A Case for Dynamic Percolation Underlying Mechanistic Crossovers in the Relaxation of Liquids

One of the outstanding questions regarding liquid dynamics is the cause of the apparent mechanistic changes in relaxation at the characteristic temperatures T_A and T_B, where, as the temperature is lowered, α relaxation times become super-Arrhenius, and the β_JG relaxation bifurcates from α, respectively. Based on system-averaged picosecond-time scale dynamic signatures in five molecular liquids and a Kob-Andersen (KA) model system, we propose that these mechanistic changes arise from the percolation of distinct dynamic environments in the liquid, where the dynamic environment of a particle is defined by the number of structural excitations in its first solvation shell. Analysis of the KA system trajectories supports this idea and suggests that the most prominent effects can be understood in terms of environments that are mobile or immobile on a picosecond time scale. Further, the existence and percolation of these dynamic environments can account for many of the characteristic dynamic signatures of glass-forming liquids.