Numerical simulation of particle size effects on contact electrification in granular systems

Based on the contact charge transfer model between two particles due to a single collision proposed by Apodaca, the contact charges carried on a particle is derived due to multiple collisions, including the repeat collisions between two particles and the collisions with different particles, in mixed-size granular system of identical material. The effect of the particle size on the charges carried on the particle is simulated. The results indicate that for a mixed-size granular system, due to multiple collisions among particles, there exists a threshold particle radius, the particles with radius higher than which and the particles with radius lower than which carry opposite charges. The threshold particle radius is equal to mean value of particle size in the mixed-size granular system. Basically, the polarity of the charges carried on the largest particle is same as the polarity of the transfer charge carrier, and in case of the positive charge transferred, the largest particle will be positively charged and the smallest particle will be negatively charged, and vice versa. In the same size region, the more dispersive the particle size is, the more the net charges can be produced. In normal-distributed granular system, the magnitude of contact charge is determined mainly by the particle size distribution, size region, total particle number and the relative impact velocity.     

Ground-state pump-probe spectroscopy

The absorption/gain spectrum of a weak probe field is theoretically analyzed for an atomic three-level Λ system driven by a strong pump field and perturbed by collisions with buffer-gas atoms. It is assumed that both pump and probe drive the same transition. When the dark state is sufficiently long lived, the probe spectrum exhibits an extremely narrow (possibly subnatural-width) resonance. It is shown that coherence-preserving collisions drastically (both qualitatively and quantitatively) change the probe absorption spectrum even at a low collision frequency, when Dicke narrowing is negligible. Strong coherence effects on the resonance structure at low gas pressures is explained by a change in refractive index due to coherence-preserving collisions for particles in the resonant velocity group.     

Deformation of the contact area and adhesive friction between a scanning frictional microscope probe and the atomic-flat surface

The determination of the equilibrium atomic structure of a nanotribocontact, formed by a hard probe to be viewed as a paraboloid of revolution and subjected to an external load, with the soft surface modeled by a set of parallel atomic planes is considered. Structural, energy, and load characteristics are calculated. In addition, dissipative static adhesive friction as a function of the normal load and the radius of probe curvature for the diamond-graphite system is derived. A number of approximations of the interatomic potentials is used. It is shown that an allowance for the deformation of the contact area causes the adhesive frictional force in the tensile (negative) load range to decrease. For positive loads in a range of 0–80 nN, the variation of the frictional force (when deformation is taken into account) depends on the radius of the probe curvature and the used approximation of the interaction potential. The dependence of friction on the radius of the probe curvature is close to a direct proportionality. The calculated results are compared with the available experimental data.     

Dynamic evolution of impaction and sticking behaviors of fly ash particle in pulverized coal combustion

This paper aims to reveal the mechanisms governing the impaction and sticking dynamics of fly ash particles in pulverized coal combustion. The modeling work is of relevance to experiments in a 25?kW self-sustained down-fired furnace, which provides a sequence of real deposit shapes as varied boundary conditions for CFD simulations. Although the formed ash deposit has a comparable length scale with the probe, it has little effect on the global impaction efficiency of newly-coming particles. However, as the deposit builds up, incident particles impact the deposit and probe at generally larger impact angles and smaller normal velocities despite the almost invariant global impaction efficiency. It results in an enhanced local sticking probability in the center region of the probe, but a decreased one in the lateral regions. The incident kinetic energy of newly sticking particles to the deposit exhibits a converse correlation with their impact angle. The relationship of the averaged local sticking probability as a function of the azimuthal angle of probe is illustrated. Finally, the effect of Reynolds number on global particle impaction efficiency is examined. A universal formula is proposed, which is of importance to bridge lab-scale experiments and practical applications.     

