Manifestations of metastable criticality in the long-range structure of model water glasses

Water’s metastable phase behavior has, for decades, been a source of interest to researchers across a broad range of the physical sciences. Much attention has been devoted to water’s polyamorphism (multiple amorphous solid phases) and to the hypothesized metastable liquid-liquid transition and associated critical point (LLCP). However, the possible relationship between these phenomena remains incompletely understood. Using molecular dynamics simulations of the realistic TIP4P/2005 model, we found a striking signature of the LLCP in the structure of water glasses, manifested as a pronounced increase in long-range density fluctuations in the vicinity of the critical pressure associated with this model’s liquid-liquid transition. By contrast, such long-range density fluctuations were absent in glasses of two model systems that lack an LLCP. We also characterized the effect of applied pressure on the departure from equilibrium upon glass formation, as quantified by the ‘non-equilibrium index’, and found that water-like systems exhibited a strong pressure dependence in this metric, whereas simple liquids did not. These results reflect a surprising relationship between the metastable equilibrium phenomenon of the LLCP and the non-equilibrium long-range structure of glassy water, with implications for our understanding of water phase behavior, and more broadly of glass physics. Furthermore, our computational approach suggests a possible experimental route to probing the existence of the LLCP in water and other fluids.