剪应力下弱作用势胶体颗粒聚团的特点

探讨了不同剪应力下，具有Ｌｅｎｎａｒｄ－Ｊｏｎｅｓ势的胶体颗粒聚团的结构特性，包括簇团的大小分布，径向分布函数，分形维数和原子配位数。研究表明，在弱作用力下，胶体簇团的分布随剪应力的增加而趋向小簇团一边；径向分布函数曲线随剪应力的增加而降低，胡在近程距离内降低得最多；分形维数随剪应力的增加表现为先增加后减小的趋势，其值随模拟条件的不同而在１．９－２．４之间变动。剪应力“场”对分形维数的大小没有太大     

具有多体效应的胶体聚团的特征

考察了多体效应（用Ｓｔｕｔｔｏｎ－Ｃｈｅｎ势，ＳＣ）对胶体聚团的影响并与双体（ＬＪ）势下的结果作了比较。研究表明，ＳＣ和ＬＪ势下簇团的性质有其相似的一面，如：随着剪应力的增加，系统里颗粒的平均势能增加，而每个簇团的颗粒数减少；在较强的剪应力场里，簇团沿剪应力方向（Ｘ轴）被明显拉长且其主轴偏离Ｘ轴等。但它们间的差异也是明显的，在剪应力下ＳＣ系统内颗粒排列得更合理，从而使得平均位能比ＬＪ系统低约１－３     

流体力学关联对胶体簇团大小的影响

用计算机模拟的方法研究了流体力学关联对胶体簇团大小的影响．研究表明,流体力学关联可使簇团尺寸大大增加,约为无流体力学关联时簇团大小的２～２０倍（在研究情况下）．把该结果与前人的研究作了对照,并解释了其中可能的原因     

铁体系催化剂制备方式对催化活性影响的研究

通过Ｔｙｎｄａｌ效应，电镜观察，电导率测定，结合聚合实验，发现铁体系催化剂在加氢汽油介质中为胶体催化剂，而且是聚结不稳定胶体体系．催化剂各组分配比，加入顺序影响胶粒的形态，因而影响到催化活性与聚合物微观结构．其中较佳配比，较佳加入顺序（Ｆｅ）∶（Ｃｌ）∶（Ａｌ）＝１∶２．５∶２０，催化剂颗粒比较小，均匀，因而比表面积大，催化活性高，相反其他配比的颗粒大，不均匀，催化活性低．陈化使胶粒聚集变大，故催化活性降低．     

甲壳质、甲壳胺衍生物保护的贵金属胶体

<正> 高分子保护金属胶体是近年来金属催化剂领域中引人注目的研究课题。它除了具有负载型金属催化剂的优点外,还具备下列新优点:a.胶体分散体系可形成“均相”溶液;b.保护高分子可以屏蔽胶体催化剂,减小毒物或空气的不良影响;c.胶体溶液的透光性能比颗粒要好得多,由此最近高分子保护金属胶体常被用于光化学研究中的催化剂;d.高分子保护金属胶体与普通金属胶体相比较,不仅具有较稳定的特点,而且尺寸小(1—10nm)、分布窄,表现出极高的催化活性和选择性;e.保护高分子对金属的修饰作用,可     

二氧化硅胶体晶体组装形貌研究

用原子力显微镜表征了二氧化硅胶体晶体的组装,探讨了二氧化硅微球用自然沉降法、抽滤法、溶剂挥发法组装时的组装行为,同时讨论了不同颗粒表面电位、不同溶剂介质及不同温度对其组装结果的影响。结果表明,颗粒表面电位是影响二氧化硅胶体晶体有序组装的重要因素之一。文中总结了最优的介质组成和温度条件,指出溶剂挥发法是较优的二氧化硅胶体晶体组装方法,其方法操作简单、周期短、得到的胶态晶体质量高,能在较大面积内高度有序。     

二氧化硅悬浮体的絮凝

微细颗粒的胶体二氧化硅广泛应用于橡胶工业、日用化学工业、无机材料工业等众多部门,它的专门用途不下千余种[1]．二氧化硅微细悬浮体是一种典型胶体分散体系,在其中加入大分子显的有机或无机絮凝剂,研究其絮凝规律,不但对研究絮凝现象的理论和实践有意义,而且有助于认识二氧化硅颗粒的表面性质和该分散体系的稳定性．1实验1·1材料和试剂二氧化硅（上海电化厂）：以燃烧法制得的）“品（白炭黑）,D＊T法测其比表面为7（）11。’·g‘．聚丙烯酸胺（＊＊M．广州南中塑料厂）：白色粉末状,用乙醇从水溶液中沉淀、提纯,粘度法测其…     

电膜过滤的效能

电膜过滤的效能李业琛，杨清娥（中国科学院广州化学研究所，５１０６５０）１前言在固液分散体中，对于悬浮液的颗粒以及溶胶的胶体粒子的分离常用过滤方法。所使用国家自然科学基金资助课题，的过滤介质分为深层过滤介质（由颗粒或纤维密堆积构成的厚层）和表面过滤介质...     