PIC/MCC Simulation of Electron and Ion Currents to Spherical Langmuir Probe

The Particle In Cell/Monte Carlo Collisions (PIC/MCC) simulation was used for the calculation of electron and ion currents to a spherical Langmuir (electrostatic) probe. This simulation took into account the collisions of collected charged particles with neutral gas particles around the probe and it can calculate the probe currents at higher neutral gas pressures. The improvements of usual simulation techniques enabled to speed up the simulation and to calculate the probe current even for neutral gas pressures above 1 kPa. The simulations were carried out for two cases: i) probe with radius of 0.5 mm in non‐thermal plasma with high electron temperature, ii) probe with radius of 10 µm in afterglow plasma with low electron temperature. The influence of probe radius on electron probe current was also studied. The simulations showed that thick sheath limit of OML theory provides incorrect values of probe current for probes with radii larger than 200 µm at plasma parameters considered even at very low neutral gas pressures. The probe characteristics were calculated for probe with 0.5 mm radius for pressures up to 500 Pa and for probe with 10 μm radius for pressures up to 3 kPa. The influence of collisions on electron and ion probe current was demonstrated and the procedure for determination of electron and ion densities from the probe measurement at higher pressures was developed. The results from PIC/MCC simulations were compared with results from continuum theory. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

空间尘埃等离子体对量子卫星通信性能的影响

为了研究尘埃等离子体中带电尘埃的粒子半径、粒子浓度和带电荷数对量子通信性能的影响,首先根据Mie散射理论得到单个带电尘埃粒子的光散射截面;然后通过粒子浓度求出总的消光截面,得出链路衰减的数学模型,提出了带电粒子特性与量子纠缠度的关系;针对退极化信道,当单个尘埃粒子所吸附带电粒子的个数为50时,给出了尘埃粒子半径、粒子浓度与信道容量和量子误码率的定量关系.仿真结果表明,当量子信号的传输距离为10km时,尘埃粒子浓度从1×10~(10) m~(-3)增加到10×10~(10) m~(-3),信道容量从0.6726降低到0.1075;尘埃粒子半径从0.1μm增加到10μm时,量子误码率由1.334×10~(-3)增加到5.309×10~(-3).由此可见,尘埃等离子体中带电尘埃粒子的半径和浓度对量子卫星通信性能有显著的影响.因此,为确保量子通信的可靠性,应根据所探测到的等离子体环境的状况,调整卫星通信系统的各项指标参数.     

Anomalous diffusion probes microstructure dynamics of entangled F-actin networks

We study the thermal motion of colloidal tracer particles in entangled actin filament (F-actin) networks, where the particle radius is comparable to the mesh size of the F-actin network. In this regime, the ensemble-averaged mean-squared displacement of the particles is proportional to tau(gamma), where 0相似文献   

结合力来自波-粒子互作用(英文)

基于ＢＢＧＫＹ方程，采用二阶扰动理论探索了可观物理量与扰动波矢的相互关系。结果发现，两个扰动带电粒子之间存在一个关联它们的非线性力。这个被称之为结合力的空间尺度是由扰动波矢的虚部决定的。这个结合力的一般表示包含一个吸引项和一个排斥项，它们随粒子的温度和它们的扰动频率而变化。包含在这个结合力中的最重要的信息是同物质的基本构成和它们之间相互作用的性质相联系的。     

Moving breather collisions in Klein-Gordon chains of oscillators

We investigate the collisions of moving breathers, with the same frequency, in three different Klein-Gordon chains of oscillators. The on-site potentials are: the asymmetric and soft Morse potential, the symmetric and soft sine-Gordon potential and the symmetric and hard φ⁴ potential. The simulation of a collision begins generating two identical moving breathers traveling with opposite velocities, they are obtained after perturbing two identical stationary breathers which centers are separated by a fixed number of particles. If this number is odd we obtain an on-site collision, but if this number is even we obtain an inter-site collision. Apart from this distinction, we have considered symmetric collisions, if the colliding moving breathers are vibrating in phase, and anti-symmetric collisions, if the colliding moving breathers are vibrating in anti-phase. The simulations show that the collision properties of the three chains are different. The main observed phenomena are: breather generation with trapping, with the appearance of two new moving breathers with opposite velocities, and a stationary breather trapped at the collision region; breather generation without trapping, with the appearance of new moving breathers with opposite velocities; breather trapping at the collision region, without the appearance of new moving breathers; and breather reflection. For each Klein-Gordon chain, the collision outcomes depend on the lattice parameters, the frequency of the perturbed stationary breathers, the internal structure of the moving breathers and the number of particles that initially separates the stationary breathers when they are perturbed.     

