纳米级随机粗糙表面微观滑动摩擦力的计算研究

表面形貌很大程度上决定了摩擦副的摩擦性能, 而所有的表面都不可能是绝对光滑的.由于摩擦表面形貌的随机性, 决定了实际的摩擦过程具有随机性的特点, 因此为了获得与随机形貌对应的摩擦特性, 建立合理的随机摩擦模型是必要的. 本文基于Lennard-Jones势能建立了纳米级随机粗糙表面和原子级光滑的刚性平面间的随机摩擦模型; 模型中, 界面势能由法向载荷和界面间平衡距离决定.通过数值计算的方法, 推导了微观滑动摩擦力的计算公式和摩擦力与法向载荷之间的关系. 研究结果表明摩擦力随着法向载荷的增加而增加, 但不是线性增长. 结果也说明界面间的表面势能可能是微观摩擦力的本质起源. 关键词： 随机粗糙表面 Lennard-Jones势能 微滑动摩擦力 微摩擦     

A molecular dynamics study of carbon dioxide in water: diffusion,structure and thermodynamics

Atomistic simulations are reported of a model of CO₂ in water. CO₂ is modelled by partial charges and Lennard-Jones interaction sites on each atom; the SPC/E model for water is used. Good agreement with experiment is found for the translational diffusion constants. The variation of the dynamics with the potential parameter was investigated. As expected, the orientational correlation times increase as the magnitude of the quadrupole moment is increased, but the translational diffusion constants are found to be surprisingly insensitive to the magnitude of the CO₂ quadrupole moment. The translational friction coefficient was resolved into electrostatic, Lennard-Jones and cross-terms; the Lennard-Jones contribution is found to be the largest. Varying the Lennard-Jones size parameter affects both translational and reorientational motion. In order to try to understand these results further, the variation of solvation free energy was investigated and the solvent structure around carbon dioxide was examined as the electrostatic and Lennard-Jones parameters were changed. The temperature dependence of the self-diffusion constant of pure SPC/E water was determined.     

汽液界面热毛细波的分子动力学模拟

用分子动力学模拟方法研究汽液界面性质,在NVT系综中,以氩原子为对象,对长方形模拟盒中汽液平衡系统进行不同温度下的模拟计算。计算结果表明,汽液界面存在热毛细波。界面上的热涨落将使界面变宽,它不仅与界面张力有关,还与温度、界面的横截面积有关。可以把汽液界面上热毛细波的均分根涨落值,作为汽液界面的宽度和粗糙度的指标。     

Adhesive contact: from atomistic model to continuum model

Two types of Lennard-Jones potential are widely used in modeling adhesive contacts. However, the relationships between the parameters of the two types of Lennard-Jones potential are not well defined. This paper employs a self-consistent method to derive the Lennard-Jones surface force law from the interatomic Lennard-Jones potential with emphasis on the relationships between the parameters. The effect of using correct parameters in the adhesion models is demonstrated in single sphere-flat contact via continuum models and an atomistic model. Furthermore, the adhesion hysteresis behaviour is investigated, and the S-shaped force-distance relation is revealed by the atomistic model. It shows that the adhesion hysteresis loop is generated by the jump-to-contact and jump-off-contact, which are illustrated by the S-shaped force-distance curve.     

The pair polarizability anisotropy of SF6 in the point-atom-polarizability approximation

The depolarized light scattering intensity and second Kerr virial coefficient for SF₆ are two to three times the values calculated with the dipole-induced-dipole model for the pair polarizability anisotropy and a Lennard-Jones pair potential. Contrary to earlier suggestions, this discrepancy is not attributable to the distribution of polarizable matter within an SF₆ molecule, at least within the point-atom-polarizability approximation. However, the calculated thermal average of the square of the polarizability anisotropy for SF₆ can be doubled by replacing the Lennard-Jones potential with a modified Lennard-Jones potential.     

Methods for constructing gas laws near critical temperatures by means of molecular dynamics

Various methods for constructing gas laws by means of molecular dynamics are considered. A model with a thermostat is shown to yield correct parameters of the Lennard-Jones gas law. Results near the condensation point are verified using the LAMMPS program on a hybrid high-performance computer cluster. Parameters of a Van der Waals approximation for the Lennard-Jones gas law are found.     

Dynamic effects of ion-lattice coupling for rare-earth ions

The adsorption isotherms for Kr monolayers on graphite measured by A. Thomy and X. Duval, J. chim. phys., 67, 1101 (1970) are transformed into the ‘spreading pressure’ as a function of the number density of adsorbed molecules and compared with the pressure of a two-dimensional Lennard-Jones fluid determined by the molecular dynamics technique. The comparison shows that the mobile-layer model for adsorption fails at low temperatures. The liquid-solid phase transitions in the adsorption monolayer appear at lower densities than in a two dimensional Lennard-Jones fluid and the adsorbed liquid layer is compressible in contrast to the two-dimensional Lennard-Jones liquid.     

