Prof Adrian Wright
Quenching to the vitreous state is discussed from the point of view of the inherent fluctuations in both density and composition. Three examples are considered, viz. a random packing of hard spheres, vitreous silica, and “normal” glasses, for which the structure of the super-cooled liquid is defined by one or more chemical equilibria. For single-component glasses (e.g. SiO2), the true measure of the (“free”) volume available for the mechanism of viscous flow (bond switching) lies with the mean square number density fluctuation <Δρ2> whereas, for glasses having more than one component, it is essential to take into account the temperature dependence of the various chemical equilibria, for which the equilibriation time is in general much longer than the times for bond switching and structural relaxation. It is also concluded that the presence of spatial fluctuations in both number density and composition has an important effect on the bulk (macroscopic) physical and chemical properties of the vitreous state.