电解质对浓悬浮液中胶体颗粒扩散特性的影响

本文利用相位调制光纤低相干动态光散射装置, 研究了不同物质量浓度下电解质 (NaCl及 BaCl₂) 对浓悬浮液中聚苯乙烯胶体颗粒扩散特性的影响. 实验结果表明, 当电解质浓度低于0.01 mol/L 时, 恒温条件下浓悬浮液中聚苯乙烯胶体颗粒扩散系数随电解质离子浓度以及离子化合价的增大而增大, 实验测量得到的扩散系数与Stern模型所得到的扩散系数符合较好. 关键词： 测量 电解质 浓悬浮液 低相干动态光散射     

电解质浓度对胶体粒子表面有效电荷的影响

胶体粒子的表面有效电荷是决定胶体性质的重要物理量,但溶液环境(如电解质溶液浓度)是否影响其数值至今尚无统一认识,近年来的一些研究工作给出了存在争议的不同结果和假设.在直接实验测量方面,由于电解质离子和胶体表面吸附离子的置换,粒子表面基团的不完全电离和胶体粒子对离子吸附的共同作用,使得对这类粒子在不同溶液环境下的表面有效电荷的测量和变化机理的认识极为困难.针对该问题,本文测定了羧基和磺酸基修饰的聚苯乙烯胶体颗粒在不同粒子浓度和HCl浓度下的电导率,由于两种粒子与HCl电离产生的阳离子相同(均为H+),可根据电导率-数密度法(迁移法)得到胶体颗粒表面有效电荷数.通过实验结果分析,明确了HCl浓度以及粒子数密度对胶体粒子表面电荷的影响规律以及表面电荷随HCl浓度增大的原因.除此之外,羧基修饰颗粒比磺酸基修饰颗粒的表面电荷随HCl浓度变化更快;对于同一HCl浓度,磺酸基修饰胶体表面电荷不受粒子数密度影响,而羧基修饰胶体颗粒却与之相关.基于粒子表面电荷的理论模型,对这些问题都给出了相应的解释.     

CONTIN算法及其在测量微凝胶颗粒粒度分布中的应用

颗粒粒径大小决定了微凝胶的相变行为,因此采用有效可靠的手段来确定胶体悬浮液中颗粒的平均粒径及粒度分布是至关重要的。CONTIN算法是分析动态光散射实验数据,获取胶体悬浮液中颗粒粒度分布的有效算法,但目前的最优正则化参数选取策略依赖于颗粒粒度分布的先验条件。为此,本文提出利用V-曲线准则获取最优正则化参数,使用CONTIN算法表征微凝胶悬浮液颗粒系的平均粒径和粒径分布信息。实验结果表明,与V-曲线正则化参数选取准则相结合,利用CONTIN算法可以有效的获取微凝胶悬浮液的颗粒粒度信息。     

碳纳米粒子悬浮液光限幅性能数值模拟

碳纳米粒子悬浮液具有良好的光限幅性质,是一种优良的宽波段光限幅材料。通过热传导方程和米氏散射理论建立了微气泡半径与入射光能量、碳纳米粒子悬浮液散射系数和透过率的理论模型。采用Matlab数值模拟了散射系数随微气泡尺寸因子的变化关系,碳纳米粒子悬浮液光限幅性能随入射光能量的变化规律。研究了气泡尺寸因子、入射激光能量以及波长对碳纳米粒子悬浮液光限幅特性的影响。研究发现当激光能量达到一定值时,微气泡的半径保持恒定,不再随入射激光能量的增加而增加。微气泡尺寸的增大对碳纳米粒子悬浮液的透过率有着显著的影响。同时,碳纳米粒子悬浮液对不同入射光波长和光能表现出不同的光限幅性能。研究结果为实验研究提供了理论指导。     

吸附作用在纳米颗粒悬浮液换热强化中的试验与机理研究

本文通过测量SiO_2纳米颗粒在不同工质、不同浓度的悬浮液在不同温度下的导热系数,结合了液氮冷冻、切割并在断面上复形的技术,并利用电镜观察了纳米颗粒在液体中的分布、团聚和液体与之的亲和性。着重分析了纳米颗粒悬浮液中SiO_2纳米颗粒与液体的亲和性对悬浮液导热性能影响的机理。文中提出了SiO_2纳米颗粒的表面吸附层和在作布朗运动时出现”微对流”而使悬浮液换热强化。     

高能α粒子对环形等离子体中低频漂移波稳定性的影响

本文利用强耦合近似,研究了高能α粒子密度梯度对环形等离子体中低频漂移波稳定性的影响。结果表明:对频率与离子反磁漂移频率ω_1相近的漂移波,α离子有降低稳定性的作用;对频率远比ω_i高的弱剪切中的漂移波,α粒子起稳定作用。     