Ultrashort microwave pulses generated due to three magnon interactions

Density-functional theory is applied to the interaction site model to study gas-liquid nucleation in a partially immiscible binary system consisting of spherical Lennard-Jones atoms (monomers) and dumbbell molecules of two Lennard-Jones atoms (dimers). Increased interaction anisotropy between the dimer sites and monomers is shown to result in mutual enhancement of nucleation. Critical nuclei with a lamellar structure are observed at high dimer activities.     

Effect of stretching on intermolecular potentials of diatomic molecules

The effect of stretching due to rotations and vibrations on the intermolecular potentials of diatomic molecules has been considered. The calculations for nitrogen and oxygen on the Lennard-Jones (12:6) model show that the effect is significant. Consequently, for polyatomic molecules the effect of stretching must be considered for a correct representation of the intermolecular potential.     

离子推力器工作性能参数控制模型

为了研究离子推力器工作参数对输出特性的影响,通过离子推力器工作性能参数的理论计算公式,建立起离子推力器输入参数与输出参数的Simulink控制模型,根据模型分别对我国研制的30 cm口径以及20 cm口径离子推力器的工作输出参数进行了理论计算,并通过推力测量试验对理论值进行了比对和分析。比对结果表明:在推力理论计算过程中引入二价Xe离子比率和束流密度分布推力修正,以及推力均方误差修正后,推力理论值与实测值符合性较好,计算误差小于1 mN,证明了推力修正方法的合理性。     

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.     

A molecular dynamics study of Lennard-Jones physisorption on W(100)

The physisorption of Xe on W(100) was modeled by Lennard-Jones pair-wise interaction potentials and the dynamics of coverages ranging from one to four adlayers obtained by molecular dynamics simulation. At 115 K, the first two layers were well-ordered and each adsorbed with c(2 × 2) symmetry. Further adsorption produced a surface similar to that of a distorted Xe(100) face. In accord with the work of Broughton and Woodcock [1], the top layers of the three- and four-adlayer coverages were “rough” and had liquid-like diffusion coefficients. The potential energies of all layers other than the first were similar, thus corroborating one of the postulates of BET theory [2]. Generally, the effect of adsorbing a layer was to reduce the entropy of all those beneath.     

Measurements of roughness noise

Experiments have been performed on the roughness noise produced by a two-dimensional turbulent wall jet boundary layer flowing over short fetches of sandpaper roughness. A range of rough surface sizes were studied from hydrodynamically smooth through fully rough. Velocity measurements were made to document the form of the wall jet boundary layer and the influence of the roughness upon it. Acoustic measurements showed background noise levels to be very low so that the sound produced by the rough surfaces could be clearly detected with signal to noise ratios as large as 20 dB. Even hydrodynamically smooth roughness was found to produce noise, conclusively indicating the presence of scattering as a source mechanism. Variations of the roughness noise spectra with flow speed and roughness size are found to be inconsistent with any simple parameter scaling. Boundary layer wall pressure fluctuation measurements made within the roughness fetches reveal a spectral form quite different than the roughness noise, and fluctuation levels some 50-70 dB higher. Despite these differences the wall pressure and roughness noise are found to be very simply related, at least at lower frequencies (<6 kHz). The roughness noise spectrum varies closely as the product of the wall pressure spectrum, the frequency squared, and the mean-square roughness height. This is the scaling predicted by scattering theory and implies a major simplification to the problem of roughness noise prediction for stochastic surfaces.     

Application of a multiregion model to the EM scattering from a rough surface with or without a target above it

An efficient multiregion model is introduced to calculate the electromagnetic scattering from a perfectly electrical conducting(PEC) rough surface with or without a PEC target above it.In the multiregion model,the rough surface is split into multiple regions depending on their position along the rough surface.Two intermediate regions are chosen as the dominant region.If a target is located above the rough surface,the target will also be included in the dominant region.The method of moments(MOM) is only adopted on the dominant region to ensure validity.Hence,the new model can greatly reduce the number of unknowns associated with full MOM analysis.The induced electric currents on the other regions are obtained by approximately considering the mutual coupling between different regions along the rough surface.Compared with the published hybrid method,this new model is not only suitable for EM scattering from a target above a rough surface but also applicable for just rough surfaces.Several numerical simulations are presented to show the validity and efficiency of the multiregion model.     

