Influence of Gravity on Structure of Colloidal Crystal Using Simulated Microgravity

将重水(D2O)和水(H2O)以一定比例混合来匹配直径为110 nm的聚苯乙烯带电小球的密度,并利用Kossel衍射方法测量了胶体晶体的结构和晶格常数等参数．通过将密度匹配(g=0)和没有进行密度匹配(g=1)的实验结果的对比,发现重力将会使样品池中胶体晶体的晶格常数随高度的变低而变小．这一结果表明,在带电粒子胶体晶体的研究中有必要考虑重力的影响     

沉降对带电胶体粒子结晶过程的影响

采用密度匹配法(重水与水按一定比例混合),以及反射光谱法,研究了重力沉降作用对直径为98 nm的带电胶体粒子结晶过程的影响. 结果表明,重力沉降在晶体生长的初期提高了晶体生长速率,而后期降低了晶体生长速率,这是由于在晶体生长初期,沉降作用可使更多的粒子结合到晶体结构中,而当晶体尺寸进一步增加,其沉降速率也相应地增大,晶相与液相间的摩擦阻力导致一部分颗粒从胶体晶体上脱落. 总的来说,重力沉降在初期加剧了晶体的生长,后期阻碍了晶体的生长. 另外,在微重力环境下形成的胶体晶体比在重力环境下形成的胶体晶体更加完整紧密.     

平板电极间胶体晶体在电场作用下的各向同性压缩

用直径 300 nm的聚苯乙烯微球配制不同浓度的胶体晶体溶液, 将其快速注入内表面镀有导电薄膜的玻璃样品池中, 形成 (111) 晶面平行于玻璃表面的面心立方单晶结构. 通过激光衍射Kossel线方法, 研究了不同体积分数的胶体晶体样品 及它们在均匀电场作用下晶体结构的变化. 实验发现, 随着电场强度的增加, 胶体晶体表现为各向同性的压缩. 胶体晶体在恒定电场下始终保持面心立方结构, 晶格常数随着电场强度的增加逐渐减小. 实验结果可用电场力、电流体力学作用力及颗粒间静电斥力共同解释: 电场力使带电微球克服静电斥力并沿电场反方向运动导致晶体压缩, 而由电场力作用引起的电流体力学液流产生的持续推力使垂直于电场平面上的胶体微球相互靠近. 本实验为天宫一号搭载科学实验的地基实验. 关键词： 胶体晶体 Kossel衍射 结构变化     

带电胶体系统中的排空作用

用Monte Carlo模拟研究弱带电胶体系统的性质.在四维超球面欧氏空间中严格表示库仑作用的基础上，通过接受率法计算带电胶球间的排空势和排空力.研究了受限于两块平行带电平板之间、淹没于带电小胶球之中的一个或两个带电大胶球系统的排空作用，并通过与不带电的硬球系统相应结果的分析、对比，发现静电势能增强带电胶球之间的排空作用. 关键词： 超球面法 接受率方法 排空作用 Monte Carlo模拟     

二分量带电胶体悬浮系统的等效硬球模型

研究由无限稀薄的靶粒子散布于有限浓度(体积分数为)的主粒子悬浮液中而组成的二分量带电胶体系统,计算了靶粒子的短时间平动和转动自扩散系数.当系统中的粒子浓度和电解质浓度都不太高时,只考虑流体力学相互作用对扩散张量的首项两体贡献.为了计算体系的对分布函数,在数值计算的基础上发展了一个等效硬球模型,近似地把主粒子和靶粒子看作等效半径为δEHS的相同硬球粒子.结果表明,靶粒子的自扩散系数随两种粒子尺寸比和主粒子体积分数变化的关系可以很好地用等效硬球模型来解释 关键词： 胶体悬浮系统 自扩散 等效硬球 流体力学作用 关联函数     

