Data from: Non-equilibrium 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.