The fragile to strong dynamical crossover in supercooled liquids. The o-terphenyl case and its ergodicity at the dynamical arrest

We report a study of the dynamic crossover in two different glass-forming materials: an adhesive hard-sphere (AHS) colloidal system and the molecular liquid o-terphenyl. Systems that present a fragile-to-strong dynamical crossover in their transport parameters. A combination of light scattering, viscosity and nuclear magnetic resonance (NMR) is used to study the dynamical arrest in these systems. In both systems the data analysis is made in the conceptual framework of the Mode Coupling Theory in its extended version. For the AHS colloid, which is characterized by clustering process, we give evidence that the evolution toward the arrested phase can be described by using both the temperature and the concentration as the order parameter. The light scattering studies conducted on the o-terphenyl confirm the suggestion of recent statistical mechanical approaches that such a molecular system remains ergodic also below the calorimetric glass-transition temperature.