Theoretical and experimental study of excess quantities in glass-forming systems

The present work deals with the correlation among a macroscopic quantity, i.e. kinematic viscosity, and some microscopic quantities, namely the mean-square displacement and the free volume. In primis, some definitions of ‘fragility’, a quantity which allows to rank different liquids on the basis of the temperature dependence of kinematic and/or thermodynamic quantities, are recalled. Successively, the temperature dependence of viscosity, mean-square displacement and free volume is taken into account for three glass-forming systems and is analyzed in order to highlight the linear relationships between the logarithm of viscosity and the excess mean-square displacement and between the logarithm of viscosity and the excess free volume. Finally, on the basis of the observed correlations, two ‘fragility’ parameters, operatively evaluated by elastic neutron scattering and positron annihilation lifetime spectroscopy, are discussed and compared.