Molecular dynamics of a dense fluid of polydisperse hard spheres

Slow dynamics in a fluid are studied in one of the most basic systems possi ble: polydisperse hard spheres. Monodisperse hard spheres cannot be studied as the slow down in dynamics as the density is increased is preempted by c rystallization. As the dynamics slow they become more heterogeneous, the sp read in the distances traveled by different particles in the same time incr eases. However, the dynamics appears to be less heterogeneous than in hard- spherelike colloids at the same volume fraction. The particles which move l east far in a characteristic relaxation time and, particularly, the particl es which move farthest in the same time are clustered, not randomly distrib uted throughout the sample. The slow dynamics (but not the statics) show st rong finite size effects, suggesting that slow dynamics are correlated over distances of several diameters. We study the dynamics at three different w idths of the distribution of diameters of the hard spheres. For each width, the relaxation time is the same function of the compressibility factor, su ggesting that this determines the relaxation time for hard spheres. (C) 200 0 American Institute of Physics. [S0021-9606(00)51135-0].