Multiple shear-banding transitions in a supramolecular polymer solution

We report on the nonlinear rheology of a reversible supramolecular polymer based on hydrogen bonding. The coupling between the flow-induced chain alignment and breakage and recombination of bonds between monomers leads to a very unusual flow behavior. Measured velocity profiles indicate three different shear-banding regimes upon increasing shear rate, each with different characteristics. While the first of these regimes has features of a mechanical instability, the second shear-banding regime is related to a shear-induced phase separation and the appearance of birefringent textures. The shear-induced phase itself becomes unstable at very high shear rates, giving rise to a third banding regime.     

Hydration of nucleic acid components in dependence on nucleotide composition and relative humidity: a Monte Carlo simulation

What role does the nucleotide composition play in the process of formation of hydrated environment of nucleic acids? Can one estimate the hydration of nucleic acids on the level of their components? In order to resolve these questions we have completed an extensive computer simulation of the hydration of nucleic acids components - deoxynucleoside monophosphates distinguished by nucleotide composition. The energetic characteristics of systems containing deoxynucleoside monophosphates and water clusters of various dimensions are received. Our results demonstrate that deoxynucleoside monophosphates containing guanine and/or cytosine residues hydrated more strongly because of formation of more hydrogen bonds with water molecules in small water clusters, i.e. at low values of relative humidity. With increasing of number of water molecules in a water cluster the energetic preference of deoxynucleoside monophosphates containing guanine and/or cytosine residues decreases, and for water clusters corresponding to a state of a dilute aqueous solution the hydration of all types of deoxynucleoside monophosphates does not differ in a great degree. Deoxynucleoside monophosphates containing guanine and/or cytosine residues cause the greater destruction of the water structure compensated by the greater interaction with the nearest water molecules for all levels of relative humidity. Received 25 December 2001 and Received in final form 22 March 2002 Published online 13 September 2002     

Shear-induced migration in colloidal suspensions

Using Brownian dynamics simulations, we perform a systematic investigation of the shear-induced migration of colloidal particles subject to Poiseuille flow in both cylindrical and planar geometry. We find that adding an attractive component to the interparticle interaction enhances the migration effect, consistent with recent simulation studies of platelet suspensions. Monodisperse, bidisperse and polydisperse systems are studied over a range of shear-rates, considering both steady-states and the transient dynamics arising from the onset of flow. For bidisperse and polydisperse systems, size segregation is observed.     

Self-Assembling Process of Colloidal Particles into Two-Dimensional Arrays Induced by Capillary Immersion Force: A Simulation Study With Discrete Element Method

This paper presents a simulation study for self-assembling process of colloidal particles into two-dimensional arrays due to capillary immersion force. Discrete element method is used to simulate the dynamics of colloidal particles trapped in a thin liquid film. The previous model is improved in the following two points: a modification of the screening effect of capillary immersion force and introduction of periodic boundary condition. Snapshots provided by the simulations agree well with experimental images taken by atomic force microscopy. The self-assembling process is quantified with pair correlation function and coordination number. At lower coverage, colloidal particles rapidly form small clusters that consist of several particles in the early stage. Subsequently, chain-like structures with some branches are mainly generated. On the other hand, at higher coverage, large domains of hexagonal close-packed (HCP) structures are gradually generated. The rate of the growth of HCP domains is much slower than that of the generation of the small clusters and the chain-like structures.     

液态金属凝固过程原子团簇结构表征的新方法（英文）

采用分子动力学方法对液态金属钠的凝固过程进行了模拟计算,运用团簇结构表征新方法――团簇类型指数法(CTIM)对凝固过程中的团簇结构进行了识别.为了阐明CTIM在识别团簇结构上的准确性和效率,将其与广为采用的Voronoi多面体方法 (VPM)进行比较.结果表明:当采用CTIM和VPM分别对液态金属钠凝固结构中的原子团簇结构进行表征时,它们所得到的微观结构特征是一致的.非晶态结构中,原子团簇类型的分布呈现明显的区段特征,每一区段都存在一种主要团簇类型,它们分别是二十面体或其缺陷结构.晶体结构中,体系形成以BCC团簇为主体的晶态结构.同时发现,VPM不易区分不同团簇构型之间的细微差别,不同构型的原子团簇可能被归为同种结构类型;而CTIM根据近邻原子之间相对位置关系,直接准确描述原子团簇的构型.不但由CTIM分析获得的凝固体系结构特征与VPM的分析结果一致;而且CTIM抓住了体系微观结构特征的主要方面,简化了团簇结构的表征形式,这在大尺度模拟体系的结构分析中将具有较高效率.     

