Patchy胶体粒子的结晶、玻璃化和凝胶化问题

本文探讨了中性多缔合位点Patchy胶体粒子系统的相图及其相关问题. 在研究中,计入了分子间的硬芯Lennard-Jones势和缔合作用,进而阐明了系统的流体相(F),无规密积相(RCP)和面心立方相(FCC)之间转变的相态结构. 在体系丰富的相结构中,F-F,F-RCP及F-FCC相转变以及描述粒子间联结性的溶胶-凝胶转变相互影响,致使一些相态在不同相互作用强度时可以呈现亚稳态和稳态. 同时,本文重点阐述了缔合能量以及patch数目对体系的临界温度、临界密度、临界三相点以及溶胶-凝胶转变等的调控机制.     

Crystallization in a dense suspension of self-propelled particles

Using Brownian dynamics computer simulations, we show that a two-dimensional suspension of self-propelled ("active") colloidal particles crystallizes at sufficiently high densities. Compared to the equilibrium freezing of passive particles, the freezing density is both significantly shifted and depends on the structural or dynamical criterion employed. In nonequilibrium the transition is accompanied by pronounced structural heterogeneities. This leads to a transition region between liquid and solid in which the suspension is globally ordered but unordered liquidlike "bubbles" still persist.     

Colloids in light fields: Particle dynamics in random and periodic energy landscapes

The dynamics of colloidal particles in potential energy landscapes have mainly been investigated theoretically. In contrast, here we discuss the experimental realization of potential energy landscapes with the help of laser light fields and the observation of the particle dynamics by video microscopy. The experimentally observed dynamics in periodic and random potentials are compared to simulation and theoretical results in terms of, e.g. the mean-squared displacement, the time-dependent diffusion coefficient or the non-Gaussian parameter. The dynamics are initially diffusive followed by intermediate subdiffusive behaviour which again becomes diffusive at long times. How pronounced and extended the different regimes are, depends on the specific conditions, in particular the shape of the potential as well as its roughness or amplitude but also the particle concentration. Here we focus on dilute systems, but the dynamics of interacting systems in external potentials, and thus the interplay between particle-particle and particle-potential interactions, are also mentioned briefly. Furthermore, the observed dynamics of dilute systems resemble the dynamics of concentrated systems close to their glass transition, with which it is compared. The effect of certain potential energy landscapes on the dynamics of individual particles appears similar to the effect of interparticle interactions in the absence of an external potential.     

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.     

Application of the dissipative particle dynamics method to magnetic colloidal dispersions

The validity of the application of the dissipative particle dynamics (DPD) method to ferromagnetic colloidal dispersions has been investigated by conducting DPD simulations for a two–dimensional system. First, the interaction between dissipative and magnetic particles has been idealized as some model potentials, and DPD simulations have been carried out using such model potentials for a two magnetic particle system. In these simulations, attention has been focused on the collision time for the two particles approaching each other and touching from an initially separated position, and such collision time has been evaluated for various cases of mass and diameter of dissipative particles and model parameters, which are included in defining the equation of motion of dissipative particles. Next, a multi–particle system of magnetic particles has been treated, and particle aggregates have been evaluated, together with the pair correlation function along an applied magnetic field direction. Such characteristics of aggregate structures have been compared with the results of Monte Carlo and Brownian dynamics simulations in order to clarify the validity of the application of the DPD method to particle dispersion systems. The present simulation results have clearly shown that DPD simulations with the model interaction potential presented here give rise to physically reasonable aggregate structures under circumstances of strong magnetic particle–particle interactions as well as a strong external magnetic field, since these aggregate structures are in good agreement with those of Monte Carlo and Brownian dynamics simulations.     

Brownian dynamics simulation of two confined colloidal particles

The dynamics of two confined colloidal particles is studied by means of Brownian dynamics simulation. The autocorrelation function and cross-correlation function of the two colloidal spheres are computed by utilizing the formulae of hydrodynamic diffusion matrix expanded to different orders, as well as the accurate tensor through numerical algorithm. Furthermore, the numerical results are compared with the experimental results and the theoretical approximation. It is found that the relatively simple theoretical approximation gives good predictions when two spheres are far away from each other, but fails when the two spheres are very close.     

