Dynamics of polydisperse hard-spheres under strong confinement

We use molecular simulation to probe the connection between local structure and the unusual re-entrant dynamics observed for polydisperse hard-sphere liquids confined in thin slit pores. The local structure is characterised by calculating 2-D bond-orientational order parameters associated with square and hexatic order for particles in the layer adjacent to the confining walls. When the wall separation is commensurate with the average particle size, the particles primarily exhibit local hexatic order, whereas local square order increases in prevalence for incommensurate geometries. The relaxation time extracted from the ensemble-averaged mean-square displacement increases exponentially with the static correlation length associated with hexatic local order in strongly confined commensurate geometries, in agreement with theoretical predictions for dynamical slowing. Square order, by contrast, is not associated with a growing length scale for either commensurate or incommensurate geometries, indicating that it is strongly geometrically frustrated. Our results suggest that the influence of bond-orientational order on dynamical slowing may be altered by changing the extent of confinement.     

The structure of strongly dipolar hard sphere fluids with extended dipoles by Monte Carlo simulations

We have investigated the structural change of dipolar hard sphere fluid while we change the dipole from an idealised point dipole (pDHS fluid) to a physically more realistic extended dipole (eDHS fluid) by increasing the distance d of the two point charges ±q while keeping the dipole moment μ = qd fixed. We discuss our results on the basis of the first- and second-rank orientational order parameters, angular distribution functions, chain-length distributions, and snapshots. At a low density, we have found chain formation with longer chains as the distance d is increased. At a high density, we have found phase transition from an orientationally ordered ferroelectric nematic phase (at low d) into an isotropic liquid containing chains (at large d).     

Comment on symmetry properties of the linear Enskog kinetic operators

The one-particle average consistent with the structure of the revised Enskog theory is introduced. Symmetry properties of the linear kinetic operators reflecting those of theN-particle pseudo-Liouville operators are derived, implying a recently proved symmetry of kinetic expressions for equilibrium time correlation functions.     

On the Brownian motion of a massive sphere suspended in a hard-sphere fluid. II. Molecular dynamics estimates of the friction coefficient

The friction coefficient exerted by a hard-sphere fluid on an infinitely massive Brownian sphere is calculated for several size ratios , where and are the diameters of the Brownian and fluid spheres, respectively. The exact microscopic expression derived in part I of this work from kinetic theory is transformed and shown to be proportional to the time integral of the autocorrelation function of the momentum transferred from the fluid to the Brownian sphere during instantaneous collisions. Three different methods are described to extract the friction coefficient from molecular dynamics simulations carried out onfinite systems. The three independent methods lead to estimates of which agree within statisticalerrors (typically 5%). The results are compared to the predictions of Enskog theory and of the hydrodynamic Stokes law. The former breaks down as the size ratio and/or the packing fraction of the fluid increase. Somewhat surprisingly, Stokes' law is found to hold withstick boundary conditions, in the range 1/4.5 explored in the present simulations, with a hydrodynamic diameterd=. The analysis of the moleuclar dynamics data on the basis of Stokes' law withslip boundary conditions is less conclusive, although the right trend is found as / increases.     

Interpretation of 1H and 2H spin–lattice relaxation dispersions: Insights from molecular dynamics simulations of polymer melts

We demonstrate that molecular dynamics simulations are a versatile tool to ascertain the interpretation of spin–lattice relaxation data. For ¹H, our simulation approach allows us to separate and to compare intra- and inter-molecular contributions to spin–lattice relaxation dispersions. Dealing with the important example of polymer melts, we show that the intramolecular parts of ¹H spectral densities and correlation functions are governed by rotational motion, while their inter-molecular counterparts provide access to translational motion, in particular, to mean-squared displacements and self-diffusion coefficients. Exploiting that the full microscopic information is available from molecular dynamics simulations, we determine the range of validity of experimental approaches, which often assume Gaussian dynamics, and we provide guidelines for the determination of free parameters required in experimental analyses. For ²H, we examine the traditional methodology to extract correlation times of complex dynamics from relaxation data. Furthermore, based on knowledge from our computational study, it is shown that measurement of ²H spin–lattice relaxation dispersions allows one to disentangle the intra- and inter-molecular contributions to the corresponding ¹H data in experimental work. Altogether, our simulation results yield a solid basis for future ¹H and ²H spin–lattice relaxation analysis.     

Pierre Résibois: 1936–1979

Main aspects of Résibois' scientific work.     

Shear viscosity of aluminum studied by shock compression considering elasto-plastic effects

The strength always exists before the material melts. In this paper, the viscoelastic-plastic model is applied to improve the finite difference method, and the numerical solutions for the disturbance amplitude damping behavior of the sinusoidal shock front in a flyer-impact experiment are obtained. When the aluminum is shocked to 101 GPa, the effect of elastoplasticity on the zero-amplitude point of the oscillatory damping curve is the same as that of viscosity when η = 700 Pa·s,and the real shear viscosity coefficient of the shocked aluminum is determined to be about 2800±100 Pa·s. Comparing the experiment data with the numerical results of the viscoelastic-plastic model, we find that the aluminum is close to melting at 101 GPa.     

