重力对溶液悬浮粒子聚集过程影响的布朗动力学模拟

采用布朗动力学的计算机模拟方法, 研究重力因素对于稀溶液中悬浮粒子聚集过程的影响. 通过在计算机模拟程序中加入和排除重力因素的影响, 对不同重力条件下的粒子团数量随时间的变化曲线进行对比研究, 得到了重力对溶液中的粒子团总数和不同大小的粒子团数量随时间变化的影响规律,可以总结为: 在聚集阶段初期,重力不影响粒子的聚集; 而在聚集阶段后期, 重力加快了粒子的聚集. 同时, 从动力学分析的角度出发, 对重力如何影响悬浮溶液中粒子聚集过程的机制也进行了更加深入的探讨.     

范德华作用和静电作用下的二元粒子自组装

采用简单粗粒化粒子模型,通过郎之万动力学模拟研究了具有范德华作用和静电作用的二元粒子自组装．研究发现,通过改变粒子尺寸和粒子间作用强度,二元粒子能够自发形成各种聚集结构,如球形、堆叠层状与管状结构．利用两亲性分子或两嵌段聚合物自组装理论,解释了二元粒子聚集结构的形成规律．当向溶液中加入反电荷离子时,模拟表明粒了聚集结构在相图中的分布出现了明显偏移．     

Patchy粒子聚集的统计力学研究

应用统计力学原理对A_aB_b型Patchy粒子的聚集过程进行研究, 考察了典型平均物理量在聚集过程中的变化情况. 首先基于配分函数导出体系的平衡自由能及描述Patchy粒子之间联接作用的质量作用定律, 进而获得团簇的数量分布函数. 进一步给出Patchy团簇的数均和重均聚合度以及物理凝胶化条件, 探讨了凝胶化区域与Patchy粒子数之间的依赖关系. 同时给出Patchy团簇生长的微分动力学方程, 并进行了相应的Monte Carlo模拟. 本文旨在揭示Patchy粒子的内在和外在因素对体系聚集态结构的影响, 为实现对Patchy粒子体系的有效调控提供理论依据.     

胶粒分形粒子簇反应控制聚集动态行为位垒影响的Monte Carlo模拟

通过对反应控制聚集过程的MonteCarlo模拟,从微观及介观层次上探讨了胶粒间相互作用位能曲线上位垒高度的变化对胶粒分形粒子簇大小分布和动态标度函数及聚集动力学行为的影响规律。     

软胶体粒子形成束晶的动力学模拟

利用广义指数模型描述软胶体粒子, 结合分子动力学模拟研究软胶体粒子形成束晶的动力学过程. 通过等温压缩和等密度降温2个不同的过程, 研究了束晶形成过程中结构变化特征和动力学路径对结构的影响规律. 研究发现, 与蒙特卡洛模拟结果相比, 分子动力学模拟得到的结构随着密度的变化有明显的迟滞现象, 这是由于考虑了真实的动力学因素引起的差异. 此外, 在相同温度和压力下通过不同的动力学路径得到的相结构不完全相同, 这是由于动力学形成过程会对相结构产生很大的影响.     

Monte Carlo模拟研究有机分子的模板诱导定位沉积

利用动力学Monte Carlo模拟方法系统研究了有机分子在模板诱导下的构图机理,从微观角度详细分析了模板诱导构图中有机分子对诱导机理的选择.结果表明,有机粒子间的相互作用力是制约沉积粒子选择构图机理的关键因素,直接决定了模板对沉积粒子的诱导控制能力和沉积粒子的最终聚集构型.通过系统的模拟,我们找到了模板完全控制形核区域,并且根据粒子聚集机理的不同将该区域细分为:台阶诱导构图区域、模板上表面定位聚集区域和模板定位区域.另外,我们还详细分析了不同形核机理下聚集构型的生长过程,发现最终的粒子聚集构型还受到沉积粒子数量的制约,不同构图区域的形核机理甚至会因沉积粒子数量的增多而发生转变.     