Particle trapping and banding in rapid colloidal solidification

We derive an expression for the nonequilibrium segregation coefficient of colloidal particles near a moving solid-liquid interface. The resulting kinetic phase diagram has applications for the rapid solidification of clay soils, gels, and related colloidal systems. We use it to explain the formation of bandlike defects in rapidly solidified alumina suspensions.     

团簇的第一原理分子动力学计算研究：价键优选法

根据团簇价键结构的特点，结合第一原理分子动力学模拟，提出价键优选法；可以在有限的 计算量之下，得到其可计算的最大团簇体系Xm的性质，以及所有团簇Xi(i关键词： 价键优选法 团簇 第一原理分子动力学 Jahn_Teller效应 Rennet_Teller效应     

利用Gupta势结合遗传算法研究ConCu55－n(n=0—55)混合团簇的结构演化及基态能量

采用半经验的Gupta多体势结合遗传算法对Co_nCu_55－n(n=0—55)混合团簇的基态结构和能量进行了研究,发现这些混合团簇的基态结构是在Co₅₅,Cu₅₅单质团簇(Mackay二十面体)的基础之上发生的畸变;从n=0(Cu₅₅)开始,Co原子从中心到表面,从棱到顶点依次、连续替换Cu原子;基态结构与键能较大键的数目及其平均键长有关;Co₁₃Cu₄₂具有最稳定的结构,13个Co原子全部位于团簇内部形成Mackay二十面体对整个团簇的稳定性有显著影响. 关键词： 团簇 结构和能量 Gupta势 遗传算法     

Exact and approximate solutions for the dilute Ising model

The ground state energy and entropy of the dilute mean field Ising model is computed exactly by a single order parameter as a function of the dilution coefficient. An analogous exact solution is obtained in the presence of a magnetic field with random locations. Results lead to a complete understanding of the geography of the associated random graph. In particular, we give the size of the giant component (continent) and the number of isolated clusters of connected spins of all given size (islands). We also compute the average number of bonds per spin in the continent and in the islands. Then, we tackle the problem of solving the dilute Ising model at strictly positive temperature. In order to obtain the free energy as a function of the dilution coefficient and the temperature, it is necessary to introduce a second order parameter. We are able to find out the exact solution in the paramagnetic region and exactly determine the phase transition line. In the ferromagnetic region we provide a solution in terms of an expansion with respect to the second parameter which can be made as accurate as necessary. All results are reached in the replica frame by a strategy which is not based on multi-overlaps.     

一种适用于原子团簇的多体展开新方案: 以氮团簇为例

多体展开方法虽然已经广泛地用于估算弱相互作用体系的能量,但是其并不适用于计算共价团簇和金属团簇的能量. 本文提出了一种适用于计算共价体系能量的相互作用多体展开(IMBE)方法. 在相互作用多体展开方法中,体系的能量表示为孤立原子的能量及该原子与其他周围原子间相互作用的和. 首先将该方法应用于计算氮团簇的能量,且多体展开截断至四体项. 结果表明：与传统的多体展开方法相比,相互作用多体展开方法可以显著地降低能量误差. 另外,以密度泛函理论计算结果为参考,相互作用多体展开方法估算能量的误差不依赖于体系的大小和结构,说明相互作用多体展开方法比较适合用于估算共价相互作用大体系的能量.     

Impact of receptor-ligand distance on adhesion cluster stability

Cells in multicellular organisms adhere to the extracellular matrix through two-dimensional clusters spanning a size range from very few to thousands of adhesion bonds. For many common receptor-ligand systems, the ligands are tethered to a surface via polymeric spacers with finite binding range, thus adhesion cluster stability crucially depends on receptor-ligand distance. We introduce a one-step master equation which incorporates the effect of cooperative binding through a finite number of polymeric ligand tethers. We also derive Fokker-Planck and mean field equations as continuum limits of the master equation. Polymers are modeled either as harmonic springs or as worm-like chains. In both cases, we find bistability between bound and unbound states for intermediate values of receptor-ligand distance and calculate the corresponding switching times. For small cluster sizes, stochastic effects destabilize the clusters at large separation, as shown by a detailed analysis of the stochastic potential resulting from the Fokker-Planck equation.     

快凝Pd82Si18合金原子团簇的演化特性及遗传机制

采用分子动力学(MD)模拟计算,对Pd82Si18合金快凝过程中基本原子团簇的遗传特性、演化趋势和结构稳定性进行了研究.团簇类型指数法(CTIM)分析表明:非晶固体中Si原子为中心的(102/14418/1551)双帽阿基米德反棱柱(BSAP)团簇数目占据优势.快凝过程中,BSAP结构团簇具有最大的遗传分数,并且其他以Si原子为中心的Kasper团簇大多都会向BSAP结构团簇转变.通过对Si原子为中心的Kasper基本团簇电子性质第一性原理计算发现,体系中BSAP团簇的结合能最低,结构稳定性较高,与分子动力学计算结果一致.     

