本文研究在粗糙壁面纳米受限条件下Roskilde简单液体及其同构线存在的可能性。同构线是热力学相图中的曲线，在适当的无量纲化单位下，沿同构线的结构和动力学保持不变。我们通过分子动力学计算机模拟考察了两种模型液体：单组分Lennard-Jones (LJ)液体和Kob-Andersen二元LJ混合物，已知两者在体相中均存在同构线。纳米受限通过采用具有fcc晶态壁面的狭缝孔几何结构实现；这导致平行和垂直于受限壁面方向均出现非均匀密度分布。尽管存在非均匀性，且与早期研究[Ingebrigtsen等, Phys. Rev. Lett. 111, 235901 (2013)]一致，我们发现这些纳米受限液体在良好近似下具有同构线。具体而言，我们展示了沿同构线的非均匀密度分布、均方位移以及通过拓扑团簇分类算法探测的高阶结构均具有良好的标度性。据此推测，在实验中，若以适当方式（例如使用碳纳米管）进行受限，Roskilde简单液体可能表现出同构线。本研究为理解纳米受限液体提供了替代框架。
英文摘要：
We study in this paper the possible existence of Roskilde-simple liquids and their isomorphs in a rough-wall nanoconfinement. Isomorphs are curves in the thermodynamic phase diagram along which structure and dynamics are invariant in suitable nondimensionalized units. Two model liquids using molecular dynamics computer simulations are considered: the single-component Lennard-Jones (LJ) liquid and the Kob-Andersen binary LJ mixture, both of which in the bulk phases are known to have isomorphs. Nanoconfinement is implemented by adopting a slit-pore geometry with fcc crystalline walls; this implies inhomogenous density profiles both parallel and perpendicular to the confining walls. Despite this fact and consistent with an earlier study [Ingebrigtsen et. al, Phys. Rev. Lett. 111, 235901 (2013)] we find that these nanoconfined liquids have isomorphs to a good approximation. More specifically, we show good scaling of inhomogenous density profiles, mean-square displacements, and higher-order structures probed using the topological cluster classification algorithm along the isomorphs. From this study, we conjecture that in experiments, Roskilde-simple liquids may exhibit isomorphs if confined in a suitable manner, for example with carbon nanotubes. Our study thus provides an alternative framework for understanding nanoconfined liquids.
Unambiguous characterization of in-plane dielectric response in nanoconfined liquids: water as a case study
Donostia International Physics Center (DIPC)（多诺西亚国际物理中心）；Department of Applied Physics, Engineering School of Gipuzkoa, Basque Country University, UPV/EHU（巴斯克地区大学工程学校吉普斯夸应用物理系）
Recent reports of surface forces across nanoconfined ionic liquids have revealed the existence of an anomalously long-ranged interaction apparently of electrostatic origin. Ionic liquids are viscous and therefore it is important to inspect rigorously whether the observed repulsive forces are indeed equilibrium forces or, rather, arise from the viscous force during drainage of the fluid between two confining surfaces. In this paper we present our direct measurements of surface forces between mica sheets approaching in the ionic liquid [C2C1Im][NTf2], exploring three orders of magnitude in appro…
Spherical confinement can alter the properties of a dipolar fluid in several different ways. In an atomistic molecular dynamics simulation study of two different dipolar liquids (SPC/E water and a model Stockmayer fluid) confined to nanocavities of different radii ranging from Rc=1nm to 4nm, we find that the Kirkwood correlation factor remains surprisingly small in water, but not so in model Stockmayer liquid. This gives rise to an anomalous ultrafast relaxation of the total dipole moment time correlation function (DMTCF). The static dielectric constant of water under nanoconfinement (computed…
Nanoconfinement can drastically change the behavior of liquids, puzzling us with counterintuitive properties. Moreover, it is relevant in applications, including decontamination and crystallization control. It still lacks a systematic analysis for fluids with different bulk properties. Here we fill this gap. We compare, by molecular dynamics simulations, three different liquids in a graphene slit pore: (A) A simple fluid, such as argon, described by a Lennard-Jones potential; (B) An anomalous fluid, such as a liquid metal, modeled with an isotropic core-softened potential; (C) Water, the proto…
Ionic liquids constrained at interfaces or restricted in subnanometric pores are increasingly employed in modern technologies, including energy applications. Understanding the details of their behavior in these conditions is therefore critical. By using molecular dynamics simulation, we clarify theoretically and numerically the effect of confinement at the nanoscale on the static and dynamic properties of an ionic liquid. In particular, we focus on the interplay among the size of the ions, the slit pore width, and the length scale associated to the long-range organization of polar and apolar d…
