Fragile-strong transitions and polyamorphism in glass forming fluids

A simple model of a glass forming fluid, consisting of a bidisperse mixture of penetrable spheres, is studied numerically by Monte Carlo simulation. The model shows a transition from fragile to strong behaviour as the temperature is reduced. This transition is driven by the competition between the two mechanisms that contribute to diffusivity in the model: collective rearrangement of particles (responsible for the fragile behaviour), and individual particle motion (which gives rise to the strong behaviour at low temperatures). Also observed is a maximum of diffusivity as a function of pressure that can be interpreted within the same framework. The connection between this behaviour and polyamorphism is addressed.