Nonequilibrium effects in fragmentation - art. no. 024606

We study, using molecular dynamics techniques, how boundary conditions affe ct the process of fragmentation of finite, highly excited, Lennard-Jones sy stems. We analyze the behavior of the caloric curves (CC), the associated t hermal response functions (TRF), and cluster mass distributions for constra ined and unconstrained hot drops. It is shown that the resulting CC for the constrained case differ from the one in the unconstrained case, mainly in the presence of a "vapor branch." This branch is absent in the free expandi ng case even at high energies. This effect is traced to the role played by the collective expansion motion. On the other hand, we found that the recen tly proposed characteristic features of a first order phase transition taki ng place in a finite isolated system, i.e., abnormally large kinetic energy fluctuations and a negative branch in the TRF, are present for the constra ined (dilute) as well as the unconstrained case. The microscopic origin of this behavior is also analyzed.