The Interfacial Transition Zone (ITZ) Between Cement Paste and Aggregate in Concrete

This paper describes the so called interfacial transition zone—ITZ—in concrete. This is the region of the cement paste around the aggregate particles, which is perturbed by the presence of the aggregate. Its origin lies in the packing of the cement grains against the much larger aggregate, which leads to a local increase in porosity and predominance of smaller cement particles in this region. The ITZ is region of gradual transition and is highly heterogeneous, nevertheless the average microstructural features may be measured by analysis of a large numbers of backscattered electron images of polished concrete samples. Such measurements show that the higher porosity present initially is significantly diminished by the migration of ions during hydration.     

中能重离子碰撞中同位旋相关核子–核子碰撞截面的灵敏探针

基于重质量弹核(放射性核或稳定核)打轻质量靶核的逆运动学效应,即反应产物主要集中在小角度的优点,利用同位旋相关的量子分子动力学模型,探寻了对于中能重离子碰撞过程中同位旋效应灵敏的物理观测量.计算表明,在入射能量约大于80MeV/u能区,中等质量碎片多重性随发射带电粒子总数之间的演化是提取介质中同位旋相关的核子–核子碰撞截面的灵敏物理量.     

气泡室中"胚胎"气泡成长为可见气泡的模型建立及初步的理论计算

建立了气泡室中“胚胎”气泡成长为可见气泡模型.初步计算表明,“胚胎”气泡在成长为可见气泡的过程中,它不仅要从其周围吸收热量,而且还有分子进入到它的里面.理论上可以合理解释能量相同的中子和质子入射到气泡室所产生的径迹上的气泡半径是不同的,中子在气泡室中产生的径迹细长,而质子在气泡室中产生的径迹粗短;可以合理解释在同一径迹上某个区域内可以有半径大小相差不多的气泡存在;也可以合理解释电荷数较多的入射粒子较能量相同但电荷数不同的入射粒子,其在气泡室中径迹上气泡的半径要大;理论上预测入射粒子刚进入气泡室时,其径迹上有大气泡破裂现象发生.通过选择合适的工作物质可以提高气泡室的灵敏度和探测效率.     

随机取向烟幕凝聚粒子的消光特性

采用蒙特卡罗方法根据团簇-团簇凝聚(CCA)模型对由球形原始微粒凝聚而成的烟幕凝聚粒子进行模拟,用离散偶极子近似(DDA)方法研究随机取向烟幕凝聚粒子的消光特性.结果表明,凝聚粒子的消光特性受到原始微粒数量以及粒径的影响,粒子的凝聚将减弱烟幕的消光性能;当凝聚结构中原始微粒的数目一定时,存在使烟幕消光性能达到最大的原始微粒粒径.     

束晕-混沌控制中的粒子跟踪模拟研究

运用PIC程序研究了强流离子束中粒子的横向运动，发现了不加控制时束晕粒子并非一直处于晕区和施加非线性控制后粒子的横向运动被限制在一定的范围内形成环状，以及均方根半径的变化近似成为周期运动等性质.根据观察到的均方根半径及其变化率的规律性，提出了一种新的自适应控制器.用该控制器不仅能在很短的时间内完全控制住束晕，而且不需要对增益因子进行精确计算，也能在系统参数改变的情况下取得较好的控制效果. 关键词： 束晕_混沌 粒子跟踪 数值模拟 自适应控制     

部分状粒子处理方法及其对云微物理参数测量的影响

作为云微物理过程测量的重要利器,机载云降水粒子成像仪在云降水物理与人工影响天气研究中具有重要的作用.从采样结果来看,机载云降水粒子成像仪所测粒子图像中含有大量的粒子图像仅是粒子的一部分而已,即部分状粒子.因其数量较多,对该类粒子所选处理方法不同,会引起测量结果的很大差异.本文介绍并分析了现有部分状粒子处理方法的优劣,通过对部分状粒子的再定义与粒子形状分类,提出了一个融合粒子形状识别技术、"粒径重构"和"中心在内"方法的新的部分状粒子处理方法;利用实测数据,对所提方法与现有方法进行了云微物理参量处理结果的对比,发现本文所提方法与"粒径重构"方法处理结果比较一致,能较好地克服"整体在内"与"中心在内"两种方法存在的缺陷;同时,在针柱状粒子占比较多情形下,本文所提方法要比"粒径重构"方法处理后的结果相对合理.因此本文所提方法对仪器所测粒子数据处理具有更好的适应性.     