基于空间光场调控技术的光学微操纵

光镊技术具有无机械接触与高精度操纵微小尺度粒子的优点,自发明以来已逐渐成为生命科学和胶体物理学等领域中强有力的研究工具。随着研究的深入,传统的单光阱光镊已难以满足更高级的应用需求。近年来,空间光场调控技术通过对光场的振幅、相位和偏振态分布的调制,极大地丰富和增强了光学微操纵技术的功能,促进了包括激光微纳加工、微粒分选与输运、胶体粒子物理特性研究等方面的发展。从光场的振幅、相位和偏振态调制技术出发,综述近年来空间光场调控技术的发展及其在光学微操纵中的应用,重点介绍全息光镊、特殊模式光束微操纵、矢量光场微操纵等光学微操纵技术的研究进展。     

基于倏逝场微小粒子驱动技术研究进展

处于倏逝场中的微小粒子会受到辐射压力的作用而朝着倏逝场的传播方向运动,基于此原理的微小粒子驱动技术可用于介质颗粒、胶体颗粒、生物细胞等微小粒子的捕获和驱动.由于倏逝场光学微操作系统不会受到物镜焦深和激光光斑尺寸的限制,因此它比自由空间系统的优越性更强,而波导形成的光学力可以应用于长距离驱动,其仅仅受限于系统的散射和吸收...     

利用胶体系统研究玻璃态

玻璃态是一种无序结构的固体,组成单元可以是原子、分子、高分子、胶体粒子等等。尽管玻璃态材料在生活中很常见,有广泛的工业应用,但相关理论,尤其是液体到玻璃态转变的理论是物理学中争议最多的领域之一。溶液中的胶体粒子可以形成晶体、液体、玻璃等各种态。在光学显微镜下可以直接看到三维内部单个微米胶体粒子,通过图像处理还可以得到粒子的布朗运动轨迹,从而得到玻璃化转变过程中的微观动力学信息,这是原子分子玻璃系统中难以测量的。文章介绍了胶体作为模型系统对玻璃态的研究,主要包括传统的过冷液体到玻璃态的转变,另外还涉及气相、凝胶、多晶等其他无序或半无序态与玻璃态之间的过渡或转变。     

一类大气非线性动力热力学反应-扩散系统解的稳定性态

研究了一类大气非线性系统. 利用大气非线性热力动力学理论, 讨论了大气非线性强迫耗散系统的速度、温度和湿度所满足的非线性反应-扩散系统, 再利用微分方程Lyapunov稳定性理论, 得到了大气非线性反应-扩散系统在均匀定态解邻域内的微扰解. 最后由系统的控制参数的变化, 得到了大气非线性强迫耗散系统有关的物理量的有序-无序-有序状态的转化过程, 从而可以预报和预测相应的局部大气非线性强迫耗散系统的稳定性态.     

Hydrodynamic Stress Tensor in Inhomogeneous Colloidal Suspensions: an Irving-Kirkwood Extension *

Based on statistical mechanics for classical fluids, general expressions for hydrodynamic stress in inhomogeneous colloidal suspension are derived on a molecular level. The result is exactly an extension of the Iving-Kirkwood stress for atom fluids to colloidal suspensions where dynamic correlation emerges. It is found that besides the inter-particle distance, the obtained hydrodynamic stress depends closely on the velocity of the colloidal particles in the suspension, which is responsible for the appearance of the solvent-mediated hydrodynamic force. Compared to Brady's stresslets for the bulk stress, our results are applicable to inhomogeneous suspension, where the inhomogeneity and anisotropy of the dynamic correlation should be taken into account. In the near-field regime where the packing fraction of colloidal particles is high, our results can reduce to those of Brady. Therefore, our results are applicable to the suspensions with low, moderate, or even high packing fraction of colloidal particles.     

Automatic detection of colloidal particles based on a shape regularized integrated active contour model

Manual segmentation of single colloidal particle in suspension encounters a bottleneck when a number of defocused particles simultaneously exist in an image. In this paper, we describe an image processing algorithm for extracting individual particle from digitized microscope images of colloidal suspensions. We propose a particle detection and location solution using a shape regularized integrated active contour model (ACM). Compared with existing methods where active contour models are not applied well to deal with multiple objects in complicated background, the proposed approach can automatically identify and locate multiple particles by combining characteristics of the particles such as shape, boundary and region. A regularization term is defined by prior information of specific shape, which is able to drive the shape of evolving curve toward the shape prior gradually. To locate the centers of the particles, the Hough transform is applied. Experimental results using polystyrene beads as sample particles reveal that the method has high efficiency and ability to deal with colloidal particles.     

