

The phase diagram of vertically agitated wet granular matter is presented, both experimentally and by simulation. We observe two phase transitions, one of which is driven by the applied force (acceleration), the other by injected energy (velocity of container walls). All observed features can be traced down to the hysteretic nature of capillary bridge formation and rupture. Other details of the interaction are remarkably irrelevant, suggesting a certain universality for the investigated phenomena. Phase transitions in condensed matter under thermally equilibrated conditions are by far the best understood examples of collective behavior. The astonishing fact that a solid melts at a precisely defined temperature, although the thermal energy is broadly distributed among the many degrees of freedom, can be completely apprehended from the principle of free energy minimization. Furthermore, the role of thermal fluctuations, which lead to the striking universality near critical points, was cast into a closed theory by means of the renormalization group [1]. Hence, the physics of phase transitions at thermal equilibrium has matured into solid textbook knowledge (see, for instance [2]). Phase transitions far from equilibrium, on the contrary, are still far from being understood on such a general basis, despite their ubiquity and striking similarity. Well-known examples that are currently of great interest range from collective pattern formation in systems of molecular [3] and micron scale [4, 5] to transitions in social behavior [6]–[8]. In particular, dynamic transitions in granular matter, such as soil liquefaction due to earthquakes, fluidization by vibration, or phase separation ([9]–[18]; [19] and references therein) are examples in systems of great practical interest. As the strength of the external drive is varied, sudden changes in the 1 Author to whom any correspondence should be addressed. New Journal of Physics 10 (2008) 05302