Dynamical phase transitions in amorphous thin films*

Despite more than two decades of study, there remain many fundamental unanswered questions about the dynamics of glass-forming materials confined to thin films. In particularly, several experiments show evidence of a qualitative change in behavior upon confinement to sufficiently thin films. For example, the viscosity of amorphous thin films has been shown to exhibit a sharp transition from glassy to liquid-like behavior when film thickness is reduced below 30 nm [Y. Zhang et al., J. Chem. Phys. 145, 114502 (2016)]. Here, we provide evidence that this transition is due to the films inability to support an inactive, low-mobility, dynamic phase near a free surface. Active to inactive dynamical phase transitions have been found for a number of bulk glassy systems by biasing trajectories to low-mobility states using a field s. For a model polymer system, we find that thin films require a dramatically larger field strength than the bulk to reach the inactive phase suggesting that it may be inaccessible for thin enough films. This sheds light on why the dynamics on the surface of amorphous materials is so different from bulk behavior.