扩散模型和凝聚模型耦合作用下胶体凝聚动力学的Monte Carlo模拟研究

以扩散模型(Ds(γ)=D0×sγ)和凝聚模型(Pij(σ)=P0×(i×j)σ)为基础,对胶体体系随时间的演变、团簇大小分布及其标度关系、团簇的重均大小S(t)的变化规律以及模型对最终分形维数的影响四个角度进行了比较研究,发现扩散指数γ0和凝聚概率指数σ0对胶体的凝聚动力学过程有相似的影响.本文在较宽的γ和σ取值范围内,对胶体的凝聚动力学进行了模拟研究,对慢速凝聚向快速凝聚的转化机理作了定量分析,并进一步分析了在团簇-团簇凝聚(CCA)模型下,得到类似扩散置限凝聚(DLA)模型的凝聚体的物理意义,结果表明:(1)γ0代表了体系中团簇或单粒做"定向运动"而非无规则的布朗运动的情况.这种"定向运动"的推动力可能来自于大团簇产生的强"长程范德华力"、"电场力"等,或来自于体系边界处的外力场的作用.(2)当σ0时,体系成为先快后慢的慢速凝聚,这可能对应大团簇为一排斥中心,即胶体颗粒存在"排斥力场"的现象.(3)证实了团簇的重均大小在凝聚过程的早期按指数规律增长,而后期按幂函数规律增长的实验现象.模拟研究还表明,胶体体系的凝聚动力学过程,在σ0时是一个存在正反馈机制的非线性动力学过程,而在σ0时则体现出负反馈的特征.     

钯团簇碰撞沉积钯/银基板的分子动力学模拟

运用分子动力学模拟方法研究了常温下较大的钯团簇以不同初始速度撞击不同硬度基板的微观过程,着重分析了沉积形貌的变化、团簇的嵌入深度和原子的扩散程度、基板碰撞接触区域的温度演变以及碰撞过程中团簇与基板间的能量转化,获得了沉积过程中变形形貌、结构特征及能量转化随团簇尺寸、初始速度及基板材质的变化规律.并进一步探究了第二颗团簇以不同时刻沉积时前一团簇的变形形貌及基板接触区域温度变化的特点,发现短时间间隔下第二颗团簇的沉积更有利于团簇与基板的结合.     

嵌入纳米Fe颗粒的Fe液凝固过程的分子动力学模拟

采用Sutton-Chen 势函数及分子动力学(MD)方法对嵌入了Fe纳米团簇(半径从0.4-1.8 nm)的Fe液凝固过程进行了模拟. 模拟结果表明只有当嵌入的纳米团簇半径超过0.82 nm才能降低凝固时所需要的临界过冷度(ΔT^*), 起到诱导凝固的作用. 同时采用原子键型指数法(CTIM-2)对样本在凝固过程中的原子结构进行了标定, 通过观察微观结构演变发现当嵌入纳米团簇能够作为凝固核心时, 体系按照hcp-fcc 交叉形核的方式长大. 同时还发现嵌入纳米团簇对体系凝固过程晶核的生长方向及凝固的最终构型存在“结构遗传效应”.     

梯恩梯(TNT)爆轰初期形成富碳团簇分子及类石墨结构的分子动力学模拟

富碳炸药在爆轰过程中可产生团簇分子,而现有的实验手段不能直接观测团簇分子的形成过程.本文采用ReaxFF/lg力场对梯恩梯（TNT）在不同温度下的热分解过程进行了模拟.研究发现：团簇分子在反应初始阶段形成缓慢,大约一次增加一个TNT相对分子质量.随着反应进行,团簇分子迅速增大,最大团簇分子相对分子质量可达8000~10000,约占体系质量的18%.分析团簇分子的结构发现,团簇分子中一部分苯环被破坏,形成五元环和夹杂N、O原子的六元环,在3500 K条件下还形成了更为复杂的七元环等结构.通过体积膨胀和直接降温的方法,研究了团簇分子的稳定性：体积膨胀使得团簇分子迅速分解;而直接降温,团簇分子又聚合成更大的团簇.分析类石墨结构的产生过程,发现先膨胀然后降温是必不可少的步骤.比较团簇分子和TNT分子中各原子质量所占比,团簇分子中C原子质量比始终在增加.     

