Electrophoretic mobility without charge driven by polarisation of the nanoparticle–water interface

Polarisation of the interface, spontaneously occurring when water is in contact with hydrophobic solutes or air, couples with the uniform external field to produce a non-zero force acting on a suspended particle. This force exists even in the absence of a net particle charge, and its direction is affected by the first-order, dipolar and the second-order, qudrupolar orientational order parameters of the interfacial water. The quadrupolar polarisation gives rise to an effectively negative charge. The corresponding surface charge density is inversely proportional to the area of the shear surface. As a result, the overall contribution from the quadrupolar polarisation to the particle mobility becomes negligible compared to experimentally reported values for particles exceeding a few nanometres in size. In contrast, the contribution of the dipolar order of the interface to the effective surface charge scales inversely with the particle size and dominates the zero-charge mobility of submicron particles. The corresponding electrokinetic charge is determined by the preferential orientation of interfacial dipoles relative to the surface normal.     

Stability of Driven Systems with Growing Gaps, Quantum Rings, and Wannier Ladders

We consider a quantum particle in a periodic structure submitted to a constant external electromotive force. The periodic background is given by a smooth potential plus singular point interactions and has the property that the gaps between its bands are growing with the band index. We prove that the spectrum is pure point—i.e., trajectories of wave packets lie in compact sets in Hilbert space—if the Bloch frequency is nonresonant with the frequency of the system and satisfies a Diophantine-type estimate, or if it is resonant. Furthermore, we show that the KAM method employed in the nonresonant case produces uniform bounds on the growth of energy for driven systems.     

Measurement of the interaction force among particles in three-dimensional plasma clusters

The interaction forces between particles have been studied in a 3D plasma cluster under weak external confinement. A suitable combination of dc and rf applied to a small electrode provided gravity compensation, uniform over dimensions much larger than the cluster itself. The forces acting on the particles could be reconstructed due to unique three-dimensional diagnostics, which allow us to obtain coordinates and velocities of all the particles simultaneously. The measurements yield a maximum (external) confinement force of 1.4 x 10(-15)N and interparticle force that is repulsive at short distances and attractive at larger distances, with a maximum attractive force of 2.4 X 10(-14)N at particle separation 195 microm.     

Asymptotic stability of stationary states in the wave equation coupled to a nonrelativistic particle

We consider the Hamiltonian system consisting of a scalar wave field and a single particle coupled in a translation invariant manner. The point particle is subjected to an external potential. The stationary solutions of the system are a Coulomb type wave field centered at those particle positions for which the external force vanishes. It is assumed that the charge density satisfies the Wiener condition, which is a version of the “Fermi Golden Rule.” We prove that in the large time approximation, any finite energy solution, with the initial state close to the some stable stationary solution, is a sum of this stationary solution and a dispersive wave which is a solution of the free wave equation.     

Effects of boundaries on structure formation in low-dimensional colloid model systems near the liquid-solid-transition in equilibrium and in external fields and under shear

A brief review focusing on low-dimensional colloidal model systems is given describing both simulation studies and complementary experiments, elucidating the interplay between phase behavior, geometric structures, and transport phenomena. These studies address the response of these very soft colloidal systems to perturbations such as uniform or uniaxial compression, laser fields, randomly quenched disorder, and shear deformation caused by moving boundaries. Binary hard-disk mixtures are studied by Monte Carlo simulation, to investigate ordering on surfaces or in monolayers, modeling the effect of a substrate by an external potential. By weak external laser fields the miscibility of the mixture can be controlled, and the underlying mechanism (laser-induced demixing) is clarified. The stability of various space-filling structures is discussed only for the case where no laser fields are present.Hard spheres interacting with repulsive screened Coulomb or dipolar interaction confined in 2D and 3D narrow constrictions are investigated by Brownian Dynamics simulation. With respect to the structural behavior, it is found that layers or planes throughout the microchannel are formed. The arrangement of the particles is disturbed by diffusion, and can also be modified by an external driving force causing a density gradient along the channel. Then the number of layers or planes gets reduced, adjusting to the density gradient, and this self-organized change of order also shows up in the particle velocities. The experimental work that is reviewed here addresses dipolar colloidal particles confined by gravity on a solid substrate on which a set of pinning sites has been randomly distributed. The dynamics of the system is studied by tracking the trajectories of individual particles, and it is found that the mean square displacements of particles that are nearest neighbors of pinned particles are strongly affected by these defects. The influence of the pinning sites on the order and microscopic mechanism of phase transitions in two dimensions is investigated.     

Magnetic particle diverter in an integrated microfluidic format

A fully integrated micromagnetic particle diverter and microfluidic system are described. Particles are diverted via an external uniform magnetic field perturbed at the microscale by underlying current straps. The resulting magnetic force deflects particles across a flow stream into one of the two channels at a Y-shaped junction. The basic theoretical framework, design, and operational demonstration of the device are presented.     

