Constant chemical potential,pressure and temperature profiles in liquid–vapour equilibrium obtained by spinodal decomposition

Constant chemical potential, pressure and temperature profiles across a slab of liquid in equilibrium with its vapour confirm that, the spinodal decomposition procedure carried on the NVT ensemble simulated via molecular dynamics produce an equilibrium system. An initial homogeneous crystalline configuration of fluid is kept in a cell with a parallelepiped shape at a density near the critical density and a temperature between the triple and critical temperatures, form a slab of liquid in equilibrium with its vapour by the spinodal decomposition phenomenon if the simulation is performed in the NVT ensemble. An elongated box favours the formation of two planar parallel surfaces along the largest side of the box. We show in this paper that the ‘three conditions’ for thermodynamic equilibrium: constant temperature, constant pressure and constant chemical potential are met for such a system.     

Adsorption compression in surface layers

A recently elucidated aspect of adsorption, compression in confined phases, is discussed. Grand Canonical Monte Carlo simulations were performed for the adsorption of Lennard–Jones molecules and new details of intermolecular interactions in adsorbed layers are analysed. It is shown that a strong attraction to a surface can cause adsorption compression not only in the first layer, but also in higher layers. Compression of the first layer creates a pattern of active sites; the second layer tends to be commensurate with this pattern and has density higher than that of a ‘free’ layer. This pattern propagates to higher layers. However, there is a wide range of chemical potentials where the first layer is compressed and the second layer is not yet formed. It was found that transition to adsorption compression results in oscillations of the isosteric heat of adsorption. These oscillations are determined by a combination of (a) changes in adsorbed layers’ structure and (b) exchange of molecules between layers. In particular, at high affinity to adsorbent, the adsorption isotherm for the first layer has a slight maximum because an increase of the chemical potential causes molecules to leave the compressed first layer and move to the second layer. For this reason, the isosteric heat of adsorption decreases and can become negative. Analysis of adsorption compression mechanisms in the context of theory and emerging experimental results indicates that the significance of this phenomenon is not limited to fundamental aspects of adsorption and capillarity. These mechanisms play a crucial role in various applications, such as heterogeneous catalysis, membrane separations, and self-assembly on surfaces. Results are discussed in a broader context of theory, experiments and previous simulations.     

Entropic collapse transition of a polymer in a solvent with a nonadditive potential

We use molecular dynamics simulations to study an entropy-driven collapse transition of a flexible polymer in a solvent. Monomers and solvent particles interact with a steeply repulsive soft-sphere potential. We consider a nonadditive potential system in which the effective diameter describing the solvent-monomer interaction is greater than or equal to the diameters corresponding to the solvent-solvent and monomer-monomer interactions, which are set equal. We examine the effects of nonadditivity of the solvent-monomer potential and solvent density on the collapse transition. We find that a small degree of nonadditivity will drive the transition at sufficiently high solvent density. Increasing the density leads to a collapse transition at lower values of nonadditivity.     

Mobile polymer connectors

We study the junction between two solid objects, formed by polymer connectors which are supposed to freely explore the two grafting surfaces. We focus on the sphere-plane and sphere-sphere geometries. Because the chains are mobile, they are able to adapt both the local grafting density and their conformation to the position of the surfaces. Using a scaling approach, we show that, in chemical equilibrium, local grafting density is non-monotonic, and the junction can be divided into two different regions: the center, where connectors are compressed and exert a repulsive force between the objects (high excluded-volume interactions) and an external corona, where chains are stretched and attract the two objects. The relative importance of these regions varies for different separations and depends on surface geometries. When an external force is applied, this spatially inhomogeneous constraint exerted by the junction leads to specific force-distance profiles, which differ from profiles in the case of fixed connectors. An effective interaction potential and the adhesion energy are computed, showing a strong dependence on the geometry of surfaces.Received: 5 March 2003, Published online: 8 July 2003PACS: 36.20.Ey Conformations (statistics and dynamics) - 82.35.Gh Polymers on surfaces; adhesion - 82.70.Dd Colloids M. Manghi: Present address: Theorie, Sektion Physik, Ludwig Maximilian University, Theresienstr. 37, 80333 Munich, Germany     

