Kinetic and Thermodynamic Fragilities of Square Well Fluids with Tunable Barriers to Bond Breaking.

An understanding of the origin of fragility, which the rapidity of change of viscosity and related dynamical quantities, has been sought by a variety of approaches over the years. Within the framework of the Adam-Gibbs relation, fragility is in principle related to both the temperature variation of configurational entropy and the high temperature activation energy. Many theoretical analyses have been focused on the variation of configuration entropy, although the importance of the high temperature activation energy in determining the fragility of a glass former has also been emphasized. We explore the latter aspect by considering a model liquid whose high temperature activation energy is modified by hand, through the introduction of a tunable barrier to bond breaking. We show that changes in such a barrier are able to modify the fragility measured from the temperature dependence of dynamical quantities, while a thermodynamic measure of fragility obtained from the configurational entropy remains unchanged. We discuss the implications of our results to our understanding of fragility, and outline open questions that merit further investigation.