刚球粒子碰撞频率和距离关系的计算机模拟

用Montecallo方法模拟了由两个刚球粒子组成的体系，得到了碰撞频率和粒子间距的定量关系。模拟结果显示，碰撞频率随着粒子间距的增大急剧下降。这一特性为文献[1]中描述的两个粒子体系的聚集过程的碰撞反应动力学提供了有用的旁证。     

布朗扩散系数对胶体聚集粒子簇大小动态标度影响的Monte Carlo模拟研究

通过对胶体扩散控制聚集机理的MonteCarlo模拟,表明粒子簇的布朗扩散系数与其大小和形状有关,粒子簇的大小分布可被标度.在微观或介观的层次上,揭示了表征粒子簇结构的几何形状因子在扩散控制聚集过程中对动态标度和粒子簇分布的影响.     

梳型嵌段共聚物微观相分离的耗散粒子动力学模拟

利用耗散粒子动力学(Dissipative particle dynamics, DPD)模拟方法研究了二维梳型嵌段共聚物的微观相分离, 得到了相形貌与侧链长度及链段间相互作用的依赖关系, 进一步与线型和星型嵌段共聚物微观相分离进行了对比. 模拟结果揭示了本体中影响梳形嵌段共聚物微观相分离的主要因素, 包括嵌段共聚物的组成\, 拓扑结构以及不同粒子间的排斥力.     

基于纳米粒子的比色检测及其在化学传感及生物检测的应用

基于纳米粒子的比色检测由于其简单性、多样性而备受关注。在该体系中,目标分析物直接或间接诱发纳米粒子的聚集和分散,从而可通过溶胶的颜色改变实现分析检测。该综述介绍了纳米粒子分散及聚集过程的物理现象,简单讨论了纳米粒子间的作用力,对两种作用机制(粒子间交联和非粒子间交联来调节纳米粒子的分散和聚集)进行了介绍。并对纳米粒子在重金属离子检测、生物检测等方面的应用及方法的分析特性进行了概述,对其未来发展趋势进行了展望。     

Computer simulation on the collision-sticking dynamics of two colloidal particles in an optical trap

Collisions of a particle pair induced by optical tweezers have been employed to study colloidal stability. In order to deepen insights regarding the collision-sticking dynamics of a particle pair in the optical trap that were observed in experimental approaches at the particle level, the authors carry out a Brownian dynamics simulation. In the simulation, various contributing factors, including the Derjaguin-Landau-Verwey-Overbeek interaction of particles, hydrodynamic interactions, optical trapping forces on the two particles, and the Brownian motion, were all taken into account. The simulation reproduces the tendencies of the accumulated sticking probability during the trapping duration for the trapped particle pair described in our previous study and provides an explanation for why the two entangled particles in the trap experience two different statuses.     

Dynamic properties of the AgI-solution interface: Implications for colloid stability

This paper discusses the dynamics (i.e. the rates of the various charge displacement processes) of electrical double layers after disequilibration, taking the AgI-electrolyte solution interface as the model system. Relaxation studies performed with Ag/AgI electrodes lead to the conclusion that there is a relatively slow step, connected with the transfer of charge through the solid-solution interface. Evidence is given that this step occurs also with colloidal particles under interaction. It is inferred that in AgI sols interaction takes place at constant total particle charge, but at variable charge density distribution. The effect of particle radius a is also considered, an important parameter being the ratio τ_l/τ_int between the characteristic time scales of lateral charge flow and interaction. This ratio is dominated by the hydrodynamic drag. It appears that these dynamic factors level off the effect of the particle radius on the rate of coagulation. Other implications for colloid stability are discussed.     

