Freezing, melting and the onset of glassiness in binary mixtures

We clarify the relationship between freezing, melting and the onset of glassy dynamics in the prototypical glass-forming Kob-Andersen mixture. Our starting point is a precise definition of the onset of glassiness, as expressed by the emergence of an inflection in the time-dependent correlation functions. By scanning the temperature-composition phase diagram of the model, we find that the onset temperature barely varies with concentration, unlike the freezing temperature, which shows a marked drop at the eutectic point. We observe, however, a surprising correspondence between the onset of glassiness and the solidus line, along which the underlying crystalline phases melt. At fixed concentration, the freezing and onset temperatures display similar pressure dependencies, which are very well predicted by the isomorph theory and are encoded in the variation of the excess entropy. While our results rule out a general connection between thermodynamic metastability and glassiness, they call for a careful assessment of the role of crystalline precursors in glass-forming liquids.