碳纳米管增强硫化天然橡胶拉伸性能的分子动力学模拟

基于分子动力学(MD)模拟方法，建立了碳纳米管/硫化天然橡胶复合材料体系模型，采用ReaxFF势函数，模拟了不同碳纳米管(CNT)含量的复合材料的拉伸过程.通过计算复合材料体系的自由体积分数、均方位移及回转半径，分析了材料基本微观性质和碳纳米管团聚的机制，计算结果与实验相符.通过碳纳米管与硫化天然橡胶界面能的计算，发现在加载过程中系统总势能的变化主要由硫化天然橡胶基体引起，其中非键能起主导作用；碳纳米管由于其自身力学性能较好，且与天然橡胶分子链相互作用产生界面能，导致材料力学性能提升，材料的屈服应力随碳纳米管含量的增加而显著升高.     

自由场空泡溃灭过程能量转化机制研究

综合应用实验与数值模拟方法, 深入讨论了自由场空泡溃灭过程中的能量转化机制. 在实验研究中, 应用纹影法记录了空泡溃灭的演变过程, 提取了空泡在溃灭过程中的半径, 溃灭速度等数据, 结合空泡势能和动能方程, 描述了空泡能量的转化过程. 在开展数值模拟分析时, 运用弱可压缩流体质量守恒方程和动量方程, 建立了三维数值模型用以模拟空泡在自由场中的溃灭过程, 并且由结果中获取了空泡溃灭过程中的压力及速度变化规律, 揭示了空泡在溃灭过程中能量转化机制. 研究结果表明: (1) 自由场空泡在溃灭过程中, 空泡势能与空泡半径具有相同的演化趋势, 空泡动能与势能变化趋势相反; 当空泡达到最大半径处时, 空泡势能最大, 流场动能为零. (2) 溃灭后期在空泡周围会形成高压区域, 该区域的压力梯度与速度梯度较高, 随着空泡收缩, 高压区域面积逐渐减小. (3) 空泡在自由场中发生溃灭时, 空泡势能不断转化为流场动能, 在溃灭时刻可以明显观察到冲击波现象, 空泡的大部分能量会在此时转化为冲击波的波能.      

接触角滞后与流体动压润滑的相关性研究

理论研究表明不同润湿性界面对流体动压润滑油膜厚度有着显著地影响，一般采用接触角(CA)来表征固液界面润湿性. 而由热力学原理推导出的界面势能垒理论模型不仅与接触角相关，也是接触角滞后(CAH)的函数. 本文作者通过对不同基体材料的滑块进行表面张力修饰，获得了不同亲和性的界面. 利用干涉法及荧光法分别测量了不同润湿性界面的流体动压润滑膜厚及油膜受剪切的流动特性，研究了接触角及接触角滞后两个界面参数对流体动压润滑油膜厚度的影响，并对势能垒与接触角滞后的关系进行了讨论. 结果表明:接触角与流体动压润滑油膜厚度的相关性较差，接触角滞后可以更好地表征界面效应对流体动压润滑油膜厚度的影响.      

汽液平衡界面微观特征与应力特性的分子动力学研究

采用Verlet-List和动态存储的分子动力学方法,对饱和汽液平衡体系中的分子行为进行了模拟,研究了汽液界面的微观物理特征和应力特性以及温度变化对汽液界面热力学性质的影响。结果表明:汽液界面在空间和时间上都是涨落起伏的非稳定区域,且分子由液态向气态转变是一个突变过程;汽液过渡区厚度与局部界面涨落区宽度相同,验证了汽液过渡区和分界面涨落区的统一性;当温度从0.7变化至1.1时,气液界面厚度从4增加到9.5,而表面张力和两相密度差则不断减小;当温度接近临界温度时,系统表面张力趋近于0。     

