Aspects of nucleation and drift processes in a one-dimensional model

We introduce a one-dimensional model involving the nucleation and the drift of many particles. The model originates from interacting kink systems and simulates time evolution in modulated systems. In this model the nucleation rate of a particle depends nonlocally on the density of preexisting particles and the drift of particles is due to a weak and repulsive interaction among them. We first study the statistics of this model in the case that the drift of particles is negligible, and then consider the effects of the drift of particles.     

Drift ratchet

We consider a silicon wafer, pierced by millions of identical pores with periodically varying diameters but without spatial inversion symmetry (ratchet profile). When a liquid is periodically pumped back and forth through the pores, our theory predicts a net transport of suspended micrometer-sized particles (drift ratchet). The direction of this particle current depends very sensitively on the size of the particles. For typical parameter values of the experiment, two different types of particles at an initially homogeneous 1:1 mixture are spatially separated with a purity beyond 1:1000 on a time scale of a few hours in comparably large quantities. This result is due to the highly parallel architecture of the device. The experimental realization of the setup, presently under construction, thus appears to be a promising new particle separation device, possibly superior to existing methods for particles sizes on the micrometer scale.     

Dielectrophoresis of nanocolloids: A molecular dynamics study

Dielectrophoresis (DEP), the motion of polarizable particles in non-uniform electric fields, has become an important tool for the transport, separation, and characterization of microparticles in biomedical and nanoelectronics research. In this article we present, to our knowledge, the first molecular dynamics simulations of DEP of nanometer-sized colloidal particles. We introduce a simplified model for a polarizable nanoparticle, consisting of a large charged macroion and oppositely charged microions, in an explicit solvent. The model is then used to study DEP motion of the particle at different combinations of temperature and electric field strength. In accord with linear response theory, the particle drift velocities are shown to be proportional to the DEP force. Analysis of the colloid DEP mobility shows a clear time dependence, demonstrating the variation of friction under non-equilibrium. The time dependence of the mobility further results in an apparent weak variation of the DEP displacements with temperature.     

Partially asymmetric exclusion models with quenched disorder

We consider the one-dimensional partially asymmetric exclusion process with random hopping rates, in which a fraction of particles (or sites) have a preferential jumping direction against the global drift. In this case, the accumulated distance traveled by the particles, x, scales with the time, t, as x approximately t(1/z), with a dynamical exponent z>0. Using extreme value statistics and an asymptotically exact strong disorder renormalization group method, we exactly calculate z(PW) for particlewise disorder, which is argued to be related as z(SW)=z(PW)/2 for sitewise disorder. In the symmetric case with zero mean drift, the particle diffusion is ultraslow, logarithmic in time.     

Anomalous drift of resonant particles in a buffer medium under pressure of light

We study theoretically the drift of resonant particles in a buffer medium when a traveling light wave impinges on the medium, with allowance for the velocity dependence of the transport collision rate. When the pressure of light dominates over the light-induced drift (low pressure of the buffer gas or the drift of conduction electrons in semiconductors), we discover a new sudden transformation of the spectral dependence of the drift velocity of the resonant particles: Instead of the ordinary bell-shaped function representing the velocity spectrum we have a double-humped curve with deep dip at the center of the absorption line. We show that the largest transformation of the drift velocity spectrum occurs in the atmosphere of a heavy buffer gas in the case of Coulomb interaction between the resonant and buffer particles. The transformation effect is caused by the variation of the transport rate of the collisions of the resonant and buffer particles due to the recoil effect in the absorption of radiation. Zh. éksp. Teor. Fiz. 112, 856–868 (September 1997)     

Brachistochrones in potential flow and the connection to Darwin's theorem

We establish the existence and the asymptotic properties of a path of minimum travel time for a line of particles starting upstream of a sphere or cylinder in potential flow. A connection is made between this brachistochrone path and Darwin's proposition which relates the added mass with the drift volume dragged by a body moving an infinite distance in the fluid. We compute an asymptotic correction to the drift volume for finite distances and show how the brachistochrone path is connected to the reflux volume. We present accurate numerical calculations for the brachistochrone position, point of zero horizontal Lagrangian displacement, reflux and partial drift volumes. These calculations are seen to agree well with the asymptotic predictions even for moderate values of the parameters. In the small Reynolds number regimes, we show that while for the case of Stokes flow past a sphere no brachistochrones exist at finite distances from the sphere, the Oseen correction is sufficient to restore such least-time trajectories. Lastly, the application to a sphere falling in a stratified fluid is discussed using the new drift volume correction formula.     

