Velocity correlations and the structure of nonequilibrium hard-core fluids

A model for the pair-distribution function of nonequilibrium hard-core fluids is proposed based on a model for the effect of velocity correlations on the structure. Good agreement is found with molecular dynamics simulations of granular fluids and of sheared elastic hard spheres. It is argued that the incorporation of velocity correlations are crucial to correctly modeling atomic scale structure in nonequilibrium fluids.     

First observation of electrorheological plasmas

We report the experimental discovery of "electrorheological (ER) complex plasmas," where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles (dust) in a plasma. We show that interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments, as well as molecular dynamics simulations, show a phase transition from an isotropic to an anisotropic (string) plasma state as the electric field is increased.     

Structures and Hydrogen Bonding Interactions in Urea-water System Studied by All-atom MD Simulation and Chemical Shifts in NMR Spectrum

采用分子动力学模拟方法结合核磁共振化学位移和粘度对尿素水溶液在稀浓度范围内的结构和弱相互作用进行研究. 从径向分布函数(RDF)分析看出,尿素水溶液中存在着几种不同类型、不同氢键形成能力的传统氢键. 氢键网络分析发现尿素水溶液体系在水富集区域,水分子倾向于自身缔合形成稳定的分子簇结构,而随着尿素浓度的逐渐增加,水的有序结构受到破坏,水分子和尿素分子发生了交叉缔合作用形成氢键,尿素分子有形成自身缔合的趋势. 分子动力学统计的平均氢键数与核磁共振化学位移和粘度数据结果进行比较,发现它们的变化趋势一致,证明了实验和理论结果有很强的可靠性.     

Application of the dissipative particle dynamics method to magnetic colloidal dispersions

The validity of the application of the dissipative particle dynamics (DPD) method to ferromagnetic colloidal dispersions has been investigated by conducting DPD simulations for a two–dimensional system. First, the interaction between dissipative and magnetic particles has been idealized as some model potentials, and DPD simulations have been carried out using such model potentials for a two magnetic particle system. In these simulations, attention has been focused on the collision time for the two particles approaching each other and touching from an initially separated position, and such collision time has been evaluated for various cases of mass and diameter of dissipative particles and model parameters, which are included in defining the equation of motion of dissipative particles. Next, a multi–particle system of magnetic particles has been treated, and particle aggregates have been evaluated, together with the pair correlation function along an applied magnetic field direction. Such characteristics of aggregate structures have been compared with the results of Monte Carlo and Brownian dynamics simulations in order to clarify the validity of the application of the DPD method to particle dispersion systems. The present simulation results have clearly shown that DPD simulations with the model interaction potential presented here give rise to physically reasonable aggregate structures under circumstances of strong magnetic particle–particle interactions as well as a strong external magnetic field, since these aggregate structures are in good agreement with those of Monte Carlo and Brownian dynamics simulations.     

Brownian dynamics simulations of a cubic haematite particle suspension with a more effective treatment of steric layer interactions

We have proposed a new repulsive layer model for describing the interaction between steric layers of coated cubic particles. This approach is an effective technique applicable to particle-based simulations such as a Brownian dynamics simulation of a suspension composed of cubic particles. 3D Brownian dynamics simulations employing this repulsive interaction model have been performed in order to investigate the equilibrium aggregate structures of a suspension composed of cubic haematite particles. It has been verified that Brownian dynamics employing the present steric interaction model are in good agreement with Monte Carlo results with respect to particle aggregate structures and particle orientational characteristics. From the viewpoint of developing a surface modification technology, we have also investigated a regime change in the aggregate structure of cubic particle in a quasi-2D system by means of Brownian dynamics simulations. If the magnetic particle–particle interaction strength is relatively strong, in zero applied magnetic field the particles aggregate in an offset face-to-face configuration. As the magnetic field strength is increased, the offset face-to-face structure is transformed into a more direct face-to-face contact configuration that extends throughout the whole simulation region.     

Material characterization of ER fluids at high frequency

In this paper, the material characterization of ER fluids at high frequencies is studied. To characterize the properties at high frequencies, an experimental apparatus is provided, based upon the wave transmission through ER fluids in the presence of electric field. Details of the experiment and how to extract the complex shear modulus of ER fluids are addressed. A moderate increase in the storage modulus and loss modulus was observed when the weight ratio of ER particles and the electric field were increased. The proposed method is a comprehensive material characterization of ER fluids in high frequencies for ER smart structures.     

