Depletion Interactions in a Cylindric Pipeline with a Small Shape Change

Stressed by external forces, it is possible for a cylindric pipeline to change into an elliptic pipeline. To expose the effect of small shape change of the pipeline on the depletion interactions, both the depletion potentials and depletion forces in the hard sphere systems confined by a cylindric pipeline or by an elliptic pipeline are studied by Monte Carlo simulations. The numerical results show that the depletion interactions are strongly affected by the small change of the shape of the pipeline in a way. Furthermore, it is also found that the depletion interactions will be strengthened if the short axis of the elliptic pipeline is decreased.     

Effects of electrostatic interactions and geometrical confinements on depletion interactions in charged colloidal system

In a charged colloidal system, the influence on depletion interaction between two like-charged macro-ions is studied through Monte Carlo simulation in this paper. The numerical results show that this depletion force is affected by both the electrostatic interactions between charged spheres and charged plates and by the geometrical factor of the two charged plates, and they further indicate that the influence of geometrical confinement on the depletion interaction is larger than that of electrostatic potential.     

胶体中排空作用的耦合效应

通过Monte Carlo模拟和接受率方法研究了限制在圆柱形管道中大胶球之间以及大胶球与管壁之间排空作用的耦合效应.研究发现,当两个大胶球靠近时,大胶球之间的排空作用因与大胶球与管道壁之间的排空作用耦合而得到加强,同样当大胶球与管道壁靠近时,大胶球与管道壁之间的排空作用也因与另一大胶球之间排空作用的耦合而得到了加强.此外,研究还发现,随着圆柱形管道直径的减小,胶球之间、胶球与管道壁之间的排空作用的耦合效应是增强的. 关键词： 排空作用的耦合效应 接受率方法 Monte Carlo模拟     

Grand Canonical Ensemble Monte Carlo Simulation of Depletion Interactions in Colloidal Suspensions

Depletion interactions in colloidal suspensions confined between two parallel plates are investigated by using acceptance ratio method with grand canonical ensemble Monte Carlo simulation. The numerical results show that both the depletion potential and depletion force are affected by the confinement from the two parallel plates. Furthermore, it is found that in the grand canonical ensemble Monte Carlo simulation, the depletion interactions are strongly affected by the generalized chemical potential.     

Polymer depletion interaction between parallel walls--a Monte Carlo study

An off-lattice bead-spring model of self-assembling equilibrium ("living") polymers is used to study the polymer-induced interaction between parallel walls immersed in polydisperse solutions of different concentration by means of Monte Carlo simulation. The two walls form an open slit in contact with an external reservoir so that the confined system may exchange monomers with the surrounding phase and adapt its polydispersity in order to relax the confinement constraint. We find that the properties of the polymers in the constrained system as well as the net force deltaF acting on the walls depend essentially on the polymer concentration in the reservoir which leads to qualitative differences in their behavior with changing inter-planar distance H: In a dilute polymer solution at concentration phi below the semi-dilute threshold phi* the force between the walls is attractive and decreases steadily with growing wall separation H, so that deltaF approximately 0 at H/ Rg> or =3 if H is measured in gyration radii Rg of the unperturbed polymers. The total monomer concentration within the slit is smaller than the concentration in the reservoir and decreases monotonically with H/Rg-->0. The ratio Nin/Nout of mean chain length Nin in the slit to that in the reservoir, Nout, decreases from unity at H-->infinity, goes through a minimum at H/Rg approximately 1, and then rises again to Nin/Nout>1 for wall separations H/Rg<1. In contrast, in a dense solution of equilibrium polymers at phi>phi* one detects no indirect wall-wall interaction, deltaF approximately 0, for H larger than the monomer size. Thus, earlier speculations about the existence of possible depletion interaction between parallel walls even in a dense polymer system cannot be confirmed. Inside the slit the monomer density is found to be always larger than in the reservoir while Nin/Nout<1 and decreases steadily as H/Rg-->0. The depletion force between parallel plates has been determined also in a monodisperse solution of conventional polymers. Qualitatively the force behavior does not differ from that of living polymers.     

The Casimir force on a piston in the spacetime with extra compactified dimensions

A Casimir piston for massless scalar fields obeying Dirichlet boundary conditions in high-dimensional spacetimes within the frame of Kaluza–Klein theory is analyzed. We derive and calculate the exact expression for the Casimir force on the piston. We also compute the Casimir force in the limit that one outer plate is moved to the extremely distant place to show that the reduced force is associated with the properties of additional spatial dimensions. The more dimensionality the spacetime has, the stronger the extra-dimension influence is. The Casimir force for the piston in the model including a third plate under the background with extra compactified dimensions always keeps attractive. Further we find that when the limit is taken the Casimir force between one plate and the piston will change to be the same form as the corresponding force for the standard system consisting of two parallel plates in the four-dimensional spacetimes if the ratio of the plate-piston distance and extra dimensions size is large enough.     