结合光学成像技术研究单颗粒碰撞电化学

近年来,单颗粒碰撞技术在纳米电化学领域受到广泛关注. 该技术通常控制超微电极处于某一电位,检测单个纳米颗粒随机碰撞到电极表面后产生的瞬时电流. 通过分析电流信号,可以研究单个纳米颗粒的性质. 尽管该技术可以检测单个纳米颗粒的电化学或电催化电流,但是传统的单颗粒碰撞技术缺乏空间分辨率,难以识别和表征特定的纳米颗粒. 因此,结合光学成像技术研究单颗粒碰撞电化学来补充电化学技术缺失的空间信息已成为一种趋势. 本文首先简要综述了单颗粒碰撞技术的三种检测原理,主要介绍了近年来单颗粒碰撞技术与荧光显微镜、表面等离激元共振显微镜、全息显微镜和电致化学发光相结合的研究进展,最后展望了单颗粒碰撞技术未来的发展趋势.     

PEG接枝PAN/PS高分子颗粒的形态控制研究

采用甲基丙烯酸单封端聚乙二醇大分子单体(MAA—PEG)为反应性稳定剂，使之与苯乙烯／丙烯腈在乙醇／水混合介质中三元分散共聚，利用单体间反应性差异及聚合物溶解性的不同，制备具有特殊形态的高分子颗粒．电子显微镜观察发现，颗粒形态与反应时间、单体配比、MAA—PEG浓度及分子量有关．X射线光电子能谱研究表明，增加反应时间，颗粒表面聚集的亲水性PEG成分增加，中心是聚苯乙烯，外部凸起部主要为聚丙烯腈，高分子颗粒具有核-壳结构．调节苯乙烯浓度、MAA—PEG浓度及介质组成可以控制颗粒大小，影响颗粒形态的主要因素是单体反应性差异和MAA—PEG的分子量及浓度．     

稳态时剪应力作用下胶体簇团形成/破裂的机理

Population correlation function P(t) has been used to examine the mechanism of breakage and coalescence of clusters at steady-state under shear, the results are in qualitative agreement with experiments. The research indicates that with a weak potential the mechanism of breakage and coalescence of clusters at steady-state under shear is predominately controlled by the particle-particle model, but that with a strong potential the mechanism shifts to that of cluster-cluster for large clusters; for small clusters, however, the mechanism of particle-particle model seems still to remain predominate, further work needs to be done.     

Population balance modeling of aggregation and breakage in turbulent Taylor-Couette flow

An experimental and computational study of aggregation and breakage processes for fully destabilized polystyrene latex particles under turbulent-flow conditions in a Taylor-Couette apparatus is presented. To monitor the aggregation and breakage processes, an in situ optical imaging technique was used. Consequently, a computational study using a population balance model was carried out to test the various parameters in the aggregation and breakage models. Very good agreement was found between the time evolution of the cluster size distribution (CSD) calculated with the model and that obtained from experiment. In order to correctly model the left-hand side of the CSD (small clusters), it was necessary to use a highly unsymmetric fragment-distribution function for breakage. As another test of the model, measurements with different solid volume fractions were performed. Within the range of the solid volume fractions considered here, the steady-state CSD was not significantly affected. In order to correctly capture the right-hand side of the CSD (large aggregates) at the higher solid volume fraction, a modified aggregation rate prefactor was used in the population balance model.     

Nonlinear behavior of polyethylene/layered double hydroxide nanocomposites under shear flow

The structural evolution of filler clusters in polyethylene/layered double hydroxide-based nanocomposites is investigated under application of a simple shear flow and is described in the framework of a modified Wagner model. Overall, the structural behavior of these polymer-clay nanocomposites is found to be similar to the behavior of filled elastomers for which breakdown of filler clusters at increasing strain and their reaggregation at decreasing strain were observed under oscillatory shear (Payne effect). Similar to the filled elastomers and other jammed systems, the polymer-clay nanocomposites demonstrate an asymmetric behavior upon approaching the steady state depending on whether the system was initially at higher or lower shear strain. In particular, the reaggregation time of filler structure in the quiescent state is found to be about one order of magnitude larger than the characteristic breakage time in the nonlinear shear regime. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A. 2008, Vol. 50, No. 5, pp. 868–881. This article was submitted by the authors in English.     