FCC结构胶体光子晶体的制备及其带隙特性测量

基于对光纤传输特性和胶体光子晶体制备方法的研究,提出了用外加电场控制的方法制备光子带隙位于通讯波段的FCC结构的胶体光子晶体,并用光纤系统测试胶体光子晶体的带隙特性.采用RSOFT模拟了胶体光子晶体的带隙,分析了带隙位于通讯波段时所需的胶体微球的基本参量(微球折射率和直径).采用自组装的方法,用步进电机控制玻璃基片向上的拉升速率.速率为5 μm/s,同时外加一电场.用扫描电镜观测胶体晶体的表面形貌,并设计了单模光纤系统测量胶体光子晶体的带隙特性.测试的透射谱线表明胶体光子晶体的带隙中心波长为1552 nm.测试结果和模拟结果具有很好的一致性,误差只有2 nm.     

流体相互作用力对剪切流动下胶体重构的影响

本文利用布朗动力学和斯托克斯动力学方法对胶体在剪切流动下的重构和断裂过程进行了数值模拟,计算并分析了颗粒间作用势以及流体相互作用力对胶体断裂的临界剪切率的影响。模拟中胶体颗粒间作用势采用了L-J势和Yukawa排斥势的组合模型。模拟结果显示,作用势最大值与最小值的差决定了胶体断裂的临界剪切率,但由于胶体的几何构型(也是作用势的函数)的影响,两者的依存关系是非线性的。布朗动力学方法虽然不包含颗粒间的流体相互作用力,但是与斯托克斯动力学方法的结果比较发现,当颗粒间作用势较强,胶体满足刚性假设时,布朗动力学方法依然能够定性地给出胶体在剪切流动下的断裂规律。     

基于二维胶体晶体刻蚀法的纳米颗粒阵列

悬浮液中的胶体球在一定条件下能够自组装成二维胶体晶体，以此为掩膜可合成纳米颗粒阵列体系，其颗粒形状、尺寸以及间距等参数易于控制。调整这些参数和相应的介质环境可以实现对颗粒阵列体系性质的有效控制，这也为研究尺寸效应提供了便利，且在一些具有特殊功能的纳米器件方面具有潜在的应用价值。文章重点介绍了这种阵列体系的合成过程、结构形态和性质，并展望了其应用前景。     

均匀带电细棒和带电圆环电场的计算机模拟

为了可以更直观地观察带电粒子在复杂电场中的运动轨迹以及描述电场的空间分布,本文基于库仑定律和电场叠加原理,应用VPython软件,对均匀带电有限长细直棒和均匀带电圆环的电场的空间分布进行了可视化模拟;并模拟探究了带电粒子在带电圆环电场中的运动形式,对于简单的周期性运动,我们在理论上计算了带电粒子运动周期,并与模拟的结果相比较做了相关的讨论;模拟探究了带电粒子多种复杂的运动形式,预测了带电粒子在圆环电场中运动可能出现的混沌现象。     

带电胶体粒子弹性有效电荷测量的理论改进

对于带高电荷的胶体粒子形成的胶体晶体,用现有的剪切模量与粒子间相互作用的理论模型去计算剪切模量时,得到的结果要比实验测量值大很多,而根据此理论模型由剪切模量测量值拟合得到的粒子表面有效电荷也比实验测量的表面有效电荷小很多.这个问题一直没有合理的解释.分析认为这是由于理论模型基于理想晶体,并没有考虑到实际晶体中存在孔隙等缺陷会造成材料机械性能的下降.针对该因素,本文对已有的理论模型进行了修正,引入了孔隙造成的影响,同时使用Sogami-Ise相互作用势代替原来常用的Yukawa作用势.研究中用扭转共振法测量了带电粒子胶体晶体的剪切模量,比较了分别根据修正前后的关系模型由所测剪切模量拟合的有效电荷(弹性有效电荷),证实修正后的理论模型更合理,与使用Alexander等的方法得到的归一化电荷相比,一致性有了明显提高.     