We report development of a novel instrument to measure tracer diffusion in water under nano-scale confinement. A direct optical access to the confinement region, where water is confined between a tapered fiber and a flat substrate, is made possible by coating the probe with metal and opening a small aperture( 0.1 $μ$m to 1 $μ$m) at its end. A well-controlled cut using an ion beam ensures desired lateral confinement area as well as adequate illumination of the confinement gap. The probe is mounted on a tuning-fork based force sensor to control the separation between the probe and the substrate …
Liquid water is not only of obvious importance but also extremely intriguing, displaying many anomalies that still challenge our understanding of such an a priori simple system. The same is true when looking at nanoconfined water: The liquid between constituents in a cell is confined to such dimensions, and there is already evidence that such water can behave very differently from its bulk counterpart. A striking finding has been reported from computer simulations for two-dimensionally confined water: The liquid displays continuous or discontinuous melting depending on its density. In order to…
Isomorphs, hidden scale invariance, and quasiuniversality
本文首先建立了具有良好同构的经典液体（Roskilde简单液体）势能函数的近似标度性质。这种“伪齐次”性质明确表明--以及在何种意义上--这样的系统具有隐藏的标度不变性。第二部分给出了单原子Roskilde简单液体的准普适性的势能公式，最近根据约化坐标常势能超曲面的准普适性单参数族的存在性将其合理化[J. C. Dyre，Phys.Rev.E87，022106（2013）]。新的配方涉及一个准通用的简化坐标势能函数。一些后果，这是讨论。
如果液体在NVT系综中的维里和势能热平衡波动超过90%相关，则该液体被称为强相关[Phys. Rev. Lett. 100，015701（2008）]。强关联液体的波动很好地近似的逆幂律分子间势。基于这一事实，我们在这里定义“同构线”的状态图中的强关联液体。它是从计算机模拟的Kob-Andersen二进制Lennard-Jones液体，没有老化与两个同构点之间的跳跃。同构状态点具有相同的过剩熵、相同的约化平均弛豫时间、相同的（约化）动力学和相同的标度径向分布函数。最后，我们计算了伦纳德-琼斯型液体的维里/势能图中的同构方程，并表明所有此类12-6液体都具有相同的同构;这些可以缩放为主同构。
In recent years lines along which structure and dynamics are invariant to a good approximation, so-called isomorphs, have been identified in the thermodynamic phase diagrams of several model liquids and solids. This paper reports computer simulations of the transverse and longitudinal collective dynamics at different length scales along an isomorph of the Lennard-Jones system. Our findings are compared to corresponding results along an isotherm and an isochore. Confirming the theoretical prediction, the reduced-unit dynamics of the transverse momentum density is invariant to a good approximati…
In the companion paper [T. S. Ingebrigtsen and H. Tanaka, J. Phys. Chem. B 119, 11052 (2015)] the effect of size polydispersity on the nature of Lennard-Jones (LJ) liquids, which represent most molecular liquids without hydrogen bonds, was studied. More specifically, it was shown that even highly size polydisperse LJ liquids are Roskilde-simple (RS) liquids. RS liquids are liquids with strong correlation between constant volume equilibrium fluctuations of virial and potential energy and are simpler than other types of liquids. Moreover, it was shown that size polydisperse LJ liquids have isomo…
This paper continues the investigation of the exponentially repulsive EXP pair-potential system of Paper I with a focus on isomorphs in the low-temperature gas and liquid phases. As expected from the EXP system's strong virial potential-energy correlations, the system's reduced-unit structure and dynamics are isomorph invariant to a good approximation. Three methods for generating isomorphs are compared: the small-step method that is exact in the limit of small density changes and two versions of the direct-isomorph-check method that allows for much larger density changes. Results from approxi…
The invariance of several structural and dynamical properties of the Lennard-Jones (LJ) system along the freezing and melting lines is interpreted in terms of the isomorph theory. First the freezing/melting lines for LJ system are shown to be approximated by isomorphs. Then we show that the invariants observed along the freezing and melting isomorphs are also observed on other isomorphs in the liquid and crystalline phase. Structure is probed by the radial distribution function and the structure factor and dynamics is probed by the mean-square displacement, the intermediate scattering function…