Liquid-crystal phase equilibria of Lennard-Jones chains

Liquid-crystal phase equilibria of Lennard-Jones chain fluids and the solubility of a Lennard-Jones gas in the coexisting phases are calculated from Monte Carlo simulations. Direct phase equilibria calculations are performed using an expanded formulation of the Gibbs ensemble. Monomer densities, order parameters, and equilibrium pressures are reported for the coexisting isotropic and nematic phases of: (1) linear Lennard-Jones chains, (2) a partially-flexible Lennard-Jones chain, and (3) a binary mixture of linear Lennard-Jones chains. The effect of chain length is determined by calculating the isotropic-nematic coexistence of linear Lennard-Jones chain fluids made of 8, 10, and 12 segments (8-, 10-, 12-mer). The effect of molecular flexibility on the isotropic-nematic equilibrium is studied for a Lennard-Jones 10-mer chain fluid with one freely-jointed segment at the end of the chain. An isotropic-nematic phase split and fractionation are reported for a binary mixture of linear 7-mer and 12-mer chains. Simulation results are compared with theoretical results as obtained from a recently developed analytical equation of state based on perturbation theory. Excellent agreement between theory and simulations is observed. The solubility of a monomer Lennard-Jones gas in the coexisting isotropic and nematic phases is estimated using the Widom test-particle insertion method. A linear relationship between solubility difference and density difference at isotropic-nematic coexistence is observed. It is shown that gas solubility is independent of the nematic ordering of the fluid, at constant temperature and density conditions.     

Electromagnetic scattering from two-layered rough interfaces with a PEC object: vertical polarization

Electromagnetic(EM) scattering from a stack of two rough interfaces separating a homogeneous medium with a perfectly electric conducting(PEC) object has been calculated through the method of moments for vertical polarization.Theoretical formulations of EM scattering from multi-layered rough interfaces with a PEC object have been derived in detail and the total fields and their normal derivatives on the rough interfaces are solved.The two-layered model is a special case.In this work,a Gaussian rough surface was applied to simulate the rough interface.A cylinder was located above,between or below the two-layered rough interfaces.Through numerical simulations,the validity of this work is demonstrated by comparing it with existing scattering models,which are special cases that include a PEC object located above/below a single-layered rough interface and two-layered rough interfaces without an object.Subsequently,the influences of characteristic parameters,such as the relative permittivity of the medium,as well as the average height between the two rough surfaces,on the bistatic scattering coefficient are discussed.     

Dirichlet form approach to infinite-dimensional Wiener processes with singular interactions

We construct infinite-dimensional Wiener processes with interactions by constructing specific quasi-regular Dirichlet forms. Our assumptions are very mild; accordingly, our results can be applied to singular interactions such as hard core potentials, Lennard-Jones type potentials, and Dyson's model. We construct nonequilibrium dynamics.Dedicated to Professor Masatoshi Fukushima on his 60th birthday     

The phases of two-dimensional matter,their transitions,and solid-state stability: A perspective via computer simulation of simple atomic systems

An extensive computer simulation investigation of the structure, thermodynamics and phase stability of the two-dimensional Lennard-Jones system is presented, with special emphasis on the low-pressure melting regime of the phase diagram. This investigation includes isobaric-isothermal Monte Carlo simulations of the various phases of the two-dimensional Lennard-Jones system and of the melting and vaporization processes in configuration space, the isodensity-isothermal Monte Carlo simulations of two-phase coexistence between crystal and liquid and between liquid and vapor, the determination of the phase diagram, the establishment of the thermodynamic melting temperature, and the determination of the physical significance of the Kosterlitz-Thouless-Feynman dislocation model for melting in relation to the stability of the crystalline phase. I conclude that th phase diagram of the Lennard-Jones system in two dimensions is qualitatively similar to that in three dimensions. Finally, I present a new simulation method for doing molecular dynamics at constant pressure and/or constant temperature, and employ this method to study the temporal-spatial evolution of two-dimensional melting and vaporization.     

Threshold and Lennard-Jones resonances and elastic lifetimes in the scattering of atoms from crystalline surfaces

In this paper we apply the GR method for solving the scattering equations of atoms from a hard corrugated surface, on accelerated particles above a hard corrugated surface and a hard corrugated surface with an attractive well. The solutions are given for the Rayleigh hypothesis that under the range of corrugation presented in this paper leads to the exact ones. Threshold resonances are studied observing that the appearance and disappearance of beams must be for a general theory with vertical tangent. The structure of the Lennard-Jones resonances given for the model mentioned above. For the first time it is stressed that Lennard-Jones resonances are not observed in metal surfaces in general, and, accordingly, they are unobserved in compact metallic surfaces. This is correlated with the fact that diffraction has not been observed. Both facts are due to the very weak corrugation of the gas-metal interaction potential. According to our results, the Lennard-Jones resonances in metals present greater difficulties to be observed experimentally. We also note that the absence of diffraction in compact metal surfaces is because they are almost plane and not because of the Debye-Waller effect. Finally, we have calculated the lifetimes of the atoms at the crystal surfaces. These are larger, the smaller the incident energy and the larger the corrugation. But the lifetimes are particularly large at resonance condition (10^?11 s).