Simple theoretical model of shear viscosity in isotopic fluid mixtures

We propose a simple hybrid model for the shear viscosity of isotopic fluid mixtures by coupling the contribution of the Stokes–Einstein relation with the existing linear model of Roults's law for the shear viscosity. The calculated values of shear viscosity using this simple hybrid model are found to be in excellent agreement with the molecular dynamics (MD) simulation results. The calculated value of the shear viscosity obtained from the theoretical model as well as the MD simulation increases with increasing mass ratio.     

基于密度泛函的伪麻黄碱拉曼光谱性质研究

研究伪麻黄碱的拉曼光谱和吸附在纳米银基底上的表面增强拉曼光谱(SERS),利用密度泛函理论B3LYP/6-311G++(d, p)方法对伪麻黄碱分子进行了计算,得到了分子构型信息和理论拉曼光谱,用Gaussview软件对分子振动模式进行了全面的归属,在伪麻黄碱的表面增强拉曼光谱中,采用了自组装方法获得了团簇银纳米表面增强基底,实现了很好的增强效应.实验结果表明：伪麻黄碱的拉曼光谱计算结果和实验结果基本一致,理论计算为伪麻黄碱分子振动峰位的归属提供了重要的依据,伪麻黄碱分子与银纳米表面化学吸附,苯环垂直于纳米基底表面,研究结果为伪麻黄碱的拉曼光谱检验分析提供了理论依据,也为苯丙胺类毒品的光谱分析研究提供了参考.     

Mode-coupling effects in the density dependence of the shear viscosities of dense argon and methane

We argue that the density dependence of the shear viscosities of dense argon at 174, 223, and 301 K and of dense methane at 298 K can be understood on the basis of the mode coupling theory for hard spheres, in particular near the fluid-solid phase transition.     

有限温度下Na_4 团簇赝转动的局域Lyapunov指数研究

:在紧束缚分子动力学模型基础上,对具有D2h对称性的Na4团簇在不同温度T=76K、112K、212K和224K下的热力学行为进行了研究。根据局域Lyapunov指数分布的变化,对由温度T=76K、112K向温度T=212K、224K的变化中会出现的赝转动所表现的混沌现象进行了分析     

Transport Coefficients in Some Stochastic Models of the Revised Enskog Equation

A stochastic model of the revised Enskog equation is considered. A choice of the smearing function suggested by the work of Leegwater is used to apply the model to the repulsive part of the Lennard-Jones potential and the inverse-power soft-sphere potential. The virial coefficients obtained from the equilibrium properties of the models are in excellent agreement with the known exact coefficients for these models. The transport coefficients for the repulsive Lennard-Jones (RLP) model are also computed and appear to be of comparable accuracy to the Enskog-theory coefficients applied directly to a hard-sphere system, although exact results for the RLP with which to make an extensive comparison are not yet available. The pressure and the transport coefficients obtained from the model (shear viscosity, thermal conductivity, and self-diffusion) are compared with the pressure and the corresponding transport coefficients predicted by the Enskog and square-well kinetic theories.     

Molecular dynamics study on superheating of Pd at high heating rates

Molecular dynamics simulations are employed here to study the melting and superheating behaviors of bulk Palladium at high heating rates. Quantum Sutton-Chen many body potential is used for these simulations. Being heated, the superheating and melting behavior is found to be strongly affected by the heating rate, and heating rate induced randomization during non-equilibrium heating processes is found to be the main driving force for phase transformation, and it eliminates the energy barrier for nucleation. Not only Pd crystals but also Pd crystals with defects are studied. And the upper limit of heating rate induced superheating is determined to be around 2100?K.     

Dependence of the viscosity of nanofluids on nanoparticle size and material

The viscosity of nanofluids as a function of nanoparticle size and material is modeled and analyzed. Dependences of the viscosity of nanofluids based on liquid argon with aluminum and lithium nanoparticles are obtained. The nanoparticle size ranges from 1 to 4 nm. The volume concentration of nanoparticles is varied from 1% to 12%. It is shown that the viscosity of the nanofluid increases with decreasing nanoparticle size and, in addition, depends on the nanoparticle material.     

Bulk and shear viscosities of a polydisperse hard-sphere fluid

The bulk and shear viscosity expressions for a multicomponent hard-sphere mixture in the Enskog transport theory are generalized to the polydisperse limit. The effect of polydispersity is expressed in terms of correction factors to the monodisperse fluid results. These correction factors have been evaluated for both a log-normal size distribution with the mass-size relation of a power-law form and a log-normal mass distribution with fixed particle size, which is the continuous limit of isotopes.     