Study of the shear-rate dependence of granular friction based on community detection

In this study, computer simulations are performed on three-dimensional granular systems under shear conditions. The system comprises granular particles that are confined between two rigid plates. The top plate is subjected to a normal force and driven by a shearing velocity. A positive shear-rate dependence of granular friction, known as velocity-strengthening, exists between the granular and shearing plate. To understand the origin of the dependence of frictional sliding, we treat the granular system as a complex network, where granular particles are nodes and normal contact forces are weighted edges used to obtain insight into the interiors of granular matter. Community structures within granular property networks are detected under different shearing velocities in the steady state. Community parameters, such as the size of the largest cluster and average size of clusters, show significant monotonous trends in shearing velocity associated with the shear-rate dependence of granular friction. Then, we apply an instantaneous change in shearing velocity. A dramatic increase in friction is observed with a change in shearing velocity in the non-steady state. The community structures in the non-steady state are different from those in the steady state. Results indicate that the largest cluster is a key factor affecting the friction between the granular and shearing plate.     

Effect of geometrical confinement on the interaction between charged colloidal suspensions

The effective interaction between charged colloidal particles confined between two planar like-charged walls is investigated using computer simulations of the primitive model describing asymmetric electrolytes. In detail, we calculate the effective force acting onto a single macroion and onto a macroion pair in the presence of slitlike confinement. For moderate Coulomb coupling, we find that this force is repulsive. Under strong-coupling conditions, however, the sign of the force depends on the distance to the plates and on the interparticle distance. In particular, the particle-plate interaction becomes strongly attractive for small distances which may explain the occurrence of colloidal crystalline layers near the plates observed in recent experiments.     

Attractive and repulsive Casimir vacuum energy with general boundary conditions

The infrared behaviour of quantum field theories confined in bounded domains is strongly dependent on the shape and structure of space boundaries. The most significant physical effect arises in the behaviour of the vacuum energy. The Casimir energy can be attractive or repulsive depending on the nature of the boundary. We calculate the vacuum energy for a massless scalar field confined between two homogeneous parallel plates with the most general type of boundary conditions depending on four parameters. The analysis provides a powerful method to identify which boundary conditions generate attractive or repulsive Casimir forces between the plates. In the interface between both regimes we find a very interesting family of boundary conditions which do not induce any type of Casimir force. We also show that the attractive regime holds far beyond identical boundary conditions for the two plates required by the Kenneth–Klich theorem and that the strongest attractive Casimir force appears for periodic boundary conditions whereas the strongest repulsive Casimir force corresponds to anti-periodic boundary conditions. Most of the analysed boundary conditions are new and some of them can be physically implemented with metamaterials.     

声场中空化气泡的耦合振动及形状不稳定性的研究

计算了两个具有非球形扰动的气泡所组成系统的能量,并基于Lagrange方程得到了有声相互作用的非球形气泡的动力学方程和形状稳定性方程,研究了声场中非球形气泡间相互作用力对非球形气泡的形状不稳定性和气泡形状模态振幅的影响.研究结果表明声场中具有非球形扰动的气泡之间的耦合方式有两种:形状耦合模式和径向耦合模式,气泡之间的耦合方式取决于气泡形状扰动模态.由形状耦合及径向耦合产生的气泡之间的相互作用力能够改变单个气泡的形状不稳定及形状模态振幅,具体影响因素取决于声场驱动条件、气泡形状模态、相邻气泡的初始半径.     

Casimir and Surface Tension Forces on a Single Interacting Bose–Einstein Condensate in Canonical Ensemble

The forces on a single Bose–Einstein condensate confined between two parallel plates consist of two components, namely, surface tension force and Casimir force. In canonical ensemble, these forces are quite different from the one in grand canonical ensemble. In small region with distance $$\ell $$ between two parallel plates, using double parabola approximation, we find that surface tension force decreases as $${{\ell }^{{ - 3}}}$$, whereas the Casimir force, in one-loop approximation of the quantum field, is proportional to $${{\ell }^{{ - 13/2}}}$$. The total force is also considered and its veer is found.     

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     

A classical density functional approach to depletion interaction of Lennard-Jones binary mixtures

In this article, we apply classical density functional theory to investigate the characteristics of depletion interaction in Lennard-Jones (LJ) binary fluid mixtures. First, to confirm the validity of our adopted density functional formalism, we calculate the radial distribution functions using a theoretical approach and compare them with results obtained by molecular dynamics simulation. Then, this approach is applied to two colloids immersed in LJ solvent systems. We investigate the variation of depletion interaction with respect to the distance of two colloids in LJ binary systems. We find that depletion interaction may be attractive or repulsive, mostly depending on the bulk density of the solvent and the temperature of the binary system. For high bulk densities, the repulsive barrier of depletion force is remarkable when the total excluded volume of colloids touches each other and reaches a maximum. The height of the repulsive barrier is related to the parameters of the LJ potential and bulk density. Moreover, the depletion force may exhibit attractive wells if the bulk density of the solvent is low. The attractive well tends to appear when the surface–surface distance of colloids is half of the size of the polymer and deepens with temperature lowering in a fixed bulk density. In contrast with the hard-sphere system, no oscillation of depletion potential around zero is observed.     