聚四氟乙烯真空摩擦学性能的分子动力学模拟

采用分子动力学模拟的方法研究聚四氟乙烯(PTFE)和铁滑动模型在不同真空度下的摩擦学性能，从原子尺度揭示聚四氟乙烯在不同真空度下的摩擦磨损机理.模拟结果表明，随着真空度的增加，摩擦系数逐渐降低，磨损率增加.通过分析径向分布函数(RDF)、原子相对浓度、摩擦界面温度和原子运动速度等的变化，发现真空度升高，更多的聚四氟乙烯分子链吸附在铁原子层表面，使得相互作用增强，摩擦界面温度升高，原子运动速度加快，在滑动过程中更多的聚四氟乙烯分子黏附在铁原子表面，导致磨损量也增大.利用空间环境地面模拟装置考察了PTFE在不同真空度下的摩擦磨损性能，从而验证了分子动力学模拟计算结果的可靠性.     

非均匀颗粒材料的类固-液相变行为及本构方程

以非均匀颗粒介质为研究对象，采用三维离散元方法对其在不同密集度和剪切速率下的动 力过程进行了数值模拟，分析了其在由瞬时接触的快速流动向持续接触的准静态流动的转变 过程及其行为特点. 通过对不同材料性质下相变过渡区内颗粒材料的宏观应力、接触时间数、 配位数、团聚颗粒数量、有效摩擦系数等参量的计算，更加全面地描述了非均匀颗粒材料在 类固-液相变过程中的基本特征. 基于以上数值计算结果，建立了一个适用于颗粒材料 类固态、类液态以及其相变过程的本构方程，并通过剪切室实验结果验证了它的合理性.     

横向磁场下侧壁加热方腔熔化的数值模拟研究

磁场下的固?液相变过程在电磁冶金和增材制造等工程应用中广泛存在, 其中的熔化过程和流动机理尚未完全探究清楚. 方腔熔化是研究固?液相变过程的基础模型, 具有良好的普适性, 研究磁场对其流动的影响可以为其他复杂相变过程提供参考. 本文基于焓方法开展了固?液相变的数值模拟研究, 得到了垂直主环流方向的横向磁场对侧壁加热方腔中流动、传热和熔化过程的影响. 首先, 对于无磁场时的方腔熔化问题, 通过与已有的实验结果和数值结果进行对比, 证实了文献中方腔宽度对固?液界面的形状及位置影响不能忽视的结论. 随后, 对小磁场情况下的三维工况进行了直接数值模拟, 发现此时磁场效应主要表现为对混乱三维流动产生整流作用, 使流动趋于二维化. 但由于固?液界面的存在, 主流区的速度在趋于一致的同时也会反作用于界面, 其形状随磁场增强而逐渐转变为二维结构. 最后, 本文采用准二维模型分析了更强磁场时的情形, 讨论了不同参数对传热效率及界面形状的影响, 并发现了横向磁场作用下的垂直最大速度仍满足磁对流中的无量纲参数标度律关系.      

热弹性马氏体相变连续介质热力学研究

介绍形状记忆合金热弹性马氏体相变连续介质热力学研究方法和最新进展, 着重分析 了在推广的非线性弹性力学的框架下, 应用变分方法研究热弹性马氏体相变的理论和方法、 存在的问题及发展趋势. 首先介绍如何计算马氏体相变24种变体的变形梯度, 然后拓展非 线性弹性力学, 引入描述相变的多阱非凸弹性势能, 进而讨论了界面能和非局部能对相变微 结构和相变过程的影响的相关研究理论方法和进展.     

相分离过程的离散Boltzmann方法研究进展

概述相分离过程的离散Boltzmann方法研究进展.这部分研究内容,根据系统的成分,可分为单介质的两相分离和多介质的相分离;根据物理建模,可分为基于传统流体模型的LBM(lattice Boltzmann method)数值模拟和相分离系统的动理学建模与模拟;根据研究的侧重点,可分为方法(模型)研究和相分离过程研究.离散Boltzmann模拟所提供的非平衡行为特征为区分相分离的两个不同阶段提供了一个简洁、有效的物理判据,可用于不同类型界面的甄别与追踪技术设计.     

