Scale-Free Interfacial Dynamic Decoupling Drives Free-Surface Effects on the Glass Transition

A quarter-century of effort has been focused on the study of glass formation in thin films, in large part motivated by the hope that the behavior of these systems will elucidate underlying length scales and mechanisms of glass formation. Here we show that these alterations in dynamics at free surfaces are driven by a scale-free local decoupling phenomenon. We find that the apparent activation barrier is truncated in a nearly temperature-invariant manner near the interface, with a power law recovery of bulk-like activation behavior in the far field. We show that this result gives rise to the temperature dependence of ‘apparent’ length scales of interfacial dynamics observed in simulation and experiment, despite the absence of any true underlying length scale in the physics of the system. This scenario is difficult to reconcile with models centering around an underlying temperature-variant thermodynamic or dynamical length scale of glass formation, potentially narrowing the range of viable theoretical explanations for this phenomenon and pointing towards an important role for elastic barriers in supercooled liquid dynamics.