拟薄水铝石胶溶过程规律及微结构分析

研究了工业拟薄水铝石粉体用硝酸作为胶溶剂的胶溶过程及结构特性.试验测定了制备工艺条件(pH值、温度)对胶体性能的影响,在pH<4时得到稳定胶体.用TEM观察到胶体是由长10nm左右的棒状胶粒相互连接组成空间网状结构.FTIR结果表明颗粒表面的-NO3-和-OH-浓度增加,-NO3-除形成A1-NO3键外,还有部分以游离态存在于体系中.提出了胶粒外层吸附模型.     

含盐胶体液滴的蒸发图案形成机理

研究了聚四氟乙烯(PTFE)胶粒与NaCl混合液滴的蒸发过程及其图案形成机理. 结果表明, PTFE颗粒对接触线具有强烈的钉扎作用, 胶体液滴蒸发伴有显著的“咖啡环”效应. 由于液滴中心液相区表面张力法向分力的作用, 使得凝胶区存在辐射状应力, 进而产生从液滴边缘向中心的辐射状裂纹, 裂纹数量随胶粒的体积分数增大而减少. NaCl与PTFE胶粒的混合液滴出现了复杂多样的蒸发图案. 盐的加入抑制了向外的毛细补偿流, 从而有利于获得宏观上厚度均匀的沉积膜. NaCl与PTFE胶粒耦合形成了凹凸不平的枝晶状形貌, 这可能是释放蒸发应力的结果.     

高磺化度聚苯胺体系中的分形结构研究

通过透射电镜的观察研究发现磺化聚苯胺的胶体聚集体和胶粒内部结构都具有分形体的特征 ,从而将分形的概念及其数学模型引入共轭导电聚合物体系之中 .磺化聚苯胺胶体的聚集体为很不均匀的分支状开放结构 ,其形成过程可用扩散控制集团聚集模型 (DLCA)进行模拟 ,计算机模拟生成的图形及其分形维数都与实验观测结果相当吻合 .胶粒由于是在分散介质所形成的平均化场中生成 ,屏蔽效应减弱 ,是比由它组成的聚集体要致密的球形结构 ,该结构的生成可用随机雨点模型模拟且结果相近 .     

PH_3配体在团簇Au_(20)的顶点和面心配位的DFT理论研究

采用DFT方法研究了在团簇Au20的顶端位点和面心位点配位PH3分子时的几何结构、电子结构以及Au-P的成键机理和能量分析.在两种配位方式下,配体PH3对团簇的几何结构影响都表现为强烈的局域形变效应.不同的配位方式下PH3与团簇的轨道作用方式不同,所形成的团簇化合物电子组态不同.两种配位方式下Au-P成键能的区别主要是来自于配体与团簇之间的Pauli排斥的不同,在面心配位时配体与团簇之间更大的Pauli排斥作用导致了该配位方式的不稳定.     

Sn_mO_n(m=1～3,n=1～2m)系列团簇的结构和电子性质

基于密度泛函理论(DFT)中的广义梯度近似(GGA)系统地研究了Snm On(m=1~3,n=1~2m)团簇的几何结构和电子性质.当m=n时,团簇的基态结构为Sn和O原子彼此相邻的环形结构,当nm时,团簇易于形成链状结构.研究发现:氧化锡团簇的物理和化学特性类似于氧化硅,主要表现为非金属性.对分裂途径、分裂能和能隙(HOMO-LUMO Gap)进行了研究,结果表明类氧化锡(Snm Om)、Sn2O3和Sn3O4团簇具有很好的稳定性,可以作为构建团簇聚合物材料的基本单元.而且,氧化锡团簇的稳定性主要与其组成成分和结构有关,与团簇大小无关.     

Zn掺杂对LaMnO3体系磁性的影响

研究了LaMn1-xZnxO3 （x=0.05, 0.10, 0.20, 0.30, 0.40）体系的M-T曲线、 M-H曲线、电子自旋共振谱（ESR）. 实验结果表明： 随掺杂浓度增高, 体系的TC单调下降, Zn替代没有引起明显的晶体结构的变化, 磁结构从长程铁磁有序向自旋团簇玻璃态转变, ESR图谱测量的微观磁性与M-T曲线测量的宏观磁性一致. 这些结果归因于Zn掺杂引起的双交换作用、磁稀释作用和晶格效应.     