Poiseuille flow of a micropolar fluid

We use non-equilibrium molecular dynamics simulations to study the flow of a micropolar fluid and to test an extended Navier-Stokes theory (ENS) for such fluids. The angular streaming velocity (which is of course missing in the classical Navier-Stokes theory) and the translational streaming velocity are found to be in good agreement with the predictions of ENS theory. Besides, owing to molecular rotation, the translational streaming velocity profile is shown to deviate from the classical parabolic profile. Finally, temperature profiles calculated using three different expressions (a kinetic translational, a kinetic rotational and a recently derived configurational expression) are found to be in excellent agreement, demonstrating that the equipartition principle still holds in this non-equilibrium system. No deviation from the classical quartic temperature profile is observed.     

Thermal equation of state, and melting and thermoelastic properties of bcc tantalum from molecular dynamics

We performed molecular dynamics simulations with the extended Finnis-Sinclair (EFS) potential to investigate thermal equation of state (EOS), and melting and thermoelastic properties of tantalum. The agreement of the obtained thermal EOS with experiments at ambient conditions is reasonably good. The EFS potential with the two-phase method also reproduced very satisfyingly the high-pressure melting curve, excellently consistent with both the experiments of melting temperature at ambient pressure and shock melting at high pressure. From molecular dynamics simulations, we also obtained the thermoelastic properties of Ta for temperatures up to 3000 K at ambient pressure. Fully including anharmonic effects in molecular dynamics, our calculated elastic constants are in excellent agreement with experimental data. Shear modulus G decreases quickly with increasing temperature.     

Surface tension of the restrictive primitive model for ionic liquids

Hybrid molecular dynamics and Monte Carlo simulations are performed to study the liquid-vapor interface of the restricted primitive model (RPM) of ionic fluids. We report for the first time simulation results of the surface tension associated to this interface. The RPM accurately predicts experimental surface tensions of ionic salts and good agreement with theoretical predictions that include the idea of ion association is found. The simulation results indicate that the structure of an ionic liquid-vapor interface is rather rough. This is reflected in the interfacial thickness, larger than that observed in simple fluids and water.     

Turbulent structures in cylindrical density currents in a rotating frame of reference

Gravity currents are flows generated by the action of gravity on fluids with different densities. In some geophysical applications, modeling such flows makes it necessary to account for rotating effects, modifying the dynamics of the flow. While previous works on rotating stratified flows focused on currents of large Coriolis number, the present work focuses on flows with small Coriolis numbers (i.e. moderate-to-large Rossby numbers). In this work, cylindrical rotating gravity currents are investigated by means of highly resolved simulations. A brief analysis of the mean flow evolution to the final state is presented to provide a complete picture of the flow dynamics. The numerical results, showing the well-known oscillatory behavior of the flow (inertial waves) and a final state lens shape (geostrophic adjustment), are in good agreement with experimental observations and theoretical models. The turbulent structures in the flow are visualized and described using, among others, a stereoscopic visualization and videos as supplementary material. In particular, the structure of the lobes and clefts at the front of the current is presented in association to local turbulent structures. In rotating gravity currents, the vortices observed at the lobes front are not of hairpin type but are rather of Kelvin-Helmholtz type.     

AT1受体拮抗剂V8、V12和BZI8结构的量子化学研究

基于Mavromoustakos等人的分子动力学和蒙特卡罗方法结果,本文利用量子化学计算的方法研究了SARTANS(沙坦)类的3种AT1受体拮抗剂(V8、V12和BZI8).在B3LYP/6-31G(d)方法下计算得到了3种药物分子的平衡几何结构,并且采用B3LYP-GIAO/6-311G(d,p)方法计算了这些药物分子中的质子化学位移.通过研究结果可以看出,本文采用量子化学方法计算得到的3种分子的结构与Mavromoustakos等人通过分子动力学和蒙特卡罗方法得到的结果比较接近,计算结果与核磁共振实验的ROE数据和1H谱相比吻合得较好,说明所获得的结构较为合理.另外通过对所获得分子结构进行叠合分析发现,同属SARTANS类的3种分子在联苯咪唑环区域具有很大的相似性.     

Quantum corrections and bound-state effects in the energy relaxation of hot dense hydrogen

Simple analytic formulas for energy relaxation (ER) in electron-ion systems, with quantum corrections, ion dynamics, and RPA-type screening are presented. ER in the presence of bound electrons is examined in view of recent simulations for ER in hydrogen in the range 10{20}-10{24} electrons/cc.     