Effect of the initial phase state of DT-matter on the compression of inertial fusion targets

We investigate the efficiency of inertial fusion target compression, where at the initial time moment the thermonuclear fuel is in a two-phase state and has the form of two adjacent layers — the external DT-liquid layer and the internal DT-ice layer. We study this problem for the fast ignition targets, where the ultimate final density of the thermonuclear matter is of a special importance. We take the simplest type of a fast ignition target, which corresponds to the technical justification of the HiPER Project aimed at demonstrating fast ignition at the compressing laser pulse energy ~100 kJ. Such a target presents a spherical DT-ice shell coated with a thin polymer film. We obtain the dependence of the final target density on the mass fraction of the DT-matter liquid phase and formulate the requirements on the admissible concentration of liquid phase if the decrease in the DT-fuel final density does not exceed 10%. We find the criterion for choosing the laser-pulse duration which provides the minimum decrease in the final density of the target containing DT-matter in the initial two-phase state.     

Direct visualization of continuous simple shear in non-Newtonian polymeric fluids

Using a particle tracking velocimetric technique, we show direct evidence of nonlinear velocity profiles during simple-shear flow of an entangled polymer solution, offering new insight into the origins of such characteristics as stress overshoot. Upon a startup shear by imposing a constant velocity on one of the two surfaces that confine the sample, the velocity field evolves from the initial linearity across the gap to a final state with a shear rate gradient. The unexpected deviation from the widely assumed linear variation of the velocity along the gap direction is most plausibly due to the entangled polymer's ability to disentangle in the presence of high shear that can orient the polymer chains leading to anisotropy in their mutual constraint.     

Volume exclusion effects in the ground-state dominance approximation for polyelectrolyte adsorption on charged interfaces

We consider the problem of polyelectrolyte molecules adsorbing on oppositely charged interfaces. For sufficiently long chains, the ground-state dominance approximation can be used which results in a (semi-)analytical solution of the self-consistent field equations (aSCF). Whereas existing aSCF theory assumes a low polyelectrolyte density, here the required electrostatic corrections for a high polymer density are implemented. Adsorbed polymer excludes volume for the solvent and small ions, a volume effect that also leads to a reduced dielectric permittivity and a resulting polarization term in the exchange potential. Calculations show the influence of volume exclusion on the polymer density profile.     

Structure of polymer solutions at interfaces: a Monte Carlo simulation study

The canonical ensemble Monte Carlo method is applied to study the structure of polymer solutions confined between surfaces. The polymer molecules are modeled as fused-sphere freely rotating chains with fixed bond length and bond angles and the solvent as hard spheres. The simulation results for the configurational and conformational properties of the chains are presented with varying interfacial distances, chain concentrations, and chain lengths. The chains are depleted at the wall at lower density, which, however, becomes less at higher density. With an increase in the interfacial distance, the enhancement/depletion of the chains at the wall becomes more marked. At all interfacial distances and chain lengths, increasing the concentration of the solvent makes the oscillation in the density profile of the chains more pronounced. Conformational properties provide important indications regarding the behaviour of chains as they approach surfaces.     

“聚龙一号”装置磁驱动准等熵压缩实验的一维磁流体力学模拟

国内首台多路并联超高功率脉冲装置"聚龙一号"(PTS)已被用于磁驱动准等熵实验研究,其分时分组放电特点为开展材料的动高压可控路径加载研究提供了便利.磁驱动准等熵实验的物理设计和结果分析需要依赖可靠的数值模拟平台.本文介绍了含强度计算的一维磁流体力学程序MADE1D的物理模型和程序特点,讨论了"聚龙一号"装置两种不同电流波形驱动条件下准等熵实验的模拟情况.结果显示,MADE1D程序能够较好地反映电磁力引起的压缩波在样品内部的产生、传播及发展过程,计算获得的"样品/窗口"界面速度同实验测量结果符合较好.分析发现,电流波形是影响加载过程的重要因素.对于目前使用的带状电极,电流上升率不宜超过40 k A/ns,否则可能在厚度1.2 mm以上的铝样品中产生冲击.     

