Density scaling of the dynamics of vitrifying liquids and its relationship to the dynamic crossover

A central question concerning glass-formation has been what governs the kinetic arrest of the quenched liquid – cooling reduces the thermal energy which molecules need to surmount local potential barriers, while the accompanying volume contraction promotes molecular crowding and congestion (and thus alters the potential). Recent experimental findings have shown that both thermal energy and density contribute significantly to the temperature-dependence of vitrifying liquids. Herein, we show that the scaling (superpositioning) of the relaxation times near the glassy state, by expressing them as a function of temperature and the specific volume, leads to a modification of the usual fragility curves, whereby differences in the extent of departure from Arrhenius behavior can be rationalized. More intriguingly, the characteristic changes in the relaxation properties (i.e., the ‘dynamic crossover’), occurring well above the liquid-to-glass transition, are shown to be related to the same function that superposes the relaxation data.