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. T he model shows a transition from fragile to strong behaviour as the tempera ture is reduced. This transition is driven by the competition between the t wo mechanisms that contribute to diffusivity in the model: collective rearr angement of particles (responsible for the fragile behaviour), and individu al particle motion (which gives rise to the strong behaviour at low tempera tures). 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.