带电胶体系统中的排空作用

用Monte Carlo模拟研究弱带电胶体系统的性质.在四维超球面欧氏空间中严格表示库仑作用的基础上，通过接受率法计算带电胶球间的排空势和排空力.研究了受限于两块平行带电平板之间、淹没于带电小胶球之中的一个或两个带电大胶球系统的排空作用，并通过与不带电的硬球系统相应结果的分析、对比，发现静电势能增强带电胶球之间的排空作用. 关键词： 超球面法 接受率方法 排空作用 Monte Carlo模拟     

Depletion interactions in cylindric pipeline

In a colloidal system confined by a small cylindric pipeline, the depletion interaction between two large spheres is different to the system confined by two plates, and the influence on depletion interaction from the pipeline is related to both the size and shape of it. In this paper, the depletion interactions in the systems confined by pipelines of different sizes or different shapes are studied by Monte Carlo simulations. The numerical results show that the influence on depletion force from the cylindric pipeline is stronger than that from two parallel plates, and the depletion force will be strengthened when the diameter of the cylinder is decreased. In addition, we also find that the depletion interaction is rather affected if the shape change of the pipeline is slightly changed, and the influence on depletion force from the shape change is stronger than that from the size change.     

Effects of unsymmetrical geometric confinements on depletion interactions in colloidal suspensions

The depletion interactions between two large-spheres immersed in a fluid of small spheres under unsymmetrical geometrical confinement are studied through the acceptance ratio method. The numerical results show that no matter whether the volume fraction is large or small, both the depletion potential and depletion force are affected by the presence of the two plates; the closer the two large spheres are to the plate, the larger the effects of the unsymmetrical confinements.     

Poisson–Boltzmann for oppositely charged bodies: an explicit derivation

The interaction between two parallel charged plates in ionic solution is a general starting point for studying colloidal complexes. An intuitive expression of the pressure exerted on the plates is usually proposed, which includes an electrostatic plus an osmotic contribution. We present here an explicit and self-consistent derivation of this formula in the only framework of the Poisson–Boltzmann (PB) theory. We also show that, depending on external constraints, the correct thermodynamic potential can differ from the usual PB free energy. For asymmetric, oppositely charged plates, the resulting expression predicts a non-trivial equilibrium position with the plates separated by a finite distance. The depth of this energy minimum is decisive for the stability of the complex. It is therefore crucial to obtain its explicit dependence on the charge densities of the plates and on the ion concentration. Analytic expressions for the position and depth of the energy minimum were derived in 1975 by Ohshima [Colloid Polym. Sci. 253, 150 (1975)] but, surprisingly, these important results seem to have been overlooked. We retrieve these expressions in a simpler formalism, more familiar to the physics community, and give a physical interpretation of the observed behavior.     

二维情况下两组分带电囊泡形变耦合相分离的理论模拟研究

结合离散空间变分方法和耗散动力学研究了二维两组分带电囊泡的形变耦合相分离,系统地考察了囊泡带电量组分含量、带电组分的电荷密度、两组分间的相容性和温度等因素对形变耦合相分离动力学的影响.模拟结果表明电荷引入可增加不同组分间的表观相溶性.当温度较高时,静电相互作用可直接抑制囊泡相分离,避免了同种组分的团聚;当温度较低时,静电相互作用则可明显增加分相相区数目,使其呈微观相分离,从而避免了同种组分大范围的团聚.     

约束条件下胶球间排空力的研究

用MonteCarlo方法对处于两平行硬板约束下三个浓度的大小胶球系统进行了模拟,通过对大胶球表面小胶球密度的统计,由密度积分公式获得了大胶球所受的排空力.研究结果显示,因为平行硬板的存在或当改变两平行硬板的距离时,同浓度下,排空力在硬板距离小的时候最明显;三个浓度中,浓度高的,排空力受硬板距离影响最大;有硬板约束比无该约束的时候,排空力效果更显著.     