Numerical simulation and experimental verification of particle coagulation dynamics for a pulsed input

Mathematical simulation of particle coagulation dynamics was carried out using improved sectional modeling techniques for a system with a pulsed input of primary particles. The methodological improvement included the modification of the size density function based on a realistic assumption of particle size distributions, the application of a new and comprehensive curvilinear collision model, and special adjustment for the mass transfer of a doublet of particles that were very different in size. The simulation results demonstrated that the rectilinear model over-predicted the rate of particle coagulation and that the degree of over-prediction increased as the particles increased in size and the system became more heterogeneous. The coagulation rate increased remarkably as the fractal dimension of the particle aggregates decreased. The curvilinear model and the fractal scaling relationship in place of the rectilinear model and the Euclidean sizing geometry are two important modifications to the conventional Smoluchowski modeling approach. However, both modifications, rather than only one of them, should be applied together to produce more accurate and realistic simulations of coagulation dynamics. As indicated by the simulation, the importance of fluid shear rate to particle coagulation is reduced according to the curvilinear model compared to that previously described with the rectilinear model. As particles increased in size, the role of shear rate in coagulation became even less significant according to the curvilinear view of particle collisions. The results of numerical simulations in terms of the evolution of particle size distributions compared reasonably well with the observations of the jar-test coagulation experiments, which suggested the applicability of the modeling system, including the modified curvilinear-fractal approach, established in the present study.     

流体力学半径对估算胶体微球聚集速率常数的影响

胶体粒子聚集速率常数实验值远低于理论值一直是被普遍关注的问题.聚集速率常数的理论推导是基于粒子的几何半径来考虑的,但决定粒子扩散速率及聚集速率的应该是粒子的流体力学半径(大于几何半径),因而它是使聚集速率常数实验值低于理论值的因素之一.影响流体力学半径的因素很多,其中,带电粒子在溶液中因表面存在双电层,会明显增大流体力学半径,造成聚集速率减慢.而双电层的厚度又随溶液中离子强度的不同而改变.本工作在聚集速率的公式中引入了修正因子,即几何半径与其流体力学半径之比,以修正由于用几何半径代替流体力学半径带来的误差.其中几何半径和流体力学半径可以分别用扫描电镜(SEM)和动态光散射(DLS)来测定.以两种粒径的聚苯乙烯带电微球为例,考察了在不同离子强度下,该误差的大小.结果发现,对于半径为30 nm的微球,用流体力学半径计算的慢聚集速率常数比理论值偏低约8%.该误差随离子强度增加而减少.对于快聚集情况,流体力学半径对聚集速率基本没有影响.     

凝并和成核机理下颗粒尺度分布的Monte Carlo求解

颗粒的凝并和成核现象影响其尺度分布,现有的MonteCarlo方法描述颗粒尺度分布的时间演变过程存在若干困难.提出了一种新的多重MonteCarlo(MMC)算法,基于时间驱动,利用加权的虚拟颗粒的思想,在模拟过程中保持虚拟颗粒总数不变和计算区域体积不变.利用该算法对“常凝并核,一阶成核”的情况下颗粒尺度分布的时间演变过程进行了数值求解,所得结果与数值解相符,表明MMC算法具有高且稳定的计算精度.另外,MMC算法由于跟踪比实际颗粒数目少得多的虚拟颗粒而具有较低的计算代价.     

Manipulation and characterization of aqueous sodium dodecyl sulfate/sodium chloride aerosol particles

Aerosol optical tweezers coupled with Raman spectroscopy can allow the detailed investigation of aerosol dynamics. We describe here measurements of the evolving size, composition, and phase of single aqueous aerosol droplets containing the surfactant sodium dodecyl sulfate and the inorganic salt sodium chloride. Not only can the evolving wet particle size be probed with nanometer accuracy, but we show that the transition to a metastable microgel particle can be followed, demonstrating that optical tweezers can be used to manipulate both spherical and non-spherical aerosol particles. Further, through the simultaneous manipulation and characterization of two aerosol droplets of different composition in two parallel optical traps, the phase behavior of a surfactant-doped particle and a surfactant-free droplet can be compared directly in situ. We also illustrate that the manipulation of two microgel particles can allow studies of the coagulation and interaction of two solid particles. Finally, we demonstrate that such parallel measurements can permit highly accurate comparative measurements of the evolving wet particle size of a surfactant-doped droplet with a surfactant-free droplet.     