Liquid–liquid phase transition and anomalous diffusion in simulated liquid GeO2

We perform molecular dynamics (MD) simulation of diffusion in liquid GeO₂ at the temperatures ranged from 3000 to 5000 K and densities ranged from 3.65 to 7.90 g/cm³. Simulations were done in a model containing 3000 particles with the new interatomic potentials for liquid and amorphous GeO₂, which have weak Coulomb interaction and Morse-type short-range interaction. We found a liquid–liquid phase transition in simulated liquid GeO₂ from a tetrahedral to an octahedral network structure upon compression. Moreover, such phase transition accompanied with an anomalous diffusion of particles in liquid GeO₂ that the diffusion constant of both Ge and O particles strongly increases with increasing density (e.g. with increasing pressure) and it shows a maximum at the density around 4.95 g/cm³. The possible relation between anomalous diffusion of particles and structural phase transition in the system has been discussed.     

Diffusion of a chemically active colloidal particle in composite channels

Diffusion of colloidal particles in microchannels has been extensively investigated, where the channel wall is either a no-slip or a slip-passive boundary. However, in the context of active fluids, driving boundary walls are ubiquitous and are expected to have a substantial effect on the particle dynamics. By mesoscale simulations, we study the diffusion of a chemically active colloidal particle in composite channels, which are constructed by alternately arranging the no-slip and diffusio-osmotic boundary walls. In this case, the chemical reaction catalyzed by the active colloidal particle creates a local chemical gradient along the channel wall, which drives a diffusio-osmotic flow parallel to the wall. We show that the diffusio-osmotic flow can significantly change the spatial distribution and diffusion dynamics of the colloidal particle in the composite channels. By modulating the surface properties of the channel wall, we can achieve different patterns of colloidal position distribution. The findings thus propose a novel possibility to manipulate colloidal diffusion in microfluidics, and highlight the importance of driving boundary walls in dynamics of colloidal particles in microchannels.     

Comparative simulation study of colloidal gels and glasses

Using computer simulations, we identify the mechanisms causing aggregation and structural arrest of colloidal suspensions interacting with a short-ranged attraction at moderate and high densities. Two different nonergodicity transitions are observed. As the density is increased, a glass transition takes place, driven by excluded volume effects. In contrast, at moderate densities, gelation is approached as the strength of the attraction increases. At high density and interaction strength, both transitions merge, and a logarithmic decay in the correlation function is observed. All of these features are correctly predicted by mode coupling theory.     

基于AAO模板的TiO2微纳结构及其光电特性

以多孔氧化铝（AAO）和导电玻璃FTO为基底,用射频（RF）磁控溅射法和溶胶凝胶（Sol-gel）法分别制备了微纳结构的TiO₂,两种TiO₂沉积技术各自体现出其特质。从XRD衍射光谱和拉曼散射光谱可以看到,在相同退火条件下,RF和Sol-gel法制备的TiO₂晶相结构不同。在以导电玻璃FTO为基底时,RF制备的薄膜颗粒分布均匀,没有团簇现象;而Sol-gel制备的薄膜由密实的颗粒构成。在以AAO为模板时,Sol-gel制备的TiO₂溶胶粒子由于胶体溶液的流动性使其对孔的填充率保持较高的水平;而RF制备的TiO₂对孔的填充率则降低。光电流实验的结果表明,前者具有更好的光电特性。     

采用溶胶凝胶协同自组装方法制备SiO2反蛋白石结构薄膜及其光学性质研究

研究了溶胶凝胶协同自组装制备大面积高质量SiO₂反蛋白石结构薄膜的方法. 向单分散的聚苯乙烯(PS)胶体溶液中添加SiO₂前驱物溶液,用垂直自组装法一步得到微球空隙中均匀填充有凝胶的 复合PS胶体晶体薄膜,在空气中烧结去除PS后得到SiO₂反蛋白石结构薄膜.通过对添加前驱物溶液比例、 自组装温度以及烧结温度等参数的研究,用不同粒径的PS微球制备了不同孔径的高质量SiO₂反蛋白石结构薄膜. 用扫描电子显微镜和X射线能量色散谱仪对制备得到的薄膜样品进行显微形貌和成分表征,并测试了其透射光谱. 结果表明:溶胶凝胶协同自组装法制备的SiO₂反蛋白石结构薄膜大面积高度有序,孔径可以控制且选择范围宽; 薄膜的透射光谱带隙明显,带隙中心波长与理论计算结果相符.     