微注射成形中表面张力效应的数值模拟

微注射成形过程是一个喂料射入膜腔挤出腔内空气的过程。在这一过程中,存在喂料和空气的移动界面。由于喂料和气体物质特性差异很大,喂料分子对界面上的分子引力明显大于气体分子的引力,故界面上的分子会受到方向指向喂料内部的合力,即表面张力。为了研究表面张力效应对粘性流微注射模拟结果的影响,在矢量化显式算法的有限元模拟软件中,实现了表面张力计算和模拟功能,并在有限元方法中以系统化方法确定了离散变量场的拉普拉斯算子值。应用植入该功能的软件,模拟了一个典型的注射填充过程的算例,探讨了表面张力作用项对粘性流体填充过程的影响。通过对不同尺寸构件的模拟结果比较,证实表面张力作用项对粘性流注射填充过程模拟结果影响不大,除了毫米级以下构件,在一般情况下可以忽略。     

Molecular dynamics study of the liquid-vapor interface of lithium bromide aqueous solutions

The molecular dynamics simulations of the liquid–vapor interface of LiBr aqueous solutions were carried out to investigate the structural and thermophysical properties. As concerns the structural properties, the results of molecular dynamics simulation show that the ions exist in the liquid apart from the surface and this tendency becomes strong as the solute concentration is lowered. This phenomenon is due to the desorption of ion. The calculated values such as density or surface tension agree with experimental ones. As concerns thermophysical properties, the number of water molecules in the bulk gas decreases with an increase of the solute concentration. This result represents the depression of vapor pressure. In addition, in order to investigate the dynamic process of water vapor absorption into LiBr aqueous solution, the molecular dynamics simulation under non-equilibrium condition was carried out. The results show that when the solute concentration is low and the temperature is also low, almost all incident water molecules become trapped at the solution surface and then easily diffuse into the bulk liquid, and when the solute concentration is high and temperature is also high, most incident water molecules become trapped at the solution surface, and the sequent processes are very complicated. Received on 28 September 1998     

Theory of sorption hysteresis in nanoporous solids: Part II Molecular condensation

Motivated by the puzzle of sorption hysteresis in Portland cement concrete or cement paste, we develop in Part II of this study a general theory of vapor sorption and desorption from nanoporous solids, which attributes hysteresis to hindered molecular condensation with attractive lateral interactions. The classical mean-field theory of van der Waals is applied to predict the dependence of hysteresis on temperature and pore size, using the regular solution model and gradient energy of Cahn and Hilliard. A simple “hierarchical wetting” model for thin nanopores is developed to describe the case of strong wetting by the first monolayer, followed by condensation of nanodroplets and nanobubbles in the bulk. The model predicts a larger hysteresis critical temperature and enhanced hysteresis for molecular condensation across nanopores at high vapor pressure than within monolayers at low vapor pressure. For heterogeneous pores, the theory predicts sorption/desorption sequences similar to those seen in molecular dynamics simulations, where the interfacial energy (or gradient penalty) at nanopore junctions acts as a free energy barrier for snap-through instabilities. The model helps to quantitatively understand recent experimental data for concrete or cement paste wetting and drying cycles and suggests new experiments at different temperatures and humidity sweep rates.     

