Compressing nearly hard sphere fluids increases glass fragility

We use molecular dynamics to investigate the glass transition occuring at large volume fraction, φ, and low temperature, T , in assemblies of soft repulsive particles. We find that equilibrium dynamics in the (φ, T ) plane obey a form of dynamic scaling in the proximity of a critical point at T = 0 and φ = φ0, which should correspond to the ideal glass transition of hard spheres. This glass point, ‘point G’, is distinct from athermal jamming thresholds. A remarkable consequence of scaling behaviour is that the dynamics at fixed φ passes smoothly from that of a strong glass to that of a very fragile glass as φ increases beyond φ0. Correlations between fragility and various physical properties are explored. Introduction. – Structureless, hard, frictionless particles pass from a mobile to an immobile state with increasing density [1]. Interacting particles and molecules in a glass-forming material also pass from a mobile fluid state to an immobile glassy state as temperature is reduced [2]. Much research in the last decade has been devoted to extracting a common geometric essence from these two classes of phenomena. One line of research approaches the threshold of immobilization or jamming via processes unrelated to thermal equilibrium [3–6]. Connections between these jamming transitions and those seen at positive temperature are suggested [7], but remain unclear. In particular, the threshold density for jamming and that for immobilization at non-zero temperature are considered identical by some researchers [8,9], distinct for others [10–12], ill-defined by some others [3, 13]. Direct measurements are not conclusive, because the location of the glass transition relies upon fitting and extrapolation [8, 10], while the jamming transition is not uniquely defined [3, 4]. The notion that temperature and density should have analogous effects on the glass transition has a long history [14, 15], although quantitative evidence supporting these analogies is limited. Recent work studying the effect of pressure on the glass transition showed that the dynamics of glass-formers is little affected by increasing the density, since a simple rescaling procedure collapses a broad range of dynamic data [16, 17]. This finding directly implies that the (isochoric) fragility [18] of most glass-formers is independent of density, at least in the range currently explored by experiments. A second conclusion is that the glass transition of molecular systems is mostly controlled by temperature, suggesting that the density-driven glass transition of hard spheres might have a different nature. In this article we study the relative influence of density and temperature on the glass transition using a model of soft repulsive particles [19]. In the zero-temperature limit, the model is equivalent to density-controlled hard spheres, while it resembles thermally driven dense fluids at large density and finite temperature. Another motivation to use compressible particles is to access densities beyond the hard sphere critical density for kinetic arrest that are unreachable with the hard sphere potential. This approach has proven useful in the context of athermal jamming [4–6], but was not extended to thermal equilibrium before. At equilibrium, issues related to the possible protocol dependence of the results [20] do not arise. Using computer simulations we have studied the equilibrium dynamics of a three-dimensional assembly of soft repulsive particles varying the volume fraction, φ, and temperature, T (see fig. 1). We have discovered a simple connection between density and temperature effects, from which several interesting results are deduced. Equilibrium dynamics obey critical scaling in the proximity of