We examine the Sastry (athermal cavitation) transitions for model monatomic liquids interacting via Lennard-Jones as well as shorter- and longer-ranged pair potentials. Low-temperature thermodynamically stable liquids have $ρ< ρ_S$ except when the attractive forces are long-ranged. For moderate- and short-ranged attractions, stable liquids with $ρ> ρ_S$ exist at higher temperatures; the pressures in these liquids are high, but the Sastry transition may strongly influence their cavitation under dynamic hydrostatic expansion. The temperature $T^*$ at which stable $ρ> ρ_S$ liquids emerge is $\sim…
Many liquids have curves (isomorphs) in their phase diagrams along which structure, dynamics, and some thermodynamic quantities are invariant in reduced units. A substantial part of their phase diagrams is thus effectively one dimensional. The shape of these isomorphs is described by a material-dependent function of density $h(ρ)$, which for real liquids is well approximated by a power law $ρ^γ$. However, in simulations, a power law is not adequate when density changes are large; typical models such as the Lennard-Jones liquid show that $γ(ρ) \equiv \mathrm{d} \ln h(ρ)/\mathrm{d} \ln ρ$ is a d…
The exponentially repulsive EXP pair potential defines a system of particles in terms of which simple liquids' quasiuniversality may be explained [A. K. Bacher et al., Nat. Commun. 5, 5424 (2014); J. C. Dyre, J. Phys. Condens. Matter 28, 323001 (2016)]. This paper and its companion present a detailed simulation study of the EXP system. Here we study how structure monitored via the radial distribution function and dynamics monitored via the mean-square displacement as a function of time evolve along the system's isotherms and isochores. The focus is on the gas and liquid phases, which are disti…
The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependence of the characteristic timescale associated with the specific heat is compared to the equivalent timescale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two timescales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids…
This paper presents computer simulations of Cu$_x$Zr$_{100-x}$ $(x=36,50,64)$ in the liquid and glass phases. The simulations are based on the effective-medium theory (EMT) potentials. We find good invariance of both structure and dynamics in reduced units along the isomorphs of the systems. The state points studied involve a density variation of almost a factor of two and temperatures going from 1500 K to above 4000 K for the liquids and from 500 K to above 1500 K for the glasses. For comparison, results are presented also for similar temperature variations along isochores, showing little inv…
This paper presents a numerical study of the Gay-Berne liquid crystal model with parameters corresponding to calamitic (rod-shaped) molecules. The focus is on the isotropic and nematic phases at temperatures above unity. There we find strong correlations between the virial and potential-energy thermal fluctuations, reflecting the hidden-scale invariance symmetry. This implies the existence of isomorphs, which are curves in the thermodynamic phase diagram of approximately invariant physics. We study numerically one isomorph in the isotropic phase and one in the nematic phase. In both cases, goo…
The binary Kob-Andersen (KA) Lennard-Jones mixture is the standard model for computational studies of viscous liquids and the glass transition. For very long simulations the viscous KA system crystallizes, however, by phase separating into a pure A particle phase forming an FCC crystal. We present the thermodynamic phase diagram for KA-type mixtures consisting of up to 50\% small (B) particles showing, in particular, that the melting temperature of the standard KA system at liquid density $1.2$ is $1.028(3)$ in A particle Lennard-Jones units. At large B particle concentrations the system cryst…
The glass transition plays a central role in nature as well as in industry, ranging from biological systems such as proteins and DNA to polymers and metals. Yet the fundamental understanding of the glass transition which is a prerequisite for optimized application of glass formers is still lacking. Glass formers show motional processes over an extremely broad range of timescales, covering more than ten orders of magnitude, meaning that a full understanding of the glass transition needs to comprise this tremendous range in timescales. Here we report on first-time simultaneous neutron and dielec…