Application of probing dusty particles to investigation of a stratified discharge with dusty plasma structures in a magnetic field

Experimental investigation of a stratified discharge in a magnetic field with the help of probe dusty particles falling through the discharge under the gravity force is described. The radial and azimuthal accelerations of probe particles in the stratum volume are measured. It is found that the particles move in the stratum without accelerations except in two regions. In the first region, the acceleration has two components directed toward the tube axis and upward, whereas, in the second region, they are directed toward the tube wall and downward.     

高散射媒质中的光程可分割动态光散射研究

为研究散射光强度随光子在散射媒质中散射光程的变化,基于单散射理论与扩散波光谱理论,采用了低相干动态光散射装置对不同粒径大小的聚苯乙烯悬浮液进行研究。将测量得到的背散射光光场强度谱的线宽与相应的理论计算结果相比较发现,在短光程区域,考虑容器壁附近拖曳效应的影响后,对于不同粒径的颗粒,光程为约5倍粒子平均自由程的区域可看成为单散射区域;对于光程大于225 m的区域可看成为扩散光波区域。实验结果表明低相干动态光散射法可实现高散射媒质从单散射区域到低次散射再到扩散区域的全光程的可分割的光场强度谱测量。     

Blueshift and spectral features of the gravitational searchlight

It is shown that a photon emitted in the forward direction by a charged particle moving in an equatorial circular orbit centred on a highly collapsed mass M, the radius being slightly in excess of one and a half times the Schwarzschild radius, is strongly blueshifted when it arrives at a distant receiver. A ring shaped emitting region composed of such orbiting particles has a power law spectrum of the formd/ as seen by a distant stationary observer.     

Slow light in photonic crystals waveguide constructed with symmetrically perturbed structure

Symmetrically perturbed photonic crystal waveguide can be constructed by inserting perturbative dielectric rods into photonic crystal waveguide structure with whose rods’ radius distributed according to a certain proportion. Slow light properties in this new structure are studied by using the plane wave expansion method (PWM). In this paper, schemes of adjusting radius of perturbative dielectric rods and adjusting the dielectric constant of perturbative dielectric rods are proposed to optimize slow light properties. The result shows that the scheme for adjusting radius of perturbative rods can realize larger average slow light bandwidth and efficiently control the NDBP value of the waveguide, but it contributes little to obtain smaller group velocity. The scheme for adjusting dielectric constant of perturbative rods can realize smaller group velocity, but can only obtain smaller slow light bandwidth and cannot efficiently enlarge NDBP value of waveguide. Both optimization schemes proposed in this paper realize group velocity that is two magnitudes smaller than the vacuum speed of light meanwhile maintaining large NDBP and low GVD region. Our results provide important theoretical basis for the potential application offered by symmetrically perturbed photonic crystal in future optical networks.     

Effect of preliminary low-temperature annealing on the kinetics of nucleation

The changes observed in the kinetics of CuCl nucleation in glass due to preliminary low-temperature annealing have been investigated using optical spectroscopy. The influence of the number of nuclei formed at a temperature of 500°C on the growth rate of the CuCl phase at 650°C has been examined. The amount of the CuCl phase in the sample is determined from the optical absorption coefficient in the range of band-to-band transitions in the CuCl nanocrystals. The average radius of CuCl particles is calculated from the position of the maximum of the exciton band. It has been demonstrated that the preliminary formation of CuCl nuclei at 500°C for 3 h makes it possible to increase the growth rate of the CuCl phase by a factor of 6. In the sample with preliminarily formed nuclei, there occur two opposite processes: thermal decomposition of part of the small nuclei, which have become subcritical at 650°C; and the growth of larger nuclei, which have become supercritical at 650°C due to the diffusive inflow of the components of the new phase. When the equilibrium concentration is reached, the new phase contains particles with a smaller radius but at a higher concentration as compared to those in the case of the conventional nucleation at 650°C.