Using relative entropy to find optimal approximations: An application to simple fluids

We develop a maximum relative entropy formalism to generate optimal approximations to probability distributions. The central results consist of (a) justifying the use of relative entropy as the uniquely natural criterion to select a preferred approximation from within a family of trial parameterized distributions, and (b) to obtain the optimal approximation by marginalizing over parameters using the method of maximum entropy and information geometry. As an illustration we apply our method to simple fluids. The “exact” canonical distribution is approximated by that of a fluid of hard spheres. The proposed method first determines the preferred value of the hard-sphere diameter, and then obtains an optimal hard-sphere approximation by a suitably weighed average over different hard-sphere diameters. This leads to a considerable improvement in accounting for the soft-core nature of the interatomic potential. As a numerical demonstration, the radial distribution function and the equation of state for a Lennard-Jones fluid (argon) are compared with results from molecular dynamics simulations.     

Colloids as model systems for metals and alloys: a case study of crystallization

Metallic systems are widely used as materials in daily human life. Their properties depend very much on the production route. In order to improve the production process and even develop novel materials a detailed knowledge of all physical processes involved in crystallization is mandatory. Atomic systems like metals are characterized by very high relaxation rates, which make direct investigations of crystallization very difficult and in some cases impossible. In contrast, phase transitions in colloidal systems are very sluggish and colloidal suspensions are optically transparent. Therefore, colloidal systems are often discussed as model systems for metals. In the present work, we study the crystallization process of charged colloidal systems from the very beginning. Charged colloids offer the advantage that the interaction potential can be systematically tuned by a variation of the particle number density and the salt concentration. We apply light scattering and ultra-small angle x-ray scattering to investigate the formation of short-range order in the liquid state even far from equilibrium, crystal nucleation and crystal growth. The results are compared with equivalent studies on metallic systems.     

Note on transport processes in dense colloidal suspensions

A new approach to transport processes in dense charged as well as neutral colloidal suspensions is presented. It is based on a far-reaching analogy between dense colloidal suspensions and dense hard-sphere fluids, implying, in turn, an analogy with atomic liquids. As a result, new expressions valid for a number of colloidal transport coefficients are predicted.     

Use of the temperature peak model for the description of track formation in semiconductor crystals irradiated by fast heavy ions

An attempt to apply the temperature peak model to describe the formation of defects and tracks in semiconductor crystals is made for the first time. The temperature dependences of model parameters, such as specific heat, thermal conductivity, and electron-phonon coupling coefficient are obtained. Agreement between the theoretical results and experimental data for InP and Ge crystals irradiated by ultrafast heavy ions indicates the adequacy of the model, with which one can evaluate the temperature of the local area near the ion trajectory, as well as the diameters of the molten region and experimentally observed track region. The diameter of the cylindrical molten region that forms along the path of 250-MeV Xe⁺ ions in InP is predicted to be 20 nm, and the measured cross-sectional diameters of the tracks fall into the range 7–15 nm.     

Disappearance of the gas-liquid phase transition for highly charged colloids

We calculate the full phase diagram of spherical charged colloidal particles using Monte Carlo free energy calculations. The system is described using the primitive model, consisting of explicit hard-sphere colloids and point counterions in a uniform dielectric continuum. We show that the gas-liquid critical point becomes metastable with respect to a gas-solid phase separation at colloid charges Q > or =20 times the counterion charge. Approximate free energy calculations with only one and four particles in the fluid and solid phases, respectively, are used to determine the critical line for highly charged colloids up to Q=2000. We propose the scaling law T*(c) approximately Q(1/2) for this critical temperature.     

Electrophoresis of colloidal dispersions in the low-salt regime

We study the electrophoretic mobility of spherical charged colloids in a low-salt suspension as a function of the colloidal concentration. Using an effective particle charge and a reduced screening parameter, we map the data for systems with different particle charges and sizes, including numerical simulation data with full electrostatics and hydrodynamics and experimental data for latex dispersions, on a single master curve. We observe two different volume fraction-dependent regimes for the electrophoretic mobility that can be explained in terms of the static properties of the ionic double layer.     

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。