Brownian motion of a charged particle in a magnetic field

We develop and numerically illustrate an exact solution of the multivariate, stochastic, differential equations that govern the velocity and position of a charged particle in a plane normal to a uniform, stationary, magnetic field. The equations self-consistently incorporate the Lorentz force into an Ornstein-Uhlenbeck collision model. Properties of the solution in the infinite dissipation limit are explored and the spectral energy density function is found     

同位旋非对称核物质性质与扩展的BHF方法(Ⅴ)三体核力对质子和中子单粒子势的影响

在扩展的同位旋相关的Brueckner Hartree Fock理论框架内,研究了三体核力对同位旋非对称核物质中质子与中子的单粒子势及其同位旋依赖性的影响.     

Fokker–Planck Equation,Molecular Friction,and Molecular Dynamics for Brownian Particle Transport near External Solid Surfaces

The Fokker–Planck (FP) equation describing the dynamics of a single Brownian particle near a fixed external surface is derived using the multiple-time-scales perturbation method, previously used by Cukier and Deutch and Nienhuis in the absence of any external surfaces, and Piasecki et al. for two Brownian spheres in a hard fluid. The FP equation includes an explicit expression for the (time-independent) particle friction tensor in terms of the force autocorrelation function and equilibrium average force on the particle by the surrounding fluid and in the presence of a fixed external surface, such as an adsorbate. The scaling and perturbation analysis given here also shows that the force autocorrelation function must decay rapidly on the zeroth-order time scale ₀, which physically requires N _Kn1, where N _Kn is the Knudsen number (ratio of the length scale for fluid intermolecular interactions to the Brownian particle length scale). This restricts the theory given here to liquid systems where N _Kn1. For a specified particle configuration with respect to the external surface, equilibrium canonical molecular dynamics (MD) calculations are conducted, as shown here, in order to obtain numerical values of the friction tensor from the force autocorrelation expression. Molecular dynamics computations of the friction tensor for a single spherical particle in the absence of a fixed external surface are shown to recover Stokes' law for various types of fluid molecule–particle interaction potentials. Analytical studies of the static force correlation function also demonstrate the remarkable principle of force-time parity whereby the particle friction coefficient is nearly independent of the fluid molecule–particle interaction potential. Molecular dynamics computations of the friction tensor for a single spherical particle near a fixed external spherical surface (adsorbate) demonstrate a breakdown in continuum hydrodynamic results at close particle–surface separation distances on the order of several molecular diameters.     

The Larmor nutation

The classical non-relativistic problem of the motion of a charged particle in an external central force field and a weak uniform magnetic field is revisited to show that the motion of the kinetic angular momentumL = r × p of the particle, in the so-called Larmor approximation, is not a simple precession but is actually a composite motion involving precession as well as a high frequency nutation. The precession-nutation motion ofL is discussed in the Larmor approximation when the Larmor-frame-orbit of the charged particle is an ellipse (or a circle) for the case of the two central forces namely the Coulomb and the Hooke-law-force, which are the only two central forces known to permit closed orbits.     

气温和颗粒密度对声场中颗粒动力学影响的数值模拟

综合考虑黏性夹带力、Basset力、虚拟质量力和压力梯度力,建立颗粒在声场中的动力学模型,利用变步长四阶RungeKutta算法和二阶隐式Adams插值算法对颗粒的受力和运动进行数值模拟。将模拟和实验得到的颗粒运动特性进行对比,验证数值模拟的正确性。在此基础上,研究气温和颗粒密度对颗粒动力学的影响规律。结果表明,黏性夹带力对颗粒运动起主导作用;气温升高,压力梯度力与黏性夹带力之间的相位差减小,Basset力、虚拟质量力与黏性夹带力之间的相位差增大。研究还发现,气温较低时,颗粒密度对颗粒运动有重要影响,夹带系数随着密度的增加而迅速下降;气温较高时,颗粒密度对颗粒运动的影响较小,颗粒位移振幅和夹带系数相对低温时明显增加。      

Neutrino in matter and external fields

A technique for describing various processes proceeding in matter and involving neutrinos and electrons is discussed. This technique is based on “the method of exact solutions,” which implies the use of solutions to proper Dirac equations for particle wave functions in matter. Exact solutions for the neutrino and the electron in the cases of uniform nonmoving and rotating matter are discussed. On studying relativistic neutrino motion and associated neutrino-energy quantization in rotating matter, a semiclassical interpretation of particle finite motion is developed. In the general case of neutrino and electron motion in matter with varying parameters, the corresponding effective force acting on the particles is determined. The possibility of electromagnetic-wave radiation by an electron that moves in a dense neutrino flux of varying density and which is accelerated by this kind of force is predicted.     