On the nonequilibrium phase transition in protein

Considerations are presented that support the idea that the nonequilibrium phase transition in protein is accompanied by the growth of a colloidal nanocrystal (or the vitrified phase of a liquid crystal). To date, the dynamic transition forms of protein, which are “the source of the catalytic power of enzymes,” have been poorly understood. In our experiments, we observed the dehydration (drying) of the colloid solution of protein in an open (far from thermodynamic equilibrium) one-component protein-water system. The protein in this state is found to acquire properties typical of matter self-organization, including the universal properties of colloidal nanocrystals of different nature, namely, nonequilibrium chaotic dynamics with self-replicability, autocatalysis, coherency, autowave fluctuations, synchronism, fractality, 3D epitaxial growth (stacking) of films, nucleation giving rise to blocks (cells) with shell nuclei, etc. It then follows that our realistic model of the nonequilibrium state of protein during growth of its colloidal nanocrystal provides an opportunity of studying the dynamics of the structural and energy-information features of the transition and solid colloidal nanocrystalline phases of protein. In addition, it allows researchers to gain fundamentally new information about the energy characteristics of protein under abiotic and biotic conditions.     

Response of colloidal liquids containing magnetic holes of different volume densities to magnetic field characterized by transmission measurement

This paper systematically investigates the response of colloidal liquids containing magnetic holes of different volume densities to magnetic field by conventional transmission measurements. It finds that the enhancement in the transmission of such a colloidal liquid under a magnetic field exhibits a strong dependence on the volume density of magnetic holes. A linear increase in the maximum enhancement factor is observed when the volume density of magnetic holes is below a critical level at which a maximum enhancement factor of ～150 is achieved in the near infrared region. Once the volume density of magnetic holes exceeds the critical level, a sharp drop of the maximum enhancement factor to ～2 is observed. After that, the maximum enhancement factor increases gradually till a large volume density of ～9%. By monitoring the arrangement of magnetic holes under a magnetic field, it reveals that the colloidal liquids can be classified into three different phases, i.e., the gas-like, liquid-like and solid-like phases, depending on the volume density of magnetic holes. The response behaviour of colloidal liquids to magnetic field is determined by the interaction between magnetic holes which is governed mainly by their volume density. A phase transition, which is manifested in the dramatic reduction in the maximum enhancement factor, is clearly observed between the liquid-like and solid-like phases. The optical switching operations for colloidal liquids in different phases are compared and the underlying physical mechanisms are discussed.     

Reentrant transitions in colloidal or dusty plasma bilayers

The phase diagram of crystalline bilayers of particles interacting via a Yukawa potential is calculated for arbitrary screening lengths and particle densities. Staggered rectangular, square, rhombic, and triangular structures are found to be stable including a first-order transition between two different rhombic structures. For varied screening length at fixed density, one of these rhombic phases exhibits both a single and even a double reentrant transition. Our predictions can be verified experimentally in strongly confined charged colloidal suspensions or dusty plasma bilayers.     

Fouling dynamics in suspension flows

A particle suspension flowing in a channel in which fouling layers are allowed to form on the channel walls is investigated by numerical simulation. A two-dimensional phase diagram with at least four different behaviors is constructed. The fouling is modeled by attachment during collision with the deposits and by detachment caused by large enough hydrodynamic drag. For fixed total number of particles and small Reynolds numbers, the relevant parameters governing the fouling dynamics are the solid volume fraction of the suspension and the detachment drag force threshold. Below a critical curve in this 2D phase space only transient fouling takes place when the suspension is accelerated from rest by a pressure gradient. Above the fouling transition line, persistent fouling layers are formed via ballistic deposition for low and via homogeneous deposition for large solid volume fractions. Close to the fouling transition line, the flow path between the deposited layers meanders, while necking appears for increasing distance from the transition. Finally, another transition to a fully blocked flow path takes place. As determined by the estimated amount of deposited particles at saturation, both transitions seem to be discontinuous. Large fluctuations and long saturation times are typical of the dynamics of the system.     