Experimental studies of a hygroscopic condenser-evaporator heat exchanger based on concentration differences of vertically falling films

A concentration-driven power cycle motivated by differences in vapor partial pressures (boiling point rise) and latent heats of brine and water is studied. The condensation of relatively low-pressure, low-temperature vapor occurs on the free interface of a relatively hot falling film of a hygroscopic salt solution due to the reduced vapor pressure of the brine. The heat released is transferred to the evaporating/cooling water film on the other side of a vertical plate separating the brine and water films. The process is maintained because the latent heat of condensation on the brine film is higher than the latent heat of evaporation of pure water. The condensation driving force is the difference between the partial pressure of condensing water vapor and that of water in the brine solution. The simultaneous mass and heat transfer mechanisms associated with this nonisothermal absorption can occur even against an opposing thermal driving force in the condensing vapor phase. Complementing earlier studies by the same authors, a vertical film-type condenser-evaporator heat exchanger is considered. The experimental study deals with the effects of the various parameters involved in this rather unique process and the mechanisms that control them. The experimental results prove the potential of operating this new heat transfer modality and provide the background for the theoretical determination of the optimal performance of this direct-contact power cycle.     

Evaporation condensation-induced bubble motion after temperature gradient set-up

Thermocapillary (Marangoni) motion of a gas bubble (or a liquid drop) under a temperature gradient can hardly be present in a one-component fluid. Indeed, in such a pure system, the vapor–liquid interface is always isothermal (at saturation temperature). However, evaporation on the hot side and condensation on the cold side can occur and displace the bubble. We have observed such a phenomenon in two different fluids submitted to a temperature gradient under reduced gravity: hydrogen under magnetic compensation of gravity in the HYLDE facility at CEA-Grenoble and water in the DECLIC facility onboard the ISS. The experiments and the subsequent analysis are performed in the vicinity of the vapor–liquid critical point to benefit from critical universality. In order to better understand the phenomena, a 1D numerical simulation has been performed. After the temperature gradient is imposed, two regimes can be evidenced. At early times, the temperatures in the bubble and the surrounding liquid become different thanks to their different compressibility and the “piston effect” mechanism, i.e. the fast adiabatic bulk thermalization induced by the expansion of the thermal boundary layers. The difference in local temperature gradients at the vapor–liquid interface results in an unbalanced evaporation/condensation phenomenon that makes the shape of the bubble vary and provoke its motion. At long times, a steady temperature gradient progressively forms in the liquid (but not in the bubble) and induces steady bubble motion towards the hot end. We evaluate the bubble velocity and compare with existing theories.     

Molecular gas dynamics approaches to interfacial phenomena accompanied with condensation

This paper deals with the condensation coefficient of methanol, which is evaluated from a condensation rate at the vapor–liquid interface. Film condensation is induced on the endwall of a vapor-filled shock tube, when a shock wave is reflected at the endwall and the vapor becomes supersaturated there. The liquid film grows with the lapse of time. The evolution in time of the liquid film thickness is measured by an optical interferometer with a high accuracy, and thereby the net condensation mass flux at the interface is obtained. The mass flux is incorporated into the kinetic boundary condition (KBC) at the interface for the Gaussian–BGK Boltzmann equation. Such a treatment of the boundary condition makes it possible to formally eliminate the evaporation and condensation coefficients in KBC and to obtain the unique numerical solution of the vapor–liquid system. In this way, the instantaneous condensation coefficient is accurately evaluated from the conformity with experiment and numerical solution. It is found that the values of condensation coefficient are, near vapor–liquid equilibrium states, close to those evaluated by molecular dynamics simulations.     

Detailed investigation on single water molecule entering carbon nanotubes

The behavior of a water molecule entering carbon nanotubes (CNTs) is studied. The Lennard-Jones potential function together with the continuum approximation is used to obtain the van der Waals interaction between a single-walled CNT (SWCNT) and a single water molecule. Three orientations are chosen for the water molecule as the center of mass is on the axis of nanotube. Extensive studies on the variations of force, energy, and velocity distributions are performed by varying the nanotube radius and the orientations of the water molecule. The force and energy distributions are validated by those obtained from molecular dynamics (MD) simulations. The acceptance radius of the nanotube for sucking the water molecule inside is derived, in which the limit of the radius is specified so that the nanotube is favorable to absorb the water molecule. The velocities of a single water molecule entering CNTs are calculated and the maximum entrance and the interior velocity for different orientations are assigned and compared.     

