Statistical geometry of cavities in a metastable confined fluid

The statistical geometry of cavities in a confined Lennard-Jones (LJ) fluid is investigated with the focus on metastable states in the vicinity of the stability limit of the liquidlike state. For a given configuration of molecules, a cavity is defined as a connected region where there is sufficient space to accommodate an additional molecule. By means of grand canonical Monte Carlo simulations, we generated a series of equilibrium stable and metastable states along the adsorption-desorption isotherm of the LJ fluid in a slit-shaped pore of ten molecular diameters in width. The geometrical parameters of the cavity distributions were studied by Voronoi-Delaunay tessellation. We show that the cavity size distribution in liquidlike states, characterized by different densities, can be approximated by a universal log-normal distribution function. The mean void volume increases as the chemical potential &mgr; and, correspondingly, the density decreases. The surface-to-volume relation for individual cavities fulfills the three-dimensional scaling S(cav)=gV(2/3)(cav) with the cavity shape factor g=8.32-9.55. The self-similarity of cavities is observed over six orders of magnitude of the cavity volumes. In the very vicinity of the stability limit, &mgr;-->&mgr;(sl), large cavities are formed. These large cavities are ramified with a fractal-like surface-to-volume relation, S(cav) approximately V(cav). Better statistics are needed to check if these ramified cavities are similar to fragments of a spanning percolation cluster. At the limit of stability, the cavity volume fluctuations are found to diverge as (-(2)) approximately [(&mgr;-&mgr;(sl))/kT](-gamma(c)) with the exponent gamma(c) approximately 0.93. This exponent can be referred to as the cavity pseudocritical exponent, in analogy with the other pseudocritical exponents characterizing the divergence of thermodynamic quantities at the spinodal point.

Towards a quantification of disorder in materials: distinguishing equilibrium and glassy sphere packings

This paper examines the prospects for quantifying disorder in simple molecular or colloidal systems. As a central element in this task, scalar measures for describing both translational and bond-orientational order are introduced. These measures are subsequently used to characterize the structures that result from a series of molecular-dynamics simulations of the hard-sphere system. The simulation results can be illustrated by a two-parameter ordering phase diagram, which indicates the relative placement of the equilibrium phases in order-parameter space. Moreover, the diagram serves as a useful tool for understanding the effect of history on disorder in nonequilibrium structures. Our investigation provides fresh insights into the types of ordering that can occur in equilibrium and glassy systems, including quantitative evidence that, at least in the case of hard spheres, contradicts the notion that glasses are simply solids with the "frozen in" structure of an equilibrium liquid. Furthermore, examination of the order exhibited by the glassy structures suggests, to our knowledge, a new perspective on the old problem of random close packing.

Is random close packing of spheres well defined?

Despite its long history, there are many fundamental issues concerning random packings of spheres that remain elusive, including a precise definition of random close packing (RCP). We argue that the current picture of RCP cannot be made mathematically precise and support this conclusion via a molecular dynamics study of hard spheres using the Lubachevsky-Stillinger compression algorithm. We suggest that this impasse can be broken by introducing the new concept of a maximally random jammed state, which can be made precise.

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.