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.     

Size dependence of freezing temperature and structure instability in simulated Lennard-Jones clusters

Liquid Lennard-Jones clusters of 14 different sizes from N=55 to 923 particles were cooled down to find their temperature of liquid-solid transition and the internal structure of the solidified clusters. The decrease of the cluster temperature was attained by a gradual change of the system temperature in Monte Carlo simulations. The liquid-to-solid transition was found by analysis of the specific heat as well as by detection of the structural units of face-centred cubic, hexagonal close-packed and decahedral type. It was observed that near the detected transition temperature the solid-like cluster structure is not always stable and fluctuates between solid and liquid states. The fluctuations of the state were observed frequently for small clusters with N ≤147, where the temporary solid structure is created by a large part of internal atoms. Manual inspection of cluster structural data and the 10%N condition for minimal number of atoms as centres of solid-like units enable detection of stable cluster solidification at freezing temperature. It was found that the freezing temperature of all clusters, with the exception of N=55, decreases linearly with N^-1/3. The extrapolated freezing temperature of the bulk LJ system is 13% lower than the experimental value of argon. After freezing, the solid phase remains but some atoms close to the cluster surface are not firmly included into the structure and oscillate mainly between solid structure and disordered one.     

液态Ni原子团簇演变的计算机模拟

采用常温常压分子动力学模拟技术,模拟了液态Ni中原子团簇在快速凝固条件下的演变过程,模型采用TB作用势.采用偶分布函数、键对和多面体等结构参数来描述快速凝固条件下团簇种类和数量的变化,并将团簇结构可视化.在2 000 K下,液态Ni中团簇数量较少,由一定数量的1551、1441及1661键对所形成的缺陷二十面体构成;在快速冷却条件下,团簇的数量随温度的降低不断增加,且出现由12个1551键型所形成的完整二十面体团簇,体系最终形成了由二十面体和缺陷二十面体团簇网络所组成的非晶结构.     

Calculation of the structure of carbon clusters based on fullerene-like C<Subscript>24</Subscript> and C<Subscript>48</Subscript> molecules

Equilibrium structures obtained by linking with valence bonds the carbon carcasses of two fullerene-like molecules have been studied by molecular dynamics simulation. In free fullerene, carbon atoms form sp² hybridized bonds, but at places of links between fullerenes, sp³ hybridized bonds are formed, which determines the changes in the properties of such structures. In the literature, the topology of diamond-like phases is described, but equilibrium clusters based on fullerene-like molecules are underexplored. The right angles between the C–C bonds are energetically unfavorable, and the reduction in the energy of clusters in the process of relaxation is connected with the optimization of valence angles, which leads to a reduction in the symmetry of clusters and, in a number of cases, even to disruption of some valence bonds. It is shown that different fashions of linking two fullerenes result in the formation of clusters with different structures and energies. Different initial conditions can lead to different configurations of clusters with the same topology. Among the analyzed clusters, a structure with the minimum potential energy per atom was found. The results of this work contribute to the study of the real structure of carbon clusters.     

Colloidal aggregation with sedimentation: concentration effects

The results of computer models for colloidal aggregation, that consider both Brownian motion and gravitational drift experienced by the colloidal particles and clusters, are extended to include concentrations spanning three orders of magnitude. In previous publications and for a high colloidal concentration, it was obtained that the aggregation crosses over from diffusion-limited colloidal aggregation (DLCA) to another regime with a higher cluster fractal dimension and a speeding up followed by a slowing down of the aggregation rate. In the present work we show, as the concentration is decreased, that we can still cross over to a similar regime during the course of the aggregation, as long as the height of the sample is increased accordingly. Among the differences between the mentioned new regimes for a high and a low colloidal concentration, the cluster fractal dimension is higher for the high concentration case and lowers its value as the concentration is decreased, presumably reaching for low enough concentrations a fixed value above the DLCA value. It is also obtained the fractal dimension of the sediments, arising from the settling clusters that reach the bottom and continue a 2D-like diffusive motion and aggregation, on the floor of the container. For these clusters we now see two and sometimes three regimes, depending on concentration and sedimentation strength, with their corresponding fractal dimensions. The first two coming from the crossover already mentioned, that took place in the bulk of the sample before the cluster deposition, while the third arises from the two-dimensional aggregation on the floor of the container. For these bottom clusters we also obtain their dynamical behavior and aggregation rate.Received: 7 January 2004, Published online: 25 March 2004PACS: 61.43.Hv Fractals; macroscopic aggregates (including diffusion-limited aggregates) - 82.70.Dd Colloids - 05.10.Ln Monte Carlo methods