Molecular gas dynamics applied to phase change processes at a vapor–liquid interface: shock-tube experiment and MGD computation for methanol

This paper deals with a molecular gas-dynamics method applied to the accurate determination of the condensation coefficient of methanol vapor. The method consisted of an experiment using a shock tube and computations using a molecular gas-dynamics equation. The experiments were performed in such situations where the shift from a vapor–liquid equilibrium state to a nonequilibrium one is realized by a shock wave in a scale of molecular mean free time of vapor molecules. The temporal evolution in thickness of a liquid film formed on the shock-tube endwall behind a reflected shock wave is measured by an optical interferometer. By comparing the measured liquid-film thickness with numerical solutions for a polyatomic version of the Gaussian–BGK model of the Boltzmann equation, the condensation coefficient of methanol vapor is accurately determined in vapor–liquid nonequilibrium states. As a result, it is clear that the condensation coefficient is just unity very near to an equilibrium state, but is smaller far from the equilibrium state.     

Recent progress in the moving contact line problem: a review

As pointed out long ago by Laplace, viscosity may become a large perturbation to capillary phenomena, especially close to solid surfaces where molecules may stick. A spectacular consequence of this is the impossibility for a triple line to move on a solid if the liquid/vapor interface is considered as a material surface and if the usual no slip boundary condition is enforced. As shown recently this specific phenomenon of contact line motion can be described by coupled van der Waals and fluid equations, yielding a rational theory that is divergence free and consistent with the equilibrium results. Far from the triple line, the equations of fluid mechanics are recovered in their usual form. In this approach, the contact line move close to the solid by evaporation or condensation, which requires (for evaporation) the molecules to jump above a high potential barrier on their way from the liquid to the vapor. An Arrhenius factor makes this process intrinsically slow, compared to molecular speeds. For (realistic) very small Arrhenius factors, the motion of the triple line induces a dynamical change of the functions in the van der Waals equations. This may lead to dynamical wetting and dewetting transitions, that is, to a change of the contact angle from a finite to a zero value or conversely. The dynamical wetting transition has been observed in liquids flowing down a plate (see Blake and Ruschak, Nature 282 (1979) 489–491) cusps on the contact line appear when it recedes faster than the speed of transition. Similar ideas account well also for the known sensitivity of contact line mobility to vapor pressure. To cite this article: Y. Pomeau, C. R. Mecanique 330 (2002) 207–222.     

双层金属纳米板界面能密度的尺寸效应

界面能密度是表征纳米复合材料与结构界面力学性质的重要物理量.采用分子动力学方法计算了不同面心立方金属晶体构成的双材料纳米薄板结构的界面能密度,分析了界面晶格结构形貌变化及界面效应对原子势能的影响.结果表明:双材料纳米薄板界面具有周期性褶皱状疏密相间的晶格结构形貌,界面上原子势能亦呈现周期性分布特性,而靠近界面的两侧原子势能与板内原子势能具有明显差异.拉格朗日界面能密度和欧拉界面能密度均随双层薄板厚度的增加而增加,最终趋向于块体双材料结构的界面能密度.     

Pure steam condensation model with laminar film in a vertical tube

A new physical model for calculating the liquid film thickness and condensation heat transfer coefficient in a vertical condenser tube is proposed by considering the effects of gravity, liquid viscosity, and vapor flow in the core region of the flow. To estimate the velocity profile in the liquid film, the liquid film was assumed to be in Couette flow forced by the interfacial velocity at the liquid–vapor interface. For simplifying the calculation procedures, the interfacial velocity was estimated by introducing an empirical power-law velocity profile. The resulting film thickness and heat transfer coefficient from the model were compared with the experimental data and the results obtained from the other condensation models. The results demonstrated that the proposed model described the liquid film thinning effect by the vapor shear flow and predicted the condensation heat transfer coefficient from experiments reasonably well.