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

Thomas E. Gartner III, Salvatore Torquato, Roberto Car, and Pablo G. Debenedetti, Nature Communications, 12, 3398 (2021)

Water‚Äôs 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‚Äôs 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‚Äôs 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 ‚Äėnon-equilibrium¬†index‚Äô, 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.