Mechanism of hydrodynamic separation of biological objects in microchannel devices

In this study, the separation mechanism employed in hydrodynamic chromatography in microchannel devices is analyzed. The main purpose of this work is to provide a methodology to develop a predictive model for hydrodynamic chromatography for biological macromolecules in microchannels and to assess the importance of various phenomenological coefficients. A theoretical model for the hydrodynamic chromatography of particles in a microchannel is investigated herein. A fully developed concentration profile for non-reactive particles in a microchannel was obtained to elucidate the hydrodynamic chromatography of these particles. The external forces acting on the particles considered in this model include the van der Waals attractive force, double-layer force as well as the gravitational force. The surface forces, such as van der Waals attractive force as well as the double-layer repulsive force, can either enhance or hinder the average velocity of the macromolecular particles. The average velocity of the particles decreases with the molecular radius because the van der Waals attractive force increases the concentration of the particles near the channel surface, which is the low-velocity region. The transport velocity of the particles is dominated by the gravity and the higher density enlarges the effect caused by gravity.     

Colloid stability of thymine-functionalized gold nanoparticles

Gold nanoparticles surface-coated with thyminethiol derivatives containing long hydrocarbon chains have been prepared. The diameter of the particles is 2.2 and 7.0 nm, respectively, with a relatively narrow size distribution. Thyminethiol derivatives are attached to the gold particle surfaces with thymine moieties as the end groups. The colloid stability of the gold nanoparticles as a function of the type and concentration of monovalent salt, pH, and particle size was investigated in alkaline, aqueous solutions. The gold particles are stable in concentrated NaCl and KCl solutions, but are unstable in concentrated LiCl and CsCl solutions. The larger gold particles are more sensitive to salt concentration and aggregate at lower salt concentrations. The reversible aggregation and dispersion of the gold particles can be controlled by changing the solution pH. The larger gold particles can be dispersed at higher pH and aggregate faster than the smaller particles, due to stronger van der Waals forces between the larger particles. Hydration forces play an important role in stabilizing the particles under conditions where electrostatic forces are negligible. The coagulation of the gold nanoparticles is attributed to van der Waals attraction and reduced hydration repulsion in the presence of LiCl and CsCl.     

The Effect of van der Waals Forces on the Deposition of Highly Dispersed Aerosol Particles on Ultrafine Fibers

The effect of van der Waals forces on the collection of highly dispersed aerosol particles with ultrafine fiber filters was studied theoretically. The capture coefficient was found from the numerical solution of the equation of convective diffusion with the account of the particle size, the effect of van der Waals forces acting between a particle and a fiber, and the gas slip effect at the surface of ultrafine fibers. It was shown that allowance being made for van der Waals forces markedly affects the capture coefficient within the maximal particle penetration range and that the radius of the most penetrating particles decreases with the rising effect of these forces.     

Specific and nonspecific interaction forces between Escherichia coli and silicon nitride, determined by poisson statistical analysis

The nature of the physical interactions between Escherichia coli JM109 and a model surface (silicon nitride) was investigated in water via atomic force microscopy (AFM). AFM force measurements on bacteria can represent the combined effects of van der Waals and electrostatic forces, hydrogen bonding, steric interactions, and perhaps ligand-receptor type bonds. It can be difficult to decouple these forces into their individual components since both specific (chemical or short-range forces such as hydrogen bonding) and nonspecific (long-range colloidal) forces may be present in the overall profiles. An analysis is presented based on the application of Poisson statistics to AFM adhesion data, to decouple the specific and nonspecific interactions. Comparisons with classical DLVO theory and a modified form of a van der Waals expression for rough surfaces were made in order to help explain the nature of the interactions. The only specific forces in the system were due to hydrogen bonding, which from the Poisson analysis were found to be -0.125 nN. The nonspecific forces of 0.155 nN represent an overall repulsive interaction. These nonspecific forces are comparable to the forces calculated from DLVO theory, in which electrostatic-double layer interactions are added to van der Waals attractions calculated at the distance of closest approach, as long as the van der Waals model for "rough" spherical surfaces is used. Calculated electrostatic-double layer and van der Waals interactions summed to 0.116 nN. In contrast, if the classic (i.e., smooth) sphere-sphere model was used to predict the van der Waals forces, the sum of electrostatic and van der Waals forces was -7.11 nN, which appears to be a large overprediction. The Poisson statistical analysis of adhesion forces may be very useful in applications of bacterial adhesion, because it represents an easy way to determine the magnitude of hydrogen bonding in a given system and it allows the fundamental forces to be easily broken into their components.     

