A Structural Approach to Relaxation in Glassy Liquids
Fig. 1: The characteristics of the softness field*
[Reprinted by permission from Macmillan Publishers Ltd: Nature Physics ©2016]
In contrast with crystallization, there is no noticeable structural change at the glass transition. Characteristic features of glassy dynamics that appear below an onset temperature, T0, are qualitatively captured by mean field theory, which assumes uniform local structure. Studies of more realistic systems have found only weak correlations between structure and dynamics. This raises the question: is structure important to glassy dynamics in three dimensions?
In the May issue of Nature Physics*, Prof. Efthimios Kaxiras, SEAS grad student Ekin Dogus Cubuk, and colleagues from University of Pennsylvania answer this question affirmatively, using machine learning to identify a new field, 'softness,' which characterizes local structure and is strongly correlated with dynamics. The authors find that the onset of glassy dynamics at T0 corresponds to the onset of correlations between softness (that is, structure) and dynamics. Moreover, they construct a simple model of relaxation that agrees well with their simulation results, showing that a theory of the evolution of softness in time would constitute a theory of glassy dynamics.
* Read S.S. Schoenholz, E.D. Cubuk, D.M. Sussman, E. Kaxiras & A.J. Liu, "A structural approach to relaxation in glassy liquids," Nature Physics 12, 469–471 (2016) doi:10.1038/nphys3644.