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.     

Exact crossover function for the Casimir force in a non-interacting Bose gas

An exact calculation of the Casimir force for a non-interacting Bose gas confined between two parallel plates is presented. The gas can be free or trapped, parallel to the plates. Depending on the finite size parameter λ/L (λ is the de Bröglie wavelength and L is the separation of the plates) and the density parameter nλ³ (n, the number density), the Casimir force crosses over from a power law to an exponential fall off is clearly seen. Since the Casimir force measurement requires very small values of L, one needs to take into account of the condensation in a finite system.     

Si组分对SiGe量子点形状演化的影响

研究了自组织生长SiGe岛（量子点）中Si组分对形状演化的影响.采用UHV/CVD方法生长了 不同Si组分的SiGe岛，用AFM对其形状和尺寸分布进行了分析，实验结果表明SiGe岛从金字 塔形向圆顶形转变的临界体积随Si组分的增大而增大.通过对量子点能量的应变能项进行修正，解释了量子点中Si组分对形状演化的影响.在特定的工艺条件下得到了单模尺寸分布的 金字塔和圆顶形量子点.结果表明，通过调节SiGe岛中的Si组分，可以实现对SiGe岛形状和 尺寸的控制. 关键词： 自组织生长SiGe岛 Si组分 临界体积     

Fluctuation-induced casimir forces in granular fluids

We numerically investigate the behavior of driven noncohesive granular media and find that two fixed large intruder particles, immersed in a sea of small particles, experience, in addition to a short-range depletion force, a long-range repulsive force. The observed long-range interaction is fluctuation-induced and we propose a mechanism similar to the Casimir effect that generates it: The hydrodynamic fluctuations are geometrically confined between the intruders, producing an unbalanced renormalized pressure. An estimation based on computing the possible Fourier modes explains the repulsive force and is in qualitative agreement with the simulations.     

Entropic stochastic resonance in a confined structure driven by dichotomous noise and white noises

The entropic stochastic resonance(ESR) in a confined system subjected to dichotomous noise and white noise and driven by a periodic sinusoidal force along the x axis of the structure and a time-dependent force in the declining direction,is investigated.Under the adiabatic approximation condition and based on the two-state theory,the expression of the output signal-to-noise ratio(SNR) is obtained.The results show that the SNR is a non-monotonic function of the strengths of dichotomous noise,white noise,and correlated strength of correlated noise.In addition,the SNR varies non-monotonically with the increase of the shape parameters of the confined structure,and also with the increase of the constant force along the y axis of the structure.The influence of the correlation rate of the dichotomous noise,and that of the frequency of the periodic force on the SNR are discussed.     

Interaction of two bubbles with distortion in an acoustic field

Expression of the secondary Bjerknes force of two bubbles is obtained by considering the distrotion of two bubbles. The secondary Bjerknes forces in different acoustic fields are simulated, and the influence factors are analyzed and discussed. It is shown that the distortion of a bubble has an important influence on the interaction of two bubbles. The strength and even the directions of the secondary Bjerknes force of two bubbles with distortion differ considerably from the predictions of the sherical symmetry theory. The results show that when two bubbles oscillated stably in an acoustic field, the secondary Bjerknes force of two bubbles with distortion is several times more than that of two spherical bubbles in the same condition. The secondary Bjerknes force of two bubble with distortion has more interaction distance than that of two spherical bubbles. The secondary Bjerknes force of two bubbles with distortion depends on the distance of two bubbles, the shape mode of two bubbles, the equilibrium radii of two bubbles and the driving acoustic filed. The nonspherical distortion effects of the secondary Bjerknes has an importance on understanding the structure formation of bubbles and evolution process of bubble group in an acoustic field.     

A note on the low-frequency noise reduction of cylindrical capsules (L)

The noise reductions provided by capsules consisting of uniform cylindrical shells whose two ends are closed off by like circular plates are analyzed for the low-frequency regime in which a capsule acts quasi-statically, much like a pressure vessel. The change in the confined volume is determined from the structural deflections produced by a net pressure difference, taking account of the interactions of the shell and end plates. Expressions for the noise reduction of a single capsule and of two nested capsules are derived. Some insights into the noise reduction effects of interaction between the shell and end plates are discussed, as are the contributions of these components.