Modeling the anomalous behaviors of supercooled tetrahedral liquids

Tetrahedral liquids such as water and silica are among the most ubiquitous and important substances in our world, yet the origin of their well-known structural, thermophysical, and dynamic anomalies still remains elusive. It has been posited that anomalous behaviors of tetrahedral liquids under ambient and supercooled conditions are due to critical fluctuations associated with a low-temperature liquidliquid critical point (LLCP), below which two distinct metastable liquids undergo a first-order liquid-liquid phase transition (LLPT). Due to rapid homogeneous nucleation of the crystalline phase, experimental probing techniques have not yet been able to verify or falsify this hypothesis. Computational studies of molecular fluids demonstrate metastable LLPTs are possible, but they have not resolved the outstanding question of how such behavior can be characterized experimentally. We present results from large-scale molecular dynamics (MD) simulations of more than 100,000 molecules of two model tetrahedral liquids that exhibit LLPTs: the ST2 model of water and an ionic model of liquid silica. The simulations reveal that both models exhibit anomalous scattering, reminiscent of that observed in experiment, which is characterized by an increase in the static structure factor at low wavenumbers. This unusual behavior is linked with coupled fluctuations in density and local tetrahedral order in the liquid. The Ornstein-Zernike correlation length estimated from the anomalous scattering component exhibits power-law growth upon cooling, consistent with the existence of a liquid-liquid critical point in both models. Further, spontaneous liquid-liquid phase separation is observed in each model upon thermally quenching large systems into the two-phase regions. Lastly, we show that nucleation of the stable crystal phase from metastable tetrahedral liquids, a rare event, can be investigated by enhanced sampling methods based on the hybrid Monte Carlo (HMC) algorithm.