A new constituent of electrostatic energy in semiconductors

The concept of electric energy is revisited in detail for semiconductors. We come to the conclusion that the main relationship used to calculate the energy related to the penetration of the electric field in semiconductors is missing a fundamental term. For instance, spatial derivate of the electrostatic energy using the traditional formula fails at giving the correct electrostatic force between semiconductor based capacitor plates, and reveals unambiguously the existence of an extra contribution to the standard electrostatic free energy. The additional term is found to be related to the generation of space charge regions which are predicted when combining electrostatics with semiconductor physics laws, such as for accumulation and inversion layers. On the contrary, no such energy is needed when relying on electrostatics only, as for instance when adopting the so-called full depletion approximation. The same holds for neutral and charged insulators that are still consistent with the customary definition, but these two examples are in fact singular cases. In semiconductors for instance, this additional energy can largely exceed the energy gained by the dipoles, thus becoming the dominant term. This unexpected result clearly asks for a generalization of electrostatic energy in matter in order to reconcile basic concepts of electrostatic energy in the framework of classical physics.     

Coupling effects of depletion interactions in a three-sphere colloidal system

In a three-sphere system, the middle sphere is acted upon by two opposite depletion forces from the other two spheres. It is found that, in this system, the two depletion forces are coupled with each other and result in a strengthened depletion force. So the difference of the depletion forces of the three-sphere system and its corresponding two two-sphere systems is introduced to describe the coupling effect of the depletion interactions. The numerical results obtained by Monte-Carlo simulations show that this coupling effect is affected by both the concentration of small spheres and the geometrical confinement. Meanwhile, it is also found that the mechanisms of the coupling effect and the effect on the depletion force from the geometry factor are the same.     

Local simulation algorithms for Coulomb interactions

Long ranged electrostatic interactions are time consuming to calculate in molecular dynamics and Monte Carlo simulations. We introduce an algorithmic framework for simulating charged particles which modifies the dynamics so as to allow equilibration using a local Hamiltonian. The method introduces an auxiliary field with constrained dynamics so that the equilibrium distribution is determined by the Coulomb interaction. We demonstrate the efficiency of the method by simulating a simple, charged lattice gas.     

Lamellar phase coexistence induced by electrostatic interactions

Membranes containing highly charged biomolecules can have a minimal free-energy state at small separations that originates in the strongly correlated electrostatic interactions mediated by counterions. This phenomenon can lead to a condensed, lamellar phase of charged membranes that coexists in thermodynamic equilibrium with a very dilute membrane phase. Although the dilute phase is mostly water, entropy dictates that this phase must contain some membranes and counterions. Thus, electrostatics alone can give rise to the coexistence of a condensed and an unbound lamellar phase. We use numerical simulations to predict the nature of this coexistence when the charge density of the membrane is large, for the case of multivalent counterions and for a membrane charge that is characteristic of biomolecules. We also investigate the effects of counterion size and salt on the two coexisting phases. With increasing salt concentration, we predict that electrostatic screening by salt can destroy the phase separation.     

Force between charged particles with ion condensation

We have numerically calculated the interaction forces between two highly charged spherical particles embedded in a cloud of small ions with or without charge redistribution on the particle surfaces. Ion condensation near the charged particles leads to reduced electrostatic interaction between the particles, and we find that the effective two-particle interaction is significantly smaller than the values expected from considering only effective single-particle potentials.     

Elastic interactions and stability of clay-filled lamellar phases

High aspect ratio clay particles dispersed in a lamellar matrix composed of a block copolymer or a lyotropic smectic are expected to orient with the lamellae. Under such conditions, the smectic medium transmits elastic forces among particles in addition to the usual forces produced by dispersion and electrostatic interactions. We compute these elastic forces and explore their influence on the thermodynamics of lamellar-clay dispersions. It turns out that the large aspect ratio of the clay implies a long range of interaction at the two particle level. Consequently, virial expansions break down at very low loadings of particles. We examine the thermodynamic behavior of assemblies of flexible and rigid clay plates in both dilute and semidilute concentration regimes. Our results should have implications for the design of nanocomposites formulated with block copolymers and lyotropic liquid crystals. Received 11 August 2000     

一种实现冷分子囚禁的可控制静电双阱方案

提出了采用双环形载荷导线和两透明电极系统实现冷分子静电囚禁的可控制静电双阱的新方案,计算了带电圆导线和带电板所产生的静电场分布,从几个方面分析了这个囚禁方案的优点. 提出了一种有效的冷分子装载方法,并研究了双阱到单阱的演化过程. 研究表明,该可控制静电双阱方案不仅方便装载与操控弱场搜寻态的极性冷分子,而且在分子物质波的干涉、纠缠、冷碰撞,甚至进行双阱分子BEC研究等分子光学领域中有着广阔的应用前景. 关键词： 极性冷分子 静电囚禁 可控制静电双阱 分子光学     

Depletion interactions between two spherocylinders

The depletion interactions between two spherocylinders as functions of their separation and their relative orientation, induced by a small hard-sphere fluid, are calculated by Monte Carlo simulations using the acceptance ratio method (ARM). The torque on the spherocylinders is determined from the resulting potential. The calculation shows that the ARM is an effective way to obtain depletion interactions of spherocylinders. The depletion interaction under the Asakura-Oosawa (also excluded-volume) approximation is also calculated numerically.     