Molecular dynamics results for the radial distribution functions of highly asymmetric hard sphere mixtures

Molecular dynamics (MD) results for the radial distribution functions of mixtures of large and small hard spheres are reported for size ratios whose (large/small) values are 1, 2.5, 5, 7.5, and 10 in the region where the concentration of the large spheres is very small. The MD contact values of these functions are compared with formulae due to Boublik, Mansoori, Carnahan, Starling, Leland, Grundke, and Henderson, Viduna and Smith, Henderson, Trokhymchuk, Woodcock, and Chan, as well as new formulae that are considered here. The new formulae give good agreement for the large–small contact values and reasonably good agreement for the large–large contact values. The Viduna–Smith formula is satisfactory for the small–small contact value and quite reasonable for the small–large contact value. Undoubtedly, further improvements are possible. These results give insight into what may be called the colloidal limit, where the size ratio is exceedingly large while the concentration of the large spheres is exceedingly small, and into the passage to this limit.     

CO_2水合物导热特性的分子动力学模拟

使用分子动力学模拟方法在NVT系综下对结构完整CO_2水合物以及结构缺陷CO_2水合物进行了导热模拟计算.对于结构完整的CO_2水合物,在200-230 K温度区间内,体系导热系数由0.4684 W·m~(-1)·K~(-1)变化到0.4836 W·m~(-1)·K~(-1),温度相关性较弱;而在230-280 K温度区间内,体系导热系数由0.4836 W·m~(-1)·K~(-1)变化到0.7494 W·m~(-1)·K~(-1),温度相关性变强;另外,通过计算功率图谱发现主体分子对水合物体系的导热贡献更大.对于结构缺陷CO_2水合物,发现晶穴占有率和笼形结构缺陷对体系导热均有一定影响,空笼晶胞导热系数约为完整晶胞导热系数的86.67%,体系的导热能力主要取决于主体结构的性质.     

Simulations of the chain length dependence of the melting mechanism in short-chain n-alkane monolayers on graphite

The melting transition in solid monolayers of a series of short-chained n-alkanes, n-octane (n-C₈H₁₈), n-decane (n-C₁₀H₂₂), and n-dodecane (n-C₁₂H₂₆) physisorbed onto the graphite basal plane are studied through use of molecular dynamics simulations. Utilizing previous experimental observations of the solid phase behavior of these monolayers, this study investigates the temperature dependence of the phases and phase transitions in these three monolayers during the solid-fluid phase transition, and compares the observed melting behavior to previous studies of hexane and butane monolayers. In particular, this study seems to indicate a greater dependence of the melting transition on the formation of gauche defects in the alkyl chains as the chain length is increased. In light of the previously proposed “footprint reduction” mechanism and variations where the formation of gauche defects are energetically negated, simulations seem to suggest that decane and dodecane monolayers are generally equally as dependent upon the formation of gauche defects for the melting transition to take place, whereas octane monolayers seem to have less dependence, but follow a trend that is established in previous studies of melting in butane and hexane monolayers. Also, the phase transition from a solid herringbone phase into an orientationally ordered “intermediate” phase is found to exhibit some differences as compared to a recent study of hexane monolayers, which may be interpreted as originating from the greater influence of gauche defects. Comparison to experimental melting temperatures is provided where possible, and applications involving thin film manipulation and lubrication is discussed.     

A molecular dynamics study of fuel droplet evaporation in sub- and supercritical conditions

Evaporation processes of a fuel droplet under sub- and supercritical ambient conditions have been studied using molecular dynamics (MD) simulations. Suspended n-dodecane droplets of various initial diameters evaporating into a nitrogen environment are considered. Both ambient pressure and temperature are varied from sub- to supercritical values, crossing the critical condition of the chosen fuel. Temporal variation in the droplet diameter is obtained and the droplet lifetime is recorded. The time at which supercritical transition happens is determined by calculating the temperature and concentration distributions of the system and comparing with the critical mixing point of the n-dodecane/nitrogen binary system. The dependence of evaporation characteristics on ambient conditions and droplet size is quantified. It is found that the droplet lifetime decreases with increasing ambient pressure and/or temperature. Supercritical transition time decreases with increasing ambient pressure and temperature as well. The droplet heat-up time as well as subcritical to supercritical transition time increases linearly with the initial droplet size d₀, while the droplet lifetime increases linearly with ${\mathrm{d}}_0^2$. A regime diagram is obtained, which indicates the subcritical and supercritical regions as a function of ambient temperature and pressure as well as the initial droplet size.     

冲击加载下铝的剪切模量

分别用Steinberg-Cochran-Guinan (SCG)模型、修正的SCG模型和有限应变理论对材料的剪切模量做了数值计算，并与一维平面应变加载下铝的实验结果进行了比较.结果表明，修正的SCG模型与实验结果较为符合.在10—80GPa的压力范围下，剪切模量随冲击压力的增加而逐渐增大，这是由于压力的影响占主要地位，发生了加工硬化.在80—125GPa的压力范围下，剪切模量随冲击压力的增大快速减小，这是因为温度的影响比较严重，发生了高温软化现象.剪切模量最终在冲击压力为125GPa处趋于零，这是由于在该压力点冲击熔化发生，剪切强度消失. 关键词： 剪切模量 Steinberg-Cochran-Guinan模型 有限应变理论 铝