Local elastic expansion model for viscous-flow activation energies of glass-forming molecular liquids

A model for the viscosity of glass-forming molecular liquids is proposed in which a ``flow event'' requires a local volume increase. The activation energy for a flow event is identified with the work done in shoving aside the surrounding liquid; this work is proportional to the high-frequency shear modulus, which increases as the temperature decreases. The model is confirmed by experiments on a number of molecular liquids. {} 1996 The American Physical Society.