AT1受体拮抗剂V8、V12和BZI8结构的量子化学研究

基于Mavromoustakos等人的分子动力学和蒙特卡罗方法结果，本文利用量子化学计算的方法研究了SARTANS（沙坦）类的3种AT1受体拮抗剂（V8、V12和BZI8）. 在B3LYP/6-31G(d)方法下计算得到了3种药物分子的平衡几何结构，并且采用B3LYP-GIAO/6-311G(d，p)方法计算了这些药物分子中的质子化学位移. 通过研究结果可以看出，本文采用量子化学方法计算得到的3种分子的结构与Mavromoustakos等人通过分子动力学和蒙特卡罗方法得到的结果比较接近，计算结果与核磁共振实验的ROE数据和¹H谱相比吻合得较好，说明所获得的结构较为合理. 另外通过对所获得分子结构进行叠合分析发现，同属SARTANS类的3种分子在联苯咪唑环区域具有很大的相似性.     

Isotope effects in water as investigated by neutron diffraction and path integral molecular dynamics

The structures of heavy and light water at 300?K were investigated by using a joint approach in which the method of neutron diffraction with oxygen isotope substitution was complemented by path integral molecular dynamics simulations. The diffraction results, which give intra-molecular O-D and O-H bond distances of 0.985(5) and 0.990(5)??, were found to be in best agreement with those obtained by using the flexible anharmonic TTM3-F water model. Both techniques show a difference of ??0.5% between the O-D and O-H intra-molecular bond lengths, and the results support a competing quantum effects model for water in which its structural and dynamical properties are governed by an offset between intra-molecular and inter-molecular quantum contributions. Further consideration of the O-O correlations is needed in order to improve agreement with experiment.     

Comment on the critical dipole moments for electron binding to a freely rotating electric dipole

In this paper the reference hypernetted-chain (RHNC) theory is solved for fluids of Stockmayer (dipolar Lennard-Jones) particles and detailed comparisons are made with computer simulation results. It is shown that the RHNC approximation significantly improves upon integral equation theories previously solved for Stockmayer systems. In particular the static dielectric constants obtained are in much better agreement with the computer simulations.     

Density waves and the effect of wall roughness in granular Poiseuille flow: Simulationand linear stability

The formation of density waves and the effect of wall roughness on them are studied using molecular dynamics simulations of gravity-driven granular Poiseuille flow. Three basic types of structures are found in moderately dense flows: a plug, a sinuous wave and a slug; a new varicose wave mode has been identified in dense flows with channels of large widths at moderate dissipations; only clump-like structures appear in dilute flows. The simulation results are contrasted with the predictions of a linear stability analysis of the kinetic-theory continuum equations for granular Poiseuille flow. The theoretical predictions on the form of density waves are in qualitative agreement with simulations in denser flows, however, there are discrepancies between simulation and theory in dilute flows.     

Sound propagation in fluids and neutron spectra

Using a model kinetic equation the properties of very short wavelength sound modes in fluids are studied over a wide range of wavenumbers and densities. The main features, in particular propagation gaps at high densities, agree with those found in real fluids and molecular dynamics simulations.     

Conformational dynamics of triradical nitroxides as studied by EPR

The EPR spectra of a flexible nitroxide triradical are recorded in a large temperature range. A detailed analysis of the spectral features is performed and the spectra are compared with computer simulations. An excellent agreement with the experimental spectra is obtained by using a dynamic model based on the conformation interconversion of different molecular structures, characterized by the electron exchange interactions between pairs of free radicals. The energy barrier for the interconversion process and the temperature variation of the constant for the equilibrium among the conformations have been obtained. The results show that EPR spectra of nitroxide triradicals spin labelled polymers could be of valuable use for studying subtle details of molecular dynamics.     

磁性液体材料微结构磁化状态数值分析

采用磁性液体边界元数值模型分析了磁性粒子微结构磁化状态及相互作用，并与相应的有限元数值结果进行比较．结果表明：虽然粒子边界层附近的磁场分布出现了一定的模糊，且变化幅度明显高于有限元结果，但在粒子边界层以外的其它区域两种数值解法具有高度一致性；当粒子边界元节点数较小时，磁力相互作用边界元结果与有限元结果相差较大，且很不稳定；随着节点数增加，边界元结果逐渐趋近于有限元虚功原理的磁力数值结果，并达到一个较稳定的量值．结果清晰地反映出边界元数值方法在磁性液体多体动力学模拟研究中的一些基本特征。     

全原子分子动力学模拟结合核磁共振波谱研究N-甘氨酰甘氨酸水溶液的结构和弱相互作用

采用分子动力学模拟方法结合核磁共振化学位移系统研究了甘氨酰甘氨酸水溶液体系饱和溶解度范围的弱相互作用. 径向分布函数表明体系中不同类型的原子显示出形成氢键的不同能力. 氢键网络分析发现了不同氢键所能形成的分子簇结构. 随温度变化核磁共振化学位移值用于研究形成氢键的变化情况,并和模拟得到的结果进行比较,模拟和实验结果得到了较好的吻合.