Effect of solvent on directional drift in Brownian motion of particle/molecule with broken symmetry

The directional drifting of particles/molecules with broken symmetry has received increasing attention. Through molecular dynamics simulations, we investigate the effects of various solvents on the time-dependent directional drifting of a particle with broken symmetry. Our simulations show that the distance of directional drift of the asymmetrical particle is reduced while the ratio of the drift to the mean displacement of the particle is enhanced with increasing mass, size, and interaction strength of the solvent atoms in a short time range. Among the parameters considered, solvent atom size is a particularly influential factor for enhancing the directional drift of asymmetrical particles, while the effects of the interaction strength and the mass of the solvent atoms are relatively weaker. These findings are of great importance to the understanding and control of the Brownian motion of particles in various physical, chemical, and biological processes within finite time spans.     

A gas of Brownian particles in statistical dynamics

We consider a large number of particles on a one-dimensional latticel Z in interaction with a heat particle; the latter is located on the bond linking the position of the particle to the point to which it jumps. The energy of a single particle is given by a potentialV(x), x∈Z. In the continuum limit, the classical version leads to Brownian motion with drift. A quantum version leads to a local drift velocity which is independent of the applied force. Both these models obey Einstein's relation between drift, diffusion, and applied force. The system obeys the first and second laws of thermodynamics, with the time evolution given by a pair of coupled non linear heat equations, one for the density of the Brownian particles and one for the heat occupation number; the equation for a tagged Brownian particle can be written as a stochastic differential equation.     

Particle drift in a resonance tube--a numerical study

The paper considers longitudinal drift of small particles in a resonance tube, caused by periodic shock waves, and its effect on particle agglomeration. It is found that depending on particle size, drift is caused by shock waves and/or gas acceleration and compression. It is also shown that the drift velocity and direction can be controlled by the frequency of the piston that causes gas oscillations in the resonance tube. The obtained numerical solutions indicate that particle drift in a resonance tube enhances aerosol agglomeration. An agglomeration kernel is derived for this case, accounting for particle drift, leading to an estimate of agglomeration time. The time predicted by present model is of the same order of magnitude as that obtained from experiments in the literature.     

负径向电场对荷电粒子运动的影响及其数值模拟

从负径向电场产生的电漂移改变荷电粒子运动的极向运动速度着手,推导出在负径向电场存在时安全因子的表达式,分析了安全因子对荷电粒子漂移位移和运动轨迹的影响。建立了在负径向电场条件下,荷电粒子在梯度磁场和曲率磁场中运动数学模型。通过数值模拟,获得了通行粒子、香蕉粒子的漂移位移和运动轨迹所呈现出的新特点和规律:负径向电场改变了荷电粒子的最大漂移位移。当荷电粒子的极向运动速度增加时,最大漂移位移减小,反之增大;改变了荷电粒子的运动轨迹,通行粒子的轨亦可能变为香蕉粒子的轨迹,香蕉粒子的轨迹可能变为通行粒子的轨迹,当电场达到足够的强度时,均成为在极向上顺时针运动的通行粒子轨迹。     

Drift velocity in the necklace model for reptation in a two-dimensional square lattice

We report the chain dynamics in the necklace model that mimics the reptation of a chain of N particles in a two-dimensional square lattice. We focus on the drift velocity under an applied static field. The characteristics of the model allow us to determine the effects of the forces on the chains and the resulting mechanisms that affect the drift velocity. Results obtained through Monte Carlo simulations were analyzed and discussed and distinct regimes as a function of the force strength and N were identified. We found that for small total applied forces, the drift velocity scales as 1/N. When the applied force to every particle is small but the total applied force is not, the tube deforms in such a way that the drift velocity does not depend on N. Large forces, applied to every particle, can straight chains such that the distance between the chain ends increases faster than the number of particles. Also, large forces can deform the chain within the tube what is directly related to a decrease of the drift velocity.     

强场和弱场侧超声分子束注入对加料的影响

报道了HL-2A装置最新的实验结果,讨论并研究了超声分子束的注入位置对分子束在等离子体中的消融和穿透的影响,其中包括电离后的分子束粒子在磁场梯度作用和 E × B 漂移下的加速或减速及由此形成的冷通道效应.研究结果表明,磁场梯度和 E × B 漂移对于超声分子束的加料效果、消融和穿透有着重要的作用.强场侧注入可使电离后的电子和离子更深地进入等离子体芯部.这些研究对于更好地理解超声分子束与等离子体的相互作用和优化设计加料系统有一定作用.     

Stochastic modeling of experimental chaotic time series

Methods developed recently to obtain stochastic models of low-dimensional chaotic systems are tested in electronic circuit experiments. We demonstrate that reliable drift and diffusion coefficients can be obtained even when no excessive time scale separation occurs. Crisis induced intermittent motion can be described in terms of a stochastic model showing tunneling which is dominated by state space dependent diffusion. Analytical solutions of the corresponding Fokker-Planck equation are in excellent agreement with experimental data.     