Simulating (electro)hydrodynamic effects in colloidal dispersions: Smoothed profile method

Previously, we have proposed a direct simulation scheme for colloidal dispersions in a Newtonian solvent (Phys. Rev. E 71, 036707 (2005)). An improved formulation called the “Smoothed Profile (SP) method” is presented here in which simultaneous time-marching is used for the host fluid and colloids. The SP method is a direct numerical simulation of particulate flows and provides a coupling scheme between the continuum fluid dynamics and rigid-body dynamics through utilization of a smoothed profile for the colloidal particles. Moreover, the improved formulation includes an extension to incorporate multi-component fluids, allowing systems such as charged colloids in electrolyte solutions to be studied. The dynamics of the colloidal dispersions are solved with the same computational cost as required for solving non-particulate flows. Numerical results which assess the hydrodynamic interactions of colloidal dispersions are presented to validate the SP method. The SP method is not restricted to particular constitutive models of the host fluids and can hence be applied to colloidal dispersions in complex fluids.     

Rheology of nanosized hairy grain suspensions

Suspensions of nanosized hairy grains have been prepared by grafting long polydimethylsiloxane chains (molecular weight ) onto silica particles (radius ), dispersed into a good solvent of PDMS. Depending on the particle volume fraction, different rheological behaviors are observed. In the very dilute regime, the suspensions are perfectly stable and the particles behave almost as hard spheres: flow penetration inside the corona is then very weak. When the particle volume fraction goes to the close packing volume fraction, the suspension viscosity does not diverge as for hard spheres due to the increase of flow penetration inside the corona and to corona entanglements. The particles have then the same behavior as polymer stars having an intermediate number of arms (). Finally, in the concentrated regime (), the suspensions form irreversible gels. We shown that this unexpected gelation phenomenon is related to the presence of the silica cores: grafted PDMS chains can adsorb onto different particles and form irreversible bonds between the cores. The viscosity and elastic modulus evolutions during gelation are well described by the scalar percolation model of sol-gel transition. Received 23 March 1998     

Sintering of a bcc structure of spherical particles of equal and different sizes

Solid-state sintering of a bcc structure of spherical particles has been studied numerically by use of simple shape parameters to describe the state of the unit cell. Both free and pressure-assisted sintering of particles of equal and different sizes for various ratios of boundary and surface diffusion have been considered. At low relative densities, the evolution of particle radius, interparticle junction radius and shrinkage predicted by the bcc model are rather similar to sintering of a simple row of particles. At higher densities, the porosity closes up; that is, the junctions start to interact, or the next-nearest neighbours in the particle structure attain contact. Quantative measures of the density at the transition from open pore space between the particles to closed porosity as well as the density at which the next-nearest neighbours start to touch are derived. Furthermore, different dihedral angles and the evolution of relative density and sintering stresses are studied.     

粉末材料堆积的物理模型与仿真系统

研究了粉末材料堆积过程仿真的物理模型和系统,并探讨了适合多种不同粒径颗粒混合堆积过程仿真的高性能计算方法.在该仿真系统中,考虑了重力、接触力、阻尼力、摩擦力和范德瓦耳斯力等多种作用力的影响,集成了多种接触力模型和阻尼模型,使其适用于三维大规模粉末材料堆积过程的计算机仿真.利用该系统对粉末材料领域中的两个典型应用进行了模拟研究.模拟了两种相同密度不同粒径颗粒（粒径比为10）的混合堆积过程.当小颗粒数为大颗粒数的300倍时,得到最大的堆积密度（体积分数）为0.82.另外,还模拟了两种不同密度相同粒径颗粒的混合堆积过程.当堆积结束时,出现了明显的分离（segregation）现象和团聚现象.所研究的物理模型和仿真系统既可用于粉末材料堆积过程研究,亦可用于普通的球形物体堆积过程的模拟研究. 关键词： 粉末堆积 物理模型 仿真系统 离散元法     

Polydisperse spherical colloidal silica particles:Preparation and application

A new industrial method has been developed to produce polydisperse spherical colloidal silica particles with a very broad particle size,ranging from 20-95 nm.The process uses a reactor in which the original seed solution is heated to 100 ℃,and then active silicic acid and the seed solution are titrated to the reactor continuously with a constant rate.The original seeds and the titrated seeds in the reactor will go through different particle growth cycles to form different particle sizes.Both the particles' size distribution and morphology have been characterized by dynamic light scattering(DLS)and the focus ion beam(FIB) system.In addition,the as-prepared polydisperse colloidal silica particle in the application of sapphire wafer's chemical mechanical polishing(CMP) process has been tested.The material removal rate(MRR) of this kind of abrasive has been tested and verified to be much faster than traditional monodisperse silica particles.Finally,the mechanism of sapphire CMP process by this kind of polydisperse silica particles has been investigated to explore the reasons for the high polishing rate.