Surface facilitated softening of rapidly heated glasses: recent experimental results and the fundamental mechanisms

Under certain conditions, e.g., during rapid heating, stable vapor-deposited, ordinary glassy, and even viscous liquid films can undergo softening by surface facilitated mechanism: the softening originates at the surface of a sample and progresses into its bulk via a transformation front. Despite years of research on this phenomenon, no simple theoretical model is currently available for quantitative description of glass softening by a propagating front. Based on our recent fast scanning calorimetry studies of surface facilitated glass softening and melting of superheated molecular crystals, we developed a comprehensive analytical description of the softening front propagation. This description is essentially a modification of well-established Wilson-Frenkel theory of non-equilibrium phase transitions. We present further experimental evidence in support of this analytical approach, and discuss the model’s advantages, formalism, limitations, and applicability under a variety of experimental conditions.