Tiny reversible rearrangement transitions in granular systems

Very dense bidimensional systems of inelastic hard disks excited from the base are studied by means of Newtonian event driven molecular dynamics. A stationary regime is reached where almost perfect crystallographic order is present in the system. When the energy injection is varied in a wide range the center of mass of the system varies smoothly except that at some points it suffers abrupt changes. There are at least two different types of changes and both show hysteresis as if it were a first order transition.