Force-driven micro-rheology

Within a microscopic formalism for the nonequilibrium response of colloidal suspensions driven by an external force, we study the active micro-rheology of a glass-forming colloidal suspensions. In this technique, a probe particle is subject to an external force, and its nonequilibrium dynamics is monitored. Strong external forcing delocalizes the particle from its nearest-neighbor cage, resulting in a pronounced force-thinning behavior of the single-particle friction. We discuss the dynamics in the vicinity of this delocalization transition, and how long-range transport is induced for a particle that is localized in the quiescent case.     

复合场中的滚轮线

带电质点在正交的匀强电场(重力场)和匀强磁场中运动,轨迹为滚轮线.处理滚轮线问题的关键是构造匀速直线运动以平衡电场力(重力),从而将带电质点的运动分解为匀速直线运动和匀速圆周运动,简化问题的处理过程.     

弱相互作用费米气体的不稳定性判据

根据由赝势法得到的弱相互作用费米气体的自由能，利用热力学方法研究了无外场时弱相互作用费米气体的稳定性.结果表明，无外场情况下理想费米气体与存在弱排斥相互作用的费米气体是稳定的；而具有弱吸引相互作用的费米气体在一定条件下可出现不稳定性.给出了不稳定性的粒子数密度判据和温度判据，就不同逸度情况下临界粒子数密度的具体表达结果以及温度、粒子质量和吸引相互作用对临界粒子数密度的影响进行了讨论. 关键词： 费米气体 相互作用 不稳定性判据     

The influence of a varying field on the non-Markovian migration of particles

The influence of an external varying field on the non-Markovian migration of particles described in the continuous-time random walk model (CTRWM) was analyzed theoretically. In terms of the Markovian representation for the CTRWM suggested earlier, a rigorous method for describing the influence of an external force was developed. This method reduced the problem to solving the non-Markovian stochastic Liouville equation (SLE) for the particle distribution function. An analysis of the derived SLE and its comparison with the earlier equations were performed. The method was used to study the characteristic features of the time dependence of the first and second moments of the distribution function for particles involved in subdiffusion motion in a uniform varying external field. Both oscillating and fluctuating fields were considered. In both cases, anomalously strong field effects on the second particle distribution moment (variance) were observed. This influence was especially strong for a fluctuating field, and in the limit of anomalously slow fluctuations at that.     

Fully self-consistent ion-drag-force calculations for dust in collisional plasmas with an external electric field

The ion drag force on a spherical dust particle immersed in a flowing plasma with an external electric field is self-consistently calculated using the particle-in-cell code SCEPTIC in the entire range of charge-exchange collisionality. Our results, not based on questionable approximations, extend prior analytic calculations valid only in a few limiting regimes. Particular attention is given to the force direction, shown never to be directed opposite to the flow except in the continuum limit, where other forces are of a much stronger magnitude.     

Electromagnetic energy, momentum, and angular momentum in an inhomogeneous linear dielectric

In a previous work, Optics Communications 284 (2011) 2460-2465, we considered a dielectric medium with an anti-reflection coating and a spatially uniform index of refraction illuminated at normal incidence by a quasimonochromatic field. Using the continuity equations for the electromagnetic energy density and the Gordon momentum density, we constructed a traceless, symmetric energy-momentum tensor for the closed system. In this work, we relax the condition of a uniform index of refraction and consider a dielectric medium with a spatially varying index of refraction that is independent of time, which essentially represents a mechanically rigid dielectric medium due to external constraints. Using continuity equations for energy density and for Gordon momentum density, we construct a symmetric energy-momentum matrix, whose four-divergence is equal to a generalized Helmholtz force density four-vector. Assuming that the energy-momentum matrix has tensor transformation properties under a symmetry group of space-time coordinate transformations, we derive the global conservation laws for the total energy, momentum, and angular momentum.     

Dynamics of a deformable self-propelled particle under external forcing

We investigate dynamics of a deformable self-propelled particle under external fields in two dimensions based on the time-evolution equations for the center of mass and a tensor variable characterizing deformations. We consider two kinds of external force. One is a gravitational force which enters additively in the time-evolution equation for the center of mass. The other is an electric force supposing that a dipole moment is induced in the particle. This force is added to the equation for the deformation tensor. It is shown that a rich variety of dynamics appears by changing the strength of the forces and the migration velocity of a self-propelled particle. A theoretical analysis is carried out to clarify the dynamics and the bifurcations.     

高压电场内细颗粒堆积机理研究

细颗粒的堆积既是电厂尾部除尘系统的核心问题,也是航天领域空间环境内限制动力或光学元件性能的关键问题。电场力因具有长程有效性和强可操作性成为控制细颗粒堆积的主要手段。本文通过微观实验研究了高压平行板电场间细颗粒堆积的机理,观察到颗粒在预极化、预荷电、变外电场电压等工况下的堆积形貌,并发展了图像处理的方法统计堆积颗粒数。研究表明,偶极力是外电场下颗粒成链的主因,而颗粒的倒伏则是来自曳力的作用,颗粒链的极限高度主要受外电场场强和颗粒堆积结构的影响。