Velocity enhancement of slow particles in lattice-gas binary mixture

We study the unidirectional flow of a binary mixture of biased-random walkers on square lattice under a periodic boundary. The lattice-gas mixture consists of two types of walkers which have different biases (drift coefficients). The characteristics of unidirectional flow are clarified numerically. The mean velocity of slow particles in the binary mixture is enhanced higher than that in lattice-gas consisting of only slow particles. The mean velocity of slow particles shows a maximal value at an intermediate density. The dependence of velocity enhancement on both drift coefficient and mixture fraction is shown. Velocity enhancement is intensified with decreasing fraction of slow particles. Also, when the bias is lower, the velocity enhancement is higher.     

漂移量反馈控制式激光准直方法

提出一种集光纤准直与光束漂移量反馈控制准直技术于一体的激光光束准直方法 ,它是在单模光纤对激光器出射光束漂移量进行初级准直的基础上 ,进一步对光束的漂移量进行反馈控制 ,进而提高出射光束的方向稳定性。准直系统中利用漂移量分离光路和光电检测系统对光束的平漂量和角漂量进行了分离检测 ,并各自构成相应的光束空间平漂量反馈控制系统和光束空间角漂量反馈控制系统 ,减小了平漂量和角漂量反馈控制中的相互耦合。引入单模光纤使激光器与光束漂移量反馈控制准直系统相隔离 ,减小了激光管热源温度变化等因素对准直系统的影响 ,同时还便于准直系统使用中的安装布置。上述方法的应用 ,提高了准直精度及准直效率。实验表明目前该方法准直精度可达 0 .7× 10 -7rad。     

Kinetic-diffusion asymptotic-preserving Monte Carlo algorithms for plasma edge neutral simulation

We developed novel Monte Carlo simulation strategies for the neutral model in plasma edge simulations where both low-collisional and high-collisional regimes are present. To maintain accuracy and reduce simulation costs in high-collisional regimes, we use hybridized particles that exhibit both kinetic and diffusive behaviour depending on the local collisionality. The method maintains an asymptotically correct distribution and a correct mean, variance, and time correlation for all values of the collisionality. We apply this scheme to a fusion case with a strongly heterogeneous background, prompting the inclusion of a diffusion-induced drift. Our numerical results show a large increase in efficiency at the expense of a minor bias.     

Ion extraction and charge exchange in laser isotope separation

In the atomic vapor laser isotope separation (AVLIS) process, a vapor is ionized by pulsed laser beams, and the ions are extracted by negatively biased collectors. We compute the unsteady dynamics of the photoplasma using a two-dimensional (2-D) particle-in-cell (PIC) code. Collisions between ions and neutral species are simulated by a Monte Carlo technique. The plasma dynamics is visualized by snapshots of particle positions showing the directions of their velocities. The three kinds of particles (electrons, photo-ions, and ions created by charge exchange) are marked by different colors. The graphic outputs illustrate the motion of the electrons toward the anodes, the vertical drift of the plasma, its erosion by the transient ion sheath, and nonselective ionization by charge exchange     

Permeability variation with fracture dissolution: Role of diffusion vs. drift

Pores and fractures in rocks are continually being reshaped through different chemical and physical processes. Fluids filling the pore space carry the different chemical species responsible for these changes. In the present work we study the relative effects of drift and diffusion of these particles, through a random walk. A bias imposed on the walker determines the competition between drift and diffusion, i.e. to a variation of the Peclet number. We find that the pore structure and hence permeability depends strongly on the bias.     

Failure and success of hydrodynamic interaction models

In suspensions with charged particles, electrostatic forces and hydrodynamic interactions are both important to describe the system. We study different models of hydrodynamic interaction for monopolarly charged particles in a non-polar liquid. In this case, there is no screening of the Coulomb repulsion, so the repulsion between all pairs must be taken into account. The particles are expected to drift away from each other, however at a lower rate when hydrodynamic interaction between the particles is taken into account. Existing, frequently used models of hydrodynamic interactions tend to overestimate the slowing down of the charged particles, even to the extent that the particles effectively attract each other. This is demonstrated for some selected particle setups. We find that these anomalies even occur in dilute systems, if they contain sufficiently many particles. We explain why these anomalies can be avoided by an approach, in which the superposition of interactions is done in the friction tensor instead of the mobility tensor.     

Asymmetric one-dimensional exclusion processes: a two-parameter exactly solvable example

We consider a two-parameter family of asymmetric exclusion processes for particles living on a continuous one-dimensional space. Using the Bethe ansatz, the exact solution to the master equation, and from that the drift and the diffusion rate in the two particle sector, are obtained.