Nonequilibrium dynamics of hard spheres in the fluid, crystalline, and glassy regimes

We investigate the response of a system of hard spheres to two classes of perturbations over a range of densities spanning the fluid, crystalline, and glassy regimes within a molecular dynamics framework. First, we consider the relaxation of a "thermal inhomogeneity," in which a central region of particles is given a higher temperature than its surroundings and is then allowed to evolve under Newtonian dynamics. In this case, the hot central "core" of particles expands and collides with the cold surrounding material, creating a transient radially expanding "compression wave," which is rapidly dissipated by particle-particle collisions and interactions with periodic images at the boundary, leading to a rapid relaxation to equilibrium. Second, we consider a rapid compression of the spheres into a disordered glassy state at high densities. Such rapidly compressed systems exhibit very slow structural relaxation times, many orders of magnitude longer than thermalization times for simple temperature inhomogeneities. We find that thermal relaxation of the velocity distribution is determined simply by the total collision rate, whereas structural relaxation requires coordinated collective motion, which is strongly suppressed at high density, although some particle rearrangement nevertheless occurs. We further find that collisions propagate significantly faster through glassy systems than through crystalline systems at the same density, which leads to very rapid relaxation of velocity perturbations, although structural relaxation remains very slow. These results extend the validity of previous observations that glassy systems exhibit a hybrid character, sharing features with both equilibrium and nonequilibrium systems. Finally, we introduce the hard-sphere causal graph, a network-based characterization of the dynamical history of a hard-sphere system, which encapsulates several useful metrics for characterizing hard-sphere systems within a single structure, and which emphasizes the role of causality in these systems.