旋涂法快速制备双层二元胶体微球有序薄膜

以较大粒径的聚苯乙烯或SiO₂胶体微球的单层有序薄膜作基膜,较小粒径的SiO₂微球作第二层,用分步旋涂法快速制备了二元双层胶体微球复合有序薄膜.膜中小粒径微球与大粒径微球的粒径比γ=020—056,大粒径与小粒径微球的排列方式可表示为LS_x（x=1,2,…,13）.旋涂速度、旋涂时间、微球悬浮介质的黏度、悬浮液中微球的数密度、旋涂衬底的可润湿性等因素均会影响旋涂组装胶粒薄膜的质量.在旋涂衬底能够被胶体微球悬浮介质完全润湿的前提下,适宜的胶体微球数密度、旋涂速度、旋涂时间是旋涂组装有序薄膜的必要条件. 关键词： 复合有序薄膜 分步旋涂 胶体微球模板     

On the influence of the hydrodynamic interactions on the aggregation rate of magnetic spheres in a dilute suspension

Magnetostatic attraction may lead to formation of aggregates in stable colloidal magnetic suspensions and magneto-rheological suspensions. The aggregation problem of magnetic composites under differential sedimentation is a key problem in the control of the instability of non-Brownian suspensions. Against these attractive forces are the electrostatic repulsion and the hydrodynamic interactions acting as stabilizing effects to the suspension. This work concerns an investigation of the pairwise interaction of magnetic particles in a dilute sedimenting suspension. We focus attention on suspensions where the Péclet number is large (negligible Brownian motion) and where the Reynolds number (negligible inertia) is small. The suspension is composed of magnetic micro-spheres of different radius and density immersed in a Newtonian fluid moving under the action of gravity. The theoretical calculations are based on direct computations of the hydrodynamic and the magnetic interactions among the rigid spheres in the regime of low particle Reynolds number. From the limiting trajectory in which aggregation occurs, we calculate the collision efficiency, representing the dimensionless rate at which aggregates are formed. The numerical results show clear evidence that the hydrodynamic interactions are of fundamental relevance in the process of magnetic particle aggregation. We compare the stabilizing effects between electrostatic repulsion and hydrodynamic interactions.     

Colloidal glass transition observed in confinement

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.     

The Colloidal Probe Technique and its Application to Adhesion Force Measurements

Knowledge of the interaction forces between colloidal particles and surfaces is a precondition for understanding the stability of dispersed systems and adhesion phenomena. One of the methods available for direct measurement of surface forces is the atomic force microscope (AFM). Based on this method the so called “colloidal probe technique” was developed more than 10 years ago. Using a micron‐sized particle glued to the end of an AFM cantilever as the force sensor, this technique is predestined for the study of colloidal interactions. In this review we describe the colloidal probe technique and give an overview of its application in the field of adhesion forces.     

Kinetics of colloidal fractal aggregation by differential dynamic microscopy

We study the kinetics of an aggregation process induced by adding salt to a stable colloidal suspension of 73 nm (diameter) particles. Despite the subdiffraction size of the colloidal particles, the process is monitored via optical microscopy, which is used here to obtain time-resolved scattering information about the colloidal aggregates. The radius of the aggregates is determined as a function of time and their fractal dimension is extracted. Our results are compatible with a diffusion limited aggregation process, as independently confirmed by spectral turbidimetry measurements on the same sample.     

椭球与圆球混合胶体体系的玻璃化转变

以椭球与圆球混合的胶体体系为研究对象,通过增加体系的面积分数,从实验上研究了混合体系发生玻璃化转变过程中结构和动力学行为的演变规律.在结构方面,通过计算和分析径向分布函数、泰森多边形以及取向序参量,发现椭球可以有效地抑制圆球结晶,整个体系在结构上始终保持无序.在动力学方面,通过计算体系的均方位移和自散射函数,发现随着面积分数的增加,体系的动力学明显变慢,弛豫时间在接近模耦合理论预测的玻璃化转变点快速增大并发散.通过考察快速粒子参与的协同重排行为,发现协同重排区域形状、大小和位置都与椭球的存在密切关联.     

Light scattering studies of Brownian motion in confined geometries

Summary Dynamic light scattering can be a useful tool to determine the confinement of Brownian particles whose motion is restricted to dimensions comparable to the wavelength of the light. The theoretical form of the correlation function of the electric field scattered from such trapped particles has been derived and compared with the signal obtained both in a simulated experiment and in a real experiment where the particles are trapped in a glass wedge. This new result can be of relevance for particles trapped in various media such as a porous (transparent) media, a gel, a suspension of lamellar phases or even a concentrated colloidal suspension where a particle is ?trapped? by its neighbours. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.