A modeling approach to describe the adhesion of rough, asymmetric particles to surfaces

A combined theoretical and experimental study of the adhesion of alumina particles and polystyrene latex spheres to silicon dioxide surfaces was performed. A boundary element technique was used to model electrostatic interactions between micron-scale particles and planar surfaces when the particles and surfaces were in contact. This method allows quantitative evaluation of the effects of particle geometry and surface roughness on the electrostatic interaction. The electrostatic interactions are combined with a previously developed model for van der Waals forces in particle adhesion. The combined model accounts for the effects of particle and substrate geometry, surface roughness and asperity deformation on the adhesion force. Predictions from the combined model are compared with experimental measurements made with an atomic force microscope. Measurements are made in aqueous solutions of varying ionic strength and solution pH. While van der Waals forces are generally dominant when particles are in contact with surfaces, results obtained here indicate that electrostatic interactions contribute to the overall adhesion force in certain cases. Specifically, alumina particles with complex geometries were found to adhere to surfaces due to both electrostatic and van der Waals interactions, while polystyrene latex spheres were not affected by electrostatic forces when in contact with various surfaces.     

Rb and Cs oligomers in different spin configurations on helium nanodroplets

The study of small clusters is intended to fill the knowledge gap between single atoms and bulk material. He nanodroplets are an ideal matrix for preparing and investigating clusters in a superfluid environment. Alkali-metal atoms are only bound very weakly to their surface by van der Waals forces. Due to the formation process, high-spin states of alkali-metal clusters on He nanodroplets are favorably observed, which is in contrast to the abundance in other preparation processes. Until now, the prevailing opinion was that stable clusters of the heavy alkali-metal atoms, rubidium (Rb) and cesium (Cs) on He nanodroplets, are limited to 5 and 3 atoms, respectively (Schulz et al., Phys. Rev. Lett. 2004, 92, 13401). Here, we present stable complexes of Rb(n)? and Cs(n)? consisting of up to n = 30 atoms, with the detection of large alkali-metal clusters being strongly enhanced by one-photon ionization. Our results also suggest that we monitored both high-spin and low-spin state clusters created on nanodroplets. The van der Waals bound high-spin alkali-metal clusters should show strong magnetic behavior, while low-spin states are predicted to exhibit metallic characteristics. Alkali-metal clusters prepared in these two configurations appear to be ideal candidates for investigating nanosized particles with ferromagnetic or metallic properties.     

Influence of very small bubbles on particle/bubble heterocoagulation

Very small bubbles which partially coat the surface of particles influence whether or not heterocoagulation between a particle and a bubble occurs. The electrostatic and van der Waals forces of interaction between particles and bubbles were calculated as a function of electrolyte concentration, particle size, and the size and distributions of these very small bubbles present on the particle surface. The height of the surface force barrier was compared with the hydrodynamic pressing force under conditions of flotation. The presence of these very small bubbles has a profound effect on the interaction between particles and bubbles and, in particular, strongly decreases the critical particle radius for heterocoagulation.     

Van der Waals Emulsions: Emulsions Stabilized by Surface‐Inactive,Hydrophilic Particles via van der Waals Attraction

Surface‐inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil‐in‐water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil–water interfaces when repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by van der Waals attraction between particle monolayer shells and oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by van der Waals attraction between single particles and oil droplets, which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion‐based technologies in practice.     

Van der Waals Emulsions: Emulsions Stabilized by Surface‐Inactive,Hydrophilic Particles via van der Waals Attraction

Surface‐inactive, highly hydrophilic particles are utilized to effectively and reversibly stabilize oil‐in‐water emulsions. This is a result of attractive van der Waals forces between particles and oil droplets in water, which are sufficient to trap the particles in close proximity to oil–water interfaces when repulsive forces between particles and oil droplets are suppressed. The emulsifying efficiency of the highly hydrophilic particles is determined by van der Waals attraction between particle monolayer shells and oil droplets enclosed therein and is inversely proportional to the particle size, while their stabilizing efficiency is determined by van der Waals attraction between single particles and oil droplets, which is proportional to the particle size. This differentiation in mechanism between emulsification and stabilization will significantly advance our knowledge of emulsions, thus enabling better control and design of emulsion‐based technologies in practice.     

The Effect of van der Waals' Forces on Aerosol Filtration with Fibrous Filters

The collection of aerosol particles with a fibrous filter under the effect of van der Waals' forces was considered with allowance for an electromagnetic retardation and a curvature of a fiber surface. To estimate van der Waals' forces, the method of pair summation of power potentials was used. The capture coefficient of inertialess particles of definite size was calculated. It was shown that as the interception parameter and the flow velocity decrease, the account for van der Waals' forces sharply increases the capture coefficient.