Viscosity in disordered media

The paper addresses the properties of glassforming melts in an attempt to predict the dependence of viscosity on temperature and on pressure. A review on the main viscosity models (free volume model, the Adam and Gibbs model etc.) is given. It is demonstrated that the activation energy can be expressed through the glass-transition temperature Tg. The main attention is concentrated on the jump frequency model of Avramov and Milchev, the AM model. According to it, viscosity η depends on the entropy S of the system and through it on temperature and on pressure. The viscosity equation η ( P , T ) = η ∞ exp [ 2.3 ( 13.5 - lg η ∞ ) ( T g ( P ) T ) α ] (η measured in [dPa s]), derived in the framework of this approach, describes the plethora of experimental data, summarized in the article, with an accuracy, superior to that of the other models. It is demonstrated that the fragility power α depends on heat capacity. Therefore, the composition dependence of α is controlled by the Deby–Neumann law.