Mobility and stability of glasses

Abstract

Recently, the mobility and stability of glasses have received a great deal of attention. In particular, there has been a resurgence in trying to understand the mechanism by which mechanical stress or deformation imparts excess mobility (faster dynamics) to glassy and jammed systems. Because there is still no recognized structural signature that identifies the glassy or jammed state, appearing as a frozen amorphous ‘‘liquid,’’ it is not completely clear what structural changes occur with increased mobility via deformation or temperature. It is also not clear if the transition to a more mobile (or more fluid-like) state via increasing mechanical load is the same as that occurring via increasing temperature. There are reports that observe some surprisingly common features between these very different systems. One of the big questions that remain is: How is the molecular glass transition obtained on decreasing temperature related to the colloidal glass transition or jamming transition in granular media obtained on increasing volume fraction? Understanding the common elements that these disparate systems display should provide insight into the underlying dynamics and packing frustration. Current efforts analyze these systems in a variety of different ways: potential energy barriers and landscapes, distribution of relaxation times, density fluctuations, distribution of force chains, shear transformation zones, and a random first order transition theory. Many of these approaches have in common the idea of local regions comprised of clusters of units (e.g., molecules, polymer segments or colloidal particles), which may be dynamically faster or slower than the average structural relaxation time, where changes in stress or strain can concentrate.