Tip and surface properties from the distance dependence of tip-surface interactions

Macroscopic "background" interactions, such as van der Waals and electrostatic forces, determine the frequency change in non-contact atomic force microscopy (NC-AFM). We demonstrate that by analysing the distance dependence of these interactions one can extract more information about the tip radius, charge and chemical composition, as well as about the surface charging and conductivity. For this purpose we calculate the interaction of different NC-AFM tips with a charged and neutral CaF₂ (111) surface and with an ideal metal surface. Force versus distance curves demonstrate a remarkably different behaviour, especially at long distances, dependent on whether the tip is conductive, oxidised or charged. Comparison with experimental curves proves that this analysis can predict tip properties.     

Novel electrostatic trap for cold polar molecules

We propose a novel scheme in which cold polar molecules are trapped by an electrostatic field generated by the combination of a pair of parallel transparent electrodes (i.e., two infinite transparent plates) and a ring electrode (i.e., a ring wire). The spatial distributions of the electrostatic fields from the above charged wire and the charged plates and the corresponding Stark potentials for cold CO molecules are calculated; the dependences of the trap centre position on the geometric parameters of the electrode are analysed. We also discuss the loading process of cold molecules from a cold molecular beam into our trap. This study shows that the proposed scheme is not only simple and convenient to trap, manipulate and control cold polar molecules in weak-field-seeking states, but also provides an opportunity to study cold collisions and collective quantum effects in a variety of cold molecular systems, etc.     

Effective screening of hydrodynamic interactions in charged colloidal suspensions

We investigate the hydrodynamic interaction in suspensions of charged colloidal silica spheres. The volume fraction as well as the range of the electrostatic repulsion between the spheres is varied. Using a combination of dynamic x-ray scattering, cross-correlated dynamic light scattering, and small angle x-ray scattering, the hydrodynamic function H(q) is determined experimentally. The effective hydrodynamic interactions are found to be screened, if the range of the direct interaction is relatively long and the static density correlations are strong. This observation of effective hydrodynamic screening is in marked contrast to hard-sphere-like systems.     

Dynamic self-assembly of rings of charged metallic spheres

This Letter describes dynamic self-assembly in a system of stainless steel spheres ( approximately 1 mm in diameter) rolling on a flat dielectric surface under the influence of an external magnetic field that rotates parallel to the plane of the surface. As the spheres move, they charge triboelectrically. Self-assembly is mediated by two types of electrostatic interactions among these charges: (i) attraction between negatively charged regions of the surface and positively charged spheres and (ii) repulsion between the like-charged spheres. The spheres organize into highly ordered rings as a result of these electrostatic interactions.     

Effective interactions in the charged colloidal system

In the charged colloidal system, with the aid of the hyper-sphere method, the effective interactions between two like-charged macro-ions are studied through Monte Carlo simulations. Numerical results show that the depletion interactions dominate the total Coulomb interactions in the region while the two macro-ions are near.     

Oppositely charged polyelectrolytes grafted onto planar surface: Mean-field lattice theory

Equilibrium structures of planar polyelectrolyte brushes formed by grafted chains carrying charges of opposite sign are examined by employing mean-field lattice theory. Two brushes of different architecture are considered: one formed by grafted diblock copolymers with oppositely charged blocks and the other being a mixed brush composed of oppositely charged homopolymers. The systems display nontrivial intrinsically inhomogeneous brush structures originating from the chain connectivity and the electrostatic interaction among the segments. In addition, a coexistence of stretched and coiled chains inside the brush is observed. The influence of the charges of the blocks, the relative length of the oppositely charged blocks, and the ionic strength of the solution on the brush inhomogeneity and structural differences between the two types of brushes are discussed. Received 14 March 2001 and Received in final form 18 June 2001