Simulation algorithm for ronchigrams of spherical and aspherical surfaces, with the lateral shear interferometry formalism

With the Ronchi test a technician controls the manufacturing process using the following procedure: first, a Ronchigram is simulated which is scale-printed and placed on the surface as a mask and is usually printed with a low spatial resolution. This simulated Ronchigram is compared visually with the experimental pattern observed. This way of comparison leads to systematic errors in the evaluation of the surface, because it depends largely on the experience of the technician. Therefore, the main objectives of this work are to increase the spatial resolution and eliminate the dependence on the technician’s experience. Therefore, we compare the simulated Ronchigrams obtained by lateral shear interferometry, whose profiles are cosine, with Ronchigrams obtained experimentally. We present the simulation algorithm for the Ronchigrams of spherical and aspherical surfaces based on the expressions of a lateral shear interferometer. We show the results, of the comparison between simulated Ronchigrams (ray tracing and lateral shear interferometry) and experimental Ronchigrams.     

Zeta potential of colloidal particle in solvent primitive model electrolyte solution: a density functional theory study

A systematic study of zeta potential for a spherical double layer (SDL) around a colloidal particle in electrolyte solutions, is performed using density functional theory and Monte Carlo simulation. The usual recipe under the solvent primitive model is employed to model the system, where macroion, counterions, and coions are represented by charged hard spheres of uniform charge density and the presence of solvent is taken into account by modelling it as neutral hard spheres. All the components of the system are embedded in a dielectric continuum in order to consider the electrostatic effect of the solvent. The density functional theory employs a suitable weighted density approximation to calculate the hard-sphere contribution, whereas the residual electrostatic interactions are calculated as a small perturbation around the uniform fluid. The zeta potential profiles of a SDL in the presence of a number of electrolytes have been calculated and are found to be considerably influenced in the presence of solvent with an increase in the concentration of the electrolyte. The theory successfully predicts the maxima and sign reversal of the zeta potential profiles at high macroion surface charge density and in the presence of multivalent counterions, as obtained from the Monte Carlo simulation.     

Analysis of direct-drive capsule compression experiments on the Iskra-5 laser facility

We have analyzed and numerically simulated our experiments on the compression of DT-gas-filled glass capsules under irradiation by a small number of beams on the Iskra-5 facility (12 beams) at the second harmonic of an iodine laser (λ = 0.66 μm) for a laser pulse energy of 2 kJ and duration of 0.5 ns in the case of asymmetric irradiation and compression. Our simulations include the construction of a target illumination map and a histogram of the target surface illumination distribution; 1D capsule compression simulations based on the DIANA code corresponding to various target surface regions; and 2D compression simulations based on the NUTCY code corresponding to the illumination conditions. We have succeeded in reproducing the shape of the compressed region at the time of maximum compression and the reduction in neutron yield (compared to the 1D simulations) to the experimentally observed values. For the Iskra-5 conditions, we have considered targets that can provide a more symmetric compression and a higher neutron yield.     

Charged colloidal particles and small mobile ions near the oil-water interface: destruction of colloidal double layer and ionic charge separation

We study suspensions of hydrophobic charged colloids in a demixed oil-water solvent with salt by means of a modified Poisson-Boltzmann theory, taking into account image-charge effects and partitioning of the monovalent ions. We find that the ion's aversion for oil can deform the double layers of the oil-dispersed colloids, which qualitatively affects the colloidal density profiles. The same theory also predicts crystallization of colloid-free micron-sized water-in-oil droplets at water volume fractions as small as approximately 10(-3) in a narrow range of the oil-dielectric constant. These findings explain recent observations by M. E. Leunissen et al. [Proc. Natl. Acad. Sci. U.S.A. 104, 2585 (2007)10.1073/pnas.0610589104].     

