Fragility of glass-forming systems and the width of the glass transition

The concept of fragility is now commonly used to classify glass-forming liquids. In the original description by Angell, the degree of fragility is most easily determined from the curvature of the viscosity on an Arrhenius plot. Other measures of fragility are discussed here which may be more accessible to experiments and computer simulations than viscosity data. Numerical values of different measures of fragility are related and experimental data are compared or a large number of systems. It is shown that the degree of fragility is independent of the glass transition temperature of the system, and that the glass transition temperature and the degree of fragility together are sufficient to specify the kinetics of the system in the glass transition regime. Based on an overview of different systems, varying degrees of fragility are tentatively explained in terms of the range of microscopic forces in the liquid.