On the surface of glasses

TL;DR

This paper uses random first order transition theory to explain why glass surfaces are more mobile than the bulk, predicting a reduced energy barrier and a limit on configurational entropy in vapor-deposited glasses, with implications for glass transition behavior.

Contribution

It provides a theoretical analysis of surface mobility in glasses and predicts a limiting configurational entropy for vapor-deposited glasses based on surface dynamics.

Findings

Surface mobility near glasses is predicted to be twice that of the bulk.

A limiting configurational entropy for vapor-deposited glasses is derived.

Surface effects extend into the bulk, affecting the glass transition temperature.

Abstract

Dynamics near the surface of glasses is generally much faster than in the bulk. Neglecting static perturbations of structure at the surface, we use random first order transition theory to show the free energy barrier for activated motion near a free surface should be half that of the bulk at the same temperature. The increased mobility allows the surface layers to descend much further on the energy landscape than the bulk ordinarily does. The simplified RFOT calculation however predicts a limiting value for the configurational entropy a vapor deposited glass may reach as a function of deposition rate. We sketch how mode coupling effects extend the excess free surface mobility into the bulk so that the glass transition temperature is measurably perturbed at depths greater than the naive length scale of dynamic cooperativity.