Finite size effect of ions and dipoles close to charged interfaces

The modified dipolar Poisson-Boltzmann(MDPB) equation,where the electrostatics of the dipolar interactions of solvent molecules and also the finite size effects of ions and dipolar solvent molecules are explicitly taken into account on a mean-field level,is studied numerically for a two-plate system with oppositely charged surfaces.The MDPB equation is solved numerically,using the nonlinear Multigrid method,for one-dimensional finite volume meshes.For a high enough surface charge density,numerical results of the MDPB equation reveal that the effective dielectric constant decreases with the increase of the surface charge density.Furthermore,increasing the salt concentration leads to the decrease of the effective dielectric constant close to the charged surfaces.This decrease of the effective dielectric constant with the surface charge density is opposite to the trend from the dipolar Poisson-Boltzmann(DPB) equation.This seemingly inconsistent result is due to the fact that the mean-field approach breaks down in such highly charged systems where the counterions and dipoles are strongly attracted to the charged surfaces and form a quasi two-dimensional layer.In the weak-coupling regime with the electrostatic coupling parameter(the ratio of Bjerrum length to Gouy-Chapman length) Ξ 1,where the MDPB equation works,the effective dielectric constant is independent of the distance from the charged surfaces and there is no accumulation of dipoles near the charged surfaces.Therefore,there are no physical and computational advantages for the MDPB equation over the modified Poisson-Boltzmann(MPB) equation where the effect of dipolar interactions of solvent dipoles is implicitly taken into account in the renormalised dielectric constant.     

Structure of a tethered polymer under flow using molecular dynamics and hybrid molecular-continuum simulations

We analyse the structure of a single polymer tethered to a solid surface undergoing a Couette flow. We study the problem using molecular dynamics (MD) and hybrid MD-continuum simulations, wherein the polymer and the surrounding solvent are treated via standard MD, and the solvent flow farther away from the polymer is solved by continuum fluid dynamics (CFD). The polymer represents a freely jointed chain (FJC) and is modelled by Lennard-Jones (LJ) beads interacting through the FENE potential. The solvent (modelled as a LJ fluid) and a weakly attractive wall are treated at the molecular level. At large shear rates the polymer becomes more elongated than predicted by existing theoretical scaling laws. Also, along the normal-to-wall direction the structure observed for the FJC is, surprisingly, very similar to that predicted for a semiflexible chain. Comparison with previous Brownian dynamics simulations (which exclude both solvent and wall potential) indicates that these effects are due to the polymer–solvent and polymer–wall interactions. The hybrid simulations are in perfect agreement with the MD simulations, showing no trace of finite size effects. Importantly, the extra cost required to couple the MD and CFD domains is negligible.     

聚龙一号装置准等熵压缩实验负载优化研究

准等熵压缩实验技术已用来研究材料在高压下的状态方程。基于聚龙一号装置平台,实现对样品的准等熵压缩和超高速飞片发射,进行了一系列实验来加深对负载构型的理解。通过对负载结构的设计,研究了构设电极尺寸与电极间隙对磁应力的大小与分布的影响。基于模拟和实验结果,带状线负载结构可以很好地提高磁压和提升装置的运行水平,其电极表面磁压分布也具有良好的均匀性和平面性。目前为止,已经可以用带状线负载在聚龙一号装置上获得峰值压力高达约100 GPa的准等熵压缩,并获得速度超过10 km/s的超高速飞片。     

溶剂对镍连二硫烯与乙烯反应的影响

用密度泛函理论在B3LYP/6-31G(d)水平上计算得到了镍连二硫烯与乙烯反应的势能面上各驻点(反应物、中间体、产物和两个过渡态)的分子几何构型、电荷分布和一些热力学参数等,研究了溶剂对镍连二硫烯与乙烯反应的影响.结果发现,随着溶剂极性的增强,乙烯和镍连二硫烯之间的成键作用增强,两个过渡态的前线轨道能量差增大,而产物和中间体的前线轨道能量差却减小,同时各驻点的溶剂稳定化能也减小.进一步,这表明溶剂极性增强能提高产物的稳定性,有利于反应的进行.此外,当溶剂相对介电常数1.00≤ε≤7.58时 关键词： 镍连二硫烯 乙烯 溶剂效应 密度泛函理论     

