Slow Relaxation, Spatial Mobility Gradients and Vitrification in Confined Films

TL;DR

This paper develops a force-level theory to understand how vapor interfaces influence relaxation dynamics and mobility gradients in confined glass-forming films, explaining key experimental observations.

Contribution

It introduces a predictive, quantitative theory that explains the effects of vapor interfaces on relaxation and mobility gradients in confined glassy films.

Findings

Vapor interfaces accelerate barrier hopping via local and elastic effects.

The theory predicts vitrification temperatures and dynamic length scales.

Results unify various experimental observations of confined glassy dynamics.

Abstract

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.