Abstract
Two decades of experimental research indicate that spatial confinement of glass-forming molecular
and polymeric liquids results in major changes of their slow dynamics beginning at large confinement
distances. A fundamental understanding remains elusive given the generic complexity of activated
relaxation in supercooled liquids and the major complications of geometric confinement, interfacial
effects, and spatial inhomogeneity. We construct a predictive, quantitative, force-level theory of relaxation
in free-standing films for the central question of the nature of the spatial mobility gradient.
The key new idea is that vapor interfaces speed up barrier hopping in two distinct, but coupled,
ways by reducing near surface local caging constraints and spatially long range collective elastic
distortion. Effective vitrification temperatures, dynamic length scales, and mobile layer thicknesses
naturally follow. Our results provide a unified basis for central observations of dynamic and pseudothermodynamic
measurements.