The Fragile-to-Strong Crossover in Simulated Supercooled Water

Glass-forming liquids usually exhibit either "fragile" or "strong" behavior approaching the glass transition. In contrast, experimental and computational evidence indicates that supercooled water exhibits a novel "fragile-to-strong crossover" on cooling at low pressure, which is related to an expected liquid-liquid transition at higher pressure. We utilize molecular dynamics simulations to investigate how the fragile-to-strong crossover alters the nature of collective molecular rearrangements in water. We compare the results from two different models, TIP4P/2005 and ST2, to identify which results are likely to be general, and not an artifact of the model. We show that the crossover can be identified with the onset of landscape-dominated dynamics. We find that the heterogeneity of molecular displacements diminishes approaching the crossover to strong behavior, which roughly coincides with the structure reaching an energetically stable random tetrahedral network. We examine how motion catalyzed by defects of the network interplays with collective motions involving larger groups of water molecules. We further consider how the changes in the heterogeneity of dynamics affect the breakdown of the Stokes-Einstein relation, which is believed to be a consequence of dynamic heterogeneity.