Viscosity in disordered media

The paper addresses the properties of glassforming melts in ail 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 T-g.The main attention is concentrated on the jump frequency model of Avramov and Milchev, the AM model. According to it, viscosity eta depends oil the entropy S of the system and through it on temperature and on pressure. The viscosity equation eta(P, T) = eta(infinity) exp[2.3(13.5 - 1g eta(infinity)) (T-g(P)/T)alpha] (eta measured in [dPa s]), derived in the T framework of this approach, describes the plethora of experimental data, summarized in the article, with ail accuracy, superior to that of the other models. It is demonstrated that the fragility power a depends oil heat capacity. Therefore. the composition dependence of alpha is controlled by the Deby-Neumann law. (c) 2005 Elsevier B.V. All rights reserved.