Monte Carlo studies of the coalescence and breakage characteristics of solid-liquid suspensions under shear

Summary A simple model for the breakage frequency of solid aggregates in a suspension under shear is developed based on the concept of bound liquid. This model is superior to the artificial models used by earlier workers. This is used along withSmoluchowski's expression for the coalescence frequency to simulate the aggregation-desaggregation behavior of suspensions using the Monte Carlo technique. The effect of initial particle size distribution, shear rate, viscosity of the medium, etc., are studied and are found to be in accord with intuitive expectations.With 5 figures     

Thermal restructuring of fractal clusters: the case of a strawberry-like core-shell polymer colloid

Thermal restructuring of fractal styrene-acrylate copolymer clusters dispersed in water has been investigated experimentally in the temperature range between 313 and 363 K. The particles constituting the clusters are of strawberry-like core-shell structure with a soft core and a rigid shell grafted on the core polymer chains. Due to the incomplete coverage of the core, the rather soft core may "flow out" through the open areas of the shell, leading to coalescence with the neighboring particles. The clusters were generated under diffusion-limited cluster aggregation conditions, and the restructuring kinetics was monitored by small-angle light scattering. Two sets of thermal restructuring experiments have been performed at various temperatures: (1) restructuring of growing clusters during aggregation and (2) restructuring of preformed clusters in the absence of aggregation. It is found that restructuring occurs only at temperature values above 323 K. In the absence of aggregation, restructuring leads to an increase of the fractal dimension and a decrease of the radius of gyration of the clusters. At sufficiently long times, both quantities reach a plateau value due to the presence of the grafted rigid shell, which constrains the coalescence of the soft core. A simple model, based on coalescence theory of liquid droplets and accounting for the incomplete coalescence and its dependence on temperature, has been developed to interpret the restructuring kinetics in the absence of aggregation. It is found that the proposed model can represent the measured experimental data well.     

Experimental and modeling study of breakage and restructuring of open and dense colloidal aggregates

In this work we present experimental and simulation analysis of the breakage and restructuring of colloidal aggregates in dilute conditions under shear. In order to cover a broad range of hydrodynamic and interparticle forces, aggregates composed of primary particles with two sizes, d(p) = 90 and 810 nm, were generated. Moreover, to understand the dependence of breakage and restructuring on the cluster structure, aggregates grown under stagnant and turbulent conditions, having substantially different initial internal structures with fractal dimension d(f) equal to 1.7 and 2.7, respectively, were used. The aggregates were broken by exposing them to a well-defined elongational flow produced in a nozzle positioned between two syringes. To investigate the evolution of aggregate size and morphology, respectively, the mean radius of gyration, , and d(f) were monitored during the breakup process using light scattering and confocal laser scanning microscopy. It was found that the evolution of aggregates' fractal dimension during breakage is solely controlled by their initial structure and is independent of the primary particles size. Similarly, the scaling of the steady-state vs the applied hydrodynamic stress is independent of primary particle size, however, depends on the history of aggregate structure. To quantitatively explain these observations, the breakage process was modeled using stokesian dynamics simulations incorporating DLVO and contact interactions among particles. The required flow-field for these simulations was obtained from computational fluid dynamics. The complex flow pattern was simplified by considering a characteristic stream line passing through the zone with the highest hydrodynamic stress inside the nozzle, this being the most critical flow condition experienced by the clusters. As the flow-field along this streamline was found to be neither pure simple shear nor pure extensional flow, the real flow was approximated as an elongational flow followed by a simple shear flow, with a stepwise transition between them. Using this approach, very good agreement between the measured and simulated aggregate size values and structure evolution was obtained. The results of this study show that the process of cluster breakup is very complex and strongly depends on the initial aggregate structure and flow-field conditions.     

Breakage rate of colloidal aggregates in shear flow through stokesian dynamics

We study the first breakage event of colloidal aggregates exposed to shear flow by detailed numerical analysis of the process. We have formulated a model, which uses stokesian dynamics to estimate the hydrodynamic interactions among the particles in a cluster, van der Waals interactions and Born repulsion to describe the normal interparticle interactions, and the tangential interactions through discrete element method to account for contact forces. Fractal clusters composed of monodisperse spherical particles were generated using different Monte Carlo methods, covering a wide range of cluster masses (N(sphere) = 30-215) and fractal dimensions (d(f) = 1.8-3.0). The breakup process of these clusters was quantified for various flow magnitudes (γ), under both simple shear and extensional flow conditions, in terms of breakage rate constant (K(B)), mass distribution of the produced fragments (FMD, f(m,k)), and critical stable aggregate mass (N(c)), defined as the largest cluster mass that does not break under defined flow conditions. The breakage rate K(B) showed a power law dependence on the product of the aggregate size and the applied stress, with values of the corresponding exponents depending only on the aggregate fractal dimension and the type of flow field, whereas the prefactor of the power law relation also depends on the size of the primary particles comprising a cluster. The FMD was fitted by Schultz-Zimm distribution, and the parameter values showed an analogous dependence on the product of the aggregate size and the applied stress similar to the rate constant. Finally, a power law relation between the applied stress and corresponding largest stable aggregate mass was found, with an exponent value depending on the aggregate fractal dimension. This unique and detailed analysis of the breakage process can be directly utilized to formulate a breakage kernel used in solving population balance equations.     

