Molecular dynamics study of the stability of the hard sphere glass

Glassy states have been observed in hard-spherelike colloidal suspensions; however, some recent work suggests that a stable, one-component hard-sphere glass doesn't exist. A possible resolution of this dilemma is that colloidal glass formation results from a small degree of particle polydispersity. In order to investigate this further, we used the molecular-dynamics method to explore the phase behavior of both one- and two-component hard-sphere systems. It was found that the metastable fluid branch of the one-component system ceased to exist at a volume fraction marginally above melting, instead this system always crystallized within a relatively short period of time. Binary systems with a size ratio gamma=0.9 were then used as the simplest approximation to model a polydisperse hard-sphere colloidal system. Here the crystallization process was slowed down dramatically for all volume fractions and the fluid state was maintained for many relaxation times. Indeed, at the lowest volume fraction straight phi=0.55 no sign of crystallization was seen on the simulation time scale. The systems at intermediate volume fractions did eventually crystallize but at the highest volume fraction of straight phi=0.58, a dramatic slowing down in the crystallization process was observed. This is qualitatively in agreement with the experimental results on colloidal suspensions. Using the insight gained from this paper, the reasons behind a polydisperse system forming a stable glass, in contrast to the one-component system, are elucidated.