Source of non-arrhenius average relaxation time in glass-forming liquids

A major mystery of glass-forming liquids is the non-Arrhenius temperature-dependence of the average relaxation time. This paper briefly reviews the classical phenomenological models for non-Arrhenius behavior – the free volume model and the entropy model – and critiques against these models. We then discuss a recently proposed model according to which the activation energy of the average relaxation time is determined by the work done in shoving aside the surrounding liquid to create space needed for a "flow event". In this model, which is based on the fact that intermolecular interactions are anharmonic, the non-Arrhenius temperature-dependence of the average relaxation time is a consequence of the fact that the instantaneous shear modulus increases upon cooling.