溶剂对镍连二硫烯与乙烯反应的影响

用密度泛函理论在B3LYP/6-31G(d)水平上计算得到了镍连二硫烯与乙烯反应的势能面上各驻点(反应物、中间体、产物和两个过渡态)的分子几何构型、电荷分布和一些热力学参数等，研究了溶剂对镍连二硫烯与乙烯反应的影响.结果发现，随着溶剂极性的增强，乙烯和镍连二硫烯之间的成键作用增强，两个过渡态的前线轨道能量差增大，而产物和中间体的前线轨道能量差却减小，同时各驻点的溶剂稳定化能也减小.进一步，这表明溶剂极性增强能提高产物的稳定性，有利于反应的进行.此外，当溶剂相对介电常数1.00≤ε≤7.58时     

Effect of slow compression on the linear stability of an accelerated shear layer

An analysis is given of the effect of a slow uniform anisotropic compression or expansion on the linear stability of a normally accelerated planar interface between two fluids with different densities and tangential velocities, i.e., a combined Kelvin-Helmholtz and Rayleigh-Taylor instability, but generalized to an arbitrary time-dependent acceleration history. The compression is presumed to be sufficiently slow that the density remains uniform within each fluid and hence depends only on time. The perturbation is taken to be sinusoidal with amplitude h(t). The time evolution of h is determined by requiring pressure continuity across the interface in the usual way. The resulting linearized stability equation is a second-order linear ordinary differential equation for h(t). Compared to the corresponding well-known result for incompressible fluids, it is found that normal compression has the effect of reducing the perturbation growth rate &hdot; by an obvious geometrical correction, while transverse compression does not directly affect the net growth rate but rather has the dynamical effect of increasing its time derivative. When attention is focused on the masses transported across the initial interface rather than h, the purely geometrical effects of compression no longer appear explicitly, while the dynamical effects remain. It is thereby shown that both normal and transverse compression dynamically enhance the mixing of material masses, in spite of the corresponding purely geometrical reduction in &hdot;.     

Anisotropic microstructure-induced reduction of the Rayleigh instability for liquid crystalline polymers

We analyze a macroscopic 3D model for flows of liquid crystalline polymers (LCPs), deduced from Doi-type [3,4] kinetic equations. The Doi model accounts for rigid-rod microstructure, which introduces elastic relaxation and polymer-induced viscosity in addition to a Newtonian solvent viscosity, thus capturing all effects contained in standard isotropic viscoelastic models for Maxwell and Oldroyd B fluids. The rod-like microstructure further introduces anisotropic effects in the form of drag on the rods, together with a short-range, Maier-Saupe intermolecular potential, whose critical points vary with LCP concentration and yield stable isotropic (at low density) and nematic (at high density) equilibrium phases. From this single model, we compare various physical mechanisms for reducing the capillary instability of inviscid cylindrical jets: solvent viscosity as studied by Rayleigh and Chandrasekhar; isotropic viscoelasticity, both with and without Newtonian solvent viscosity; anisotropic polymer friction; and finally, the nematic, highly aligned prolate phase at high LCP density. Realistic parameter values for LCPs correspond to a regime in which the LCP capillary number (polymer bulk free energy relative to surface tension) is above an identified critical value; in such regimes, the unstable growth rates of the isotropic and nematic phases are lowered arbitrarily close to zero if the molecular drag is sufficiently anisotropic even in the absence of solvent viscosity. In low capillary number regimes, where surface tension dominates LCP bulk free energy, the LCP growth rates are sandwiched below the inviscid Rayleigh curve and above an explicit positive lower bound.