  Two decades of experimental research indicates 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 pseudo-thermodynamic measurements.
