In simulations of a water-like model (ST2) that exhibits a liquid-liquid phase transition, we test for the occurrence of a thermodynamic region in which the liquid can be modelled as a two-component mixture. We assign each molecule to one of two species based on the distance to its fifth-nearest neighbor, and evaluate the concentration of each species over a wide range of temperature and density. Our concentration data compare well with mixture-model predictions in a region between the liquid-liquid critical temperature and the temperature of maximum density. Fits of the model to the data in this region yield accurate estimates for the location of the critical point. We also show that the liquid outside the region of density anomalies is poorly modelled as a simple mixture.
We use the recently-proposed \emph{compressible cell} Ising-like model [Phys. Rev. Lett. \textbf{120}, 120603 (2018)] to estimate the ratio between thermal expansivity and specific heat (the Grüneisen parameter $Γ$) in supercooled water. Near the critical pressure and temperature, $Γ$ increases. The $Γ$ value diverges near the pressure-induced finite-$T$ critical end-point [Phys. Rev. Lett. \textbf{104}, 245701 (2010)] and quantum critical points [Phys. Rev. Lett. \textbf{91}, 066404 (2003)], which indicates that two energy scales are governing the system. This enhanced behavior of $Γ$ is caus…
Free energy of formation of small ice nuclei near the Widom line in simulations of supercooled water
ST 2相互作用势已被用于大量的模拟研究，以探索在过冷水中的液-液相变（LLPT）的可能性。使用伞采样的ST 2水的蒙特卡罗模拟，我们评估的自由能形成的小冰核的过冷液体中的Widom线附近，该区域以上的临界温度的LLPT在那里发生了一些热力学异常。我们的研究结果表明，在这一地区有大量的自由能成本形成的小冰核，表明与Widom线在ST 2水的热力学异常发生在一个定义明确的亚稳液相。通过Widom线，我们确定的自由能的变化，形成小的冰核，说明如何与LLPT相关的热力学异常可能会影响冰成核过程。
Recent computational studies have reported evidence of a metastable liquid-liquid phase transition (LLPT) in molecular models of water under deeply supercooled conditions. A competing hypothesis suggests, however, that non-equilibrium artifacts associated with coarsening of the stable crystal phase have been mistaken for an LLPT in these models. Such artifacts are posited to arise due to a separation of time scales in which density fluctuations in the supercooled liquid relax orders of magnitude faster than those associated with bond-orientational order. Here, we use molecular simulation to in…
One of the most promising frameworks for understanding the anomalies of cold and supercooled water postulates the existence of two competing, interconvertible local structures. If the non-ideality in the Gibbs energy of mixing overcomes the ideal entropy of mixing of these two structures, a liquid-liquid phase transition, terminated at a liquid-liquid critical point, is predicted. Various versions of the "two-structure equation of state" (TSEOS) based on this concept have shown remarkable agreement with both experimental data for metastable, deeply supercooled water and simulations of molecula…
Metastable liquid mixtures of parahydrogen and orthodeuterium are studied theoretically by means of computer simulations. No reduced propensity of the mixture to undergo crystallization is observed, compared to that of pure liquid parahydrogen. Demixing of the two species as a precursor of crystallization is not observed either.
Water shows intriguing thermodynamic and dynamic anomalies in the supercooled liquid state. One possible explanation of the origin of these anomalies lies in the existence of a metastable liquid-liquid phase transition (LLPT) between two (high and low density) forms of water. While the anomalies are observed in experiments on bulk and confined water and by computer simulation studies of water-like models, the existence of a LLPT in water is still debated. Unambiguous experimental proof of the existence of a LLPT in bulk supercooled water is hampered by fast ice nucleation which is a precursor …
We present a model for glassy dynamics in supercooled liquid mixtures. Given the relaxation behavior of individual supercooled liquids, the model predicts the relaxation times of their mixtures as temperature is decreased. The model is based on dynamical facilitation theory for glassy dynamics, which provides a physical basis for relaxation and vitrification of a supercooled liquid. This is in contrast to empirical linear interpolations such as the Gordon-Taylor equation typically used to predict glass transition temperatures of liquid mixtures. To understand the behavior of supercooled liquid…
Twenty years ago Poole et al. (Nature 360, 324, 1992) suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of liquid-liquid separation of lower-density and higher-density water. Here we present an explicit thermodynamic model based on this hypothesis, which describes all available experimental data for supercooled water with better quality and with fewer adjustable parameters than any other model suggested so far. Liquid water at low temperatures is viewed as an 'athermal solution' of two molecular structures with different entro…
Two-state thermodynamics of the ST2 model for supercooled water
ST 2模型的热力学性质的过冷液体水表现出类似于在真实的水观察到的异常。这些异常现象的一个可能的解释是存在一个亚稳态的，由临界点终止的液-液转变。这种现象，其可能存在于真实的水是目前许多实验工作的主题，已经明确地证明了这个特定的模型，最新的模拟。在这项工作中，我们再现了两个版本的ST 2模型的状态方程描述水作为一个非理想的“混合物”的两种不同类型的本地分子秩序的异常。我们表明，在ST 2水的液-液转变是能量驱动的。这与另一种流行的模型mW相反，在mW中，两种替代的局部分子顺序的混合的非理想性是熵驱动的，并且不足以强到诱导液-液转变。