Slow dynamics of a colloidal lamellar phase

We used x-ray photon correlation spectroscopy to study the dynamics in the lamellar phase of a platelet suspension as a function of the particle concentration. We measured the collective diffusion coefficient along the director of the phase, over length scales down to the interparticle distance, and quantified the hydrodynamic interaction between the particles. This interaction sets in with increasing concentration and can be described qualitatively by a simplified model. No change in the microscopic structure or dynamics is observed at the transition between the fluid and the gel-like lamellar phases.     

Synthesis of large-scale,narrowly dispersed,highly cross-linked,and spherical latex particles via one-step emulsion polymerization through particle coagulation

Particle coagulation technology is a facile approach to prepare large-scale and narrowly dispersed polymer particles. However, diverse shapes such as ellipsolid, snowman, dumbbell, and trimer among others were obtained if the cross-linker was directly added into the initial reaction mixtures due to the restriction of the highly cross-linking particle fusion process. In this study, we prepared sub-200?nm, narrowly dispersed, highly cross-linked, and spherical latex particles using particle coagulation technology by controlling the relation between the cross-linking net formation and particle coagulation. Depending on the addition time or feeding rate of the cross-linker (divinylbenzene, DVB), the particles with different sizes or shapes were obtained. The later the addition start time of DVB, the narrower the particle size distribution of the latex particles. Alternatively, the increase of the continuing feeding time could also be used to decrease the width of particle size distribution of the ultimate latex. In addition, narrowly dispersed and spherical latex particles also could be directly obtained by advancing the particle coagulation time using 2, 2′-Azobis (2-methylpropionamidine) dihydrochloride as a cationic initiator. Our study presents a new method that will further widen the fields of application of particle coagulation technology.     

Interaction of micrometer-scale particles with nanotextured surfaces in shear flow

Dynamic particle adhesion from flow over collecting surfaces with nanoscale heterogeneity occurs in important natural systems and current technologies. Accurate modeling and prediction of the dynamics of particles interacting with such surfaces will facilitate their use in applications for sensing, separating, and sorting colloidal-scale objects. In this paper, the interaction of micrometer-scale particles with electrostatically heterogeneous surfaces is analyzed. The deposited polymeric patches that provide the charge heterogeneity in experiments are modeled as 11-nm disks randomly distributed on a planar surface. A novel technique based on surface discretization is introduced to facilitate computation of the colloidal interactions between a particle and the heterogeneous surface based on expressions for parallel plates. Combining these interactions with hydrodynamic forces and torques on a particle in a low Reynolds number shear flow allows particle dynamics to be computed for varying net surface coverage. Spatial fluctuations in the local surface density of the deposited patches are shown responsible for the dynamic adhesion phenomena observed experimentally, including particle capture on a net-repulsive surface.     

Particle bridging between oil and water interfaces

Particle bridging between a water drop and a flat oil-water interface has been observed when the drop is brought into contact with the interface, leading to the formation of a dense particle monolayer of disc shape (namely, particle disc) that prevents the drop from coalescing into the bulk water phase. Unlike previous observations where particles from opposite interfaces appear to register with each other before bridging, the present experiment demonstrates that the particle registry is not a necessity for bridging. In many cases, the particles from one of the interfaces were repelled away from the contact region, leaving behind the particles from the other interface to bridge the two interfaces. This is confirmed by particle bridging experiments between two interfaces covered with different sized particles, and between a particle-covered interface and a clean interface. The dynamics associated with the growth of the particle disc due to particle bridging follows a power law relationship between the radius of the disc and time: r proportional, variant t0.32+/-0.03. A scaling analysis assuming capillary attraction as the driving force and a hydrodynamic resistance leads to the power law r proportional, variant t1/3, in good agreement with the experiment. In addition, we found that binary mixtures of two different sized particles can undergo phase segregation driven by the particle bridging process.