The morphology of immiscible PDMS/PIB blends filled with silica nanoparticles under shear flow

The influence of surface nature (hydrophobic and hydrophilic) and concentration of silica nanoparticles on the coalescence behavior of immiscible polydimethylsiloxane (PDMS)/polyisobutylene (PIB) (90/10) blends under simple low-rate shear flow were investigated via optical shear technique. It was found that the coalescence of PIB droplets in PDMS matrix was suppressed efficiently by incorporating hydrophobic silica nanoparticles, and a constant droplet size was obtained at high particle contents. The addition of a small amount (<0.4 wt.%) of hydrophilic silica nanoparticles also decreased the size of PIB droplets. Clusters of small PIB droplets were formed at low filler concentration. When the filler concentration exceeded 0.8 wt.%, the clusters of PIB drops disappeared and elongated PIB threads with large size were formed, which suggest that the coalescence of PIB droplets was promoted. The results indicate that the discrepancy in the morphology evolution of PDMS/PIB blends upon the addition of silica nanoparticles is controlled not only by the surface chemistry of nanoparticles but also by their concentration in the blends.     

Confined flow of polymer blends

The influence of confinement on the steady-state morphology of two different emulsions is investigated. The blends, made from polybutene (PB) in polydimethylsiloxane (PDMS) and polybutadiene (PBD) in PDMS, are sheared between two parallel plates, mostly with a standard gap spacing of 40 microm, in the range of shear rates at which the transition from "bulk" behavior toward "confined" behavior is observed. For both cases, the influence of the concentration was systematically investigated, as well as the shear rate effects on the final steady-state morphology. By decreasing the shear rate, for each blend, the increasing droplets, i.e., increasing confinement for a fixed gap spacing, arrange themselves first into two layers, and when the degree of confinement reaches an even higher value, a single layer of droplets is formed. The ratio between the drop diameters and the gap spacing at which this transition occurs is always lower than 0.5. While decreasing the shear rate, the degree of confinement increases due to drop coalescence. Droplets arrange themselves in superstructures like ordered pearl necklaces and, at the lower shear rates, strings. The aspect ratio and the width of the droplet obtained from optical micrographs are compared to predictions of the single droplet Maffettone-Minale model (MM model(1)). It is found that the theory, meant for unconfined shear flow, is not able to predict the drop deformation when the degree of confinement is above a critical value that depends on the blends considered and the shear rate applied. A recently developed extension of the MM model is reported by Minale (M model(2)) where the effect of the confinement is included by using the Shapira-Haber correction.3 Further extending this M model, by incorporating an effective viscosity as originally proposed by Choi and Showalter,4 we arrive at the mM model that accurately describes the experiments of blends in confined flow.     

Analysis of the aggregation-fragmentation population balance equation with application to coagulation

Coagulation of small particles in agitated suspensions is governed by aggregation and breakage. These two processes control the time evolution of the cluster mass distribution (CMD) which is described through a population balance equation (PBE). In this work, a PBE model that includes an aggregation rate function, which is a superposition of Brownian and flow induced aggregation, and a power law breakage rate function is investigated. Both rate functions are formulated assuming the clusters are fractals. Further, two modes of breakage are considered: in the fragmentation mode a particles splits into w2 fragments of equal size, and in the erosion mode a particle splits into two fragments of different size. The scaling theory of the aggregation-breakage PBE is revised which leads to the result that under the negligence of Brownian aggregation the steady state CMD is self-similar with respect to a non-dimensional breakage coefficient theta. The self-similarity is confirmed by solving the PBE numerically. The self-similar CMD is found to deviate significantly from a log-normal distribution, and in the case of erosion it exhibits traces of multimodality. The model is compared to experimental data for the coagulation of a polystyrene latex. It is revealed that the model is not flexible enough to describe coagulation over an extended range of operation conditions with a unique set of parameters. In particular, it cannot predict the correct behavior for both a variation in the solid volume fraction of the suspension and in the agitation rate (shear rate).