We numerically investigate the metastable equilibrium structure of deep supercooled and glassy water under pressure, covering the range of densities corresponding to the experimentally produced high-density and very-high-density amorphous phases. At $T = 188$ K, a continuous increase in density is observed on varying pressure from $2.5$ to $13$ kbar, with no signs of first-order transitions. Exploiting a recently proposed approach to the analysis of the radial distribution function -- based on topological properties of the hydrogen-bond network -- we are able to identify well-defined local geo…
Motivated by an experimental finding on the density of supercooled water at high pressure [O. Mishima, J. Chem. Phys. 133, 144503 (2010)] we performed atomistic molecular dynamics simulations study of bulk water in the isothermal-isobaric ensemble. Cooling and heating cycles at different isobars and isothermal compression at different temperatures are performed on the water sample with pressures that range from 0 to 1.0 GPa. The cooling simulations are done at temperatures that range from 40 K to 380 K using two different cooling rates, 10 K/ns and 10 K/5 ns. For the heating simulations we use…
We present a phase diagram for water confined to cylindrical silica nanopores in terms of pressure, temperature and pore radius. The confining cylindrical wall is hydrophilic and disordered, which has a destabilizing effect on ordered water structure. The phase diagram for this class of systems is derived from general arguments, with parameters taken from experimental observations and computer simulations and with assumptions tested by computer simulation. Phase space divides into three regions: a single liquid, a crystal-like solid, and glass. For large pores, radii exceeding 1 nm, water exhi…
Liquid water is well-known for its intriguing thermodynamic anomalies in the supercooled state. The phenomenological two-state models - based on the assumption of the existence of two types of competing local states (or, structures) in liquid water - have been extremely successful in describing water's thermodynamic anomalies. However, the precise structural features of these competing local states in liquid water still remain elusive. Here, we have employed a geometrical order parameter-free approach to unambiguously identify the two types of competing local states -- entropically and energet…
Importance of nuclear quantum effects on the structure of supercooled water around its liquid--liquid critical point
Center for Advanced Systems Understanding (CASUS)（先进系统理解中心）；Helmholtz Zentrum Dresden-Rossendorf（德累斯顿-罗斯多夫亥姆霍兹中心）；Paderborn University（帕德博恩大学）；Technische Universität Dresden（德累斯顿工业大学）
The paper discusses the features of supercooled water thin film of width $d=3.97$~nm contained by the perfect graphene layers and crystallizing under external stationary electric field. It was found that the electric field applied perpendicular to graphene layers impedes structural ordering, while the electric field applied in lateral direction contributes to formation of the cubic ice ($Ic$) phase, which is thermodynamically less stable compared to the hexagonal ice ($Ih$) phase. It is shown that the growth of the $Ic$ crystalline phase occurs without formation of intermediate crystalline pha…
Many successful theories of liquids near the melting temperature assume that small length scale density fluctuations follow Gaussian statistics. In this paper I present numerical investigations of fluctuations in the supercooled viscous regime using an enhance sampling method. I present results for the single component Lennard-Jones liquid, the Kob-Andersen binary mixture, the Wahnström binary mixture, the Lewis-Wahnström model of ortho-terphenyl and for the TIP4P/Ice model of water. Results show that the Gaussian approximation persist to a good degree into the supercooled viscous regime, howe…
One of the most promising frameworks for understanding the anomalies of cold and especially supercooled water is that of two-structure thermodynamics, in which water is viewed as a non-ideal mixture of two interconvertible local structures. The non-ideality of this mixture may give rise, at very low temperatures, to a liquid-liquid phase transition (LLPT) and a liquid-liquid critical point (LLCP), at which thermodynamic response functions diverge. Various versions of the "two-structure equation of state" (TSEOS) based on this concept have shown remarkable agreement with both experimental data …
Liquid-liquid phase transition (LLPT) in supercooled water has been a long-standing controversial issue. We show simulation results of real stable first-order phase transitions between high and low density liquid (HDL and LDL)-like structures in confined supercooled water in both positive and negative pressures. These topological phase transitions originate from H-bond network ordering in molecular rotational mode after molecular exchanges are frozen. It is explained by the order parameter-dependent free energy change upon mixing liquid-like and ice-like moieties of H-bond orientations which i…
Molecular dynamics simulations have been performed on TIP4P/2005 supercooled water to investigate the molecular diffusion and shear viscosity at various timescales and assess the Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations. For this purpose, we calculated various time correlation functions, such as the mean-squared displacement, stress relaxation function, density correlation function, hydrogen-bond correlation function, rotational correlation function of molecular orientation, non-Gaussian parameter, and four-point correlation function. Our study of the SE and SED relations…
