Flow overshooting in crossing flow of lattice gas

We study the lattice gas flow of two components of biased-random walkers at a crossing under a periodic boundary. The lattice gas mixture consists of two components of particles (walkers) in which one component of particles moves north and the other component of particles moves east. The current (flow) increases with ρ_x (density of the east-bound particles) at low density and displays overshooting at an intermediate density. The flow overshooting occurs only for a certain range of ρ_y (density of the north-bound particles). Then clogging occurs and the current saturates. Furthermore, when the density is high, the current decreases with increasing density. The overshooting shown in the current-density (fundamental) diagram is due to the formation of an unstable oscillating jam just before clogging occurs. It is shown that flow overshooting does not occur in unidirectional flow through a porous medium but occurs in unidirectional flow through a group of Brownian particles.     

Crystallization in a dense suspension of self-propelled particles

Using Brownian dynamics computer simulations, we show that a two-dimensional suspension of self-propelled ("active") colloidal particles crystallizes at sufficiently high densities. Compared to the equilibrium freezing of passive particles, the freezing density is both significantly shifted and depends on the structural or dynamical criterion employed. In nonequilibrium the transition is accompanied by pronounced structural heterogeneities. This leads to a transition region between liquid and solid in which the suspension is globally ordered but unordered liquidlike "bubbles" still persist.     

Entropic collapse transition of a polymer in a solvent with a nonadditive potential

We use molecular dynamics simulations to study an entropy-driven collapse transition of a flexible polymer in a solvent. Monomers and solvent particles interact with a steeply repulsive soft-sphere potential. We consider a nonadditive potential system in which the effective diameter describing the solvent-monomer interaction is greater than or equal to the diameters corresponding to the solvent-solvent and monomer-monomer interactions, which are set equal. We examine the effects of nonadditivity of the solvent-monomer potential and solvent density on the collapse transition. We find that a small degree of nonadditivity will drive the transition at sufficiently high solvent density. Increasing the density leads to a collapse transition at lower values of nonadditivity.     

温稠密钛电导率计算

温稠密物质是惯性约束核聚变、重离子聚变、Z箍缩动作过程中物质发展和存在的重要阶段. 其热力学性质和辐射输运参数在聚变实验和内爆驱动力学模拟过程中有至关重要的作用. 本文通过建立非理想Saha方程, 结合线性混合规则的理论方法模拟了温稠密钛从10^-5-10 g·cm^-3, 10⁴ K到3×10⁴ K区间的粒子组分分布和电导率随温度密度的变化, 其中粒子组分分布由非理想Saha方程求解得到. 线性混合规则模型计算温稠密钛的电导率时考虑了包括电子、原子和离子之间的多种相互作用. 钛的电导率的计算结果与已有的爆炸丝实验数据相符. 通过电导率随温度密度变化趋势判断, 钛在整个温度区间, 密度0.56 g·cm^-3时发生非金属相到金属相相变. 对于简并系数和耦合系数的计算分析, 钛等离子体在整个温度和密度区间逐渐从弱耦合、非简并状态过渡到强耦合部分简并态.     

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.     

Binary fluid mixture of hard ellipses: Integral equation and weighted density functional theory

We study a two-dimensional (2D) classical fluid mixture of hard convex shapes. The components of the mixture are two kinds of hard ellipses with different aspect ratios. Two different approaches are used to calculate the direct, pair and total correlation functions of this fluid and results are compared. We first use a formalism based on the weighted density functional theory (WDFT), introduced by Chamoux and Perera [Phys. Rev. E 58 (1998) 1933]. Second, in general the Percus-Yevick (PY) and the hypernetted chain (HNC) integral equations are solved numerically for the 2D fluid mixtures of hard noncircular particles. Explicit results are obtained for the fluid mixtures of hard ellipses and comparisons are made by the two approaches. Also, the results are compared with the recent Monte Carlo simulation for the one-component fluids of hard ellipses. Finally we obtained the equation of state of hard ellipses for the aspect ratio sufficiently close to 1 and compared our results with the simulations of the fluid mixtures of hard disks.     

Excitation of gap discrete breathers in an A<Subscript>3</Subscript>B crystal with a flux of particles

The generation of discrete breathers in an A₃B crystal has been modeled by the method of molecular dynamics using Pt₃Al as an example via the application of random unidirectional momenta, which simulate the action of a particle flux, to atoms. Two possible mechanisms of the excitation of gap discrete breathers with a soft type of nonlinearity have been revealed depending on the energy of particles in a flux. If a particle is able to transfer energy of more than 1.4 eV to the Al atom, a discrete breather can be excited by the only particle. Otherwise, a discrete breather is formed upon numerous particle–Al atom collisions, which are possible only at a sufficiently high density of particles, as each following particle must transfer its momentum to the Al atom before its oscillations provoked by previous particles attenuate.     

On the nonequilibrium statistical mechanics of a binary mixture. I. The distribution functions

In this paper, we obtain the generalization of the BBGKY hierarchy for a binary mixture of chemically neutral particles. Using modified boundary conditions different from the ones proposed by Bogoliubov, we solve the hierarchy, and obtain explicitly the set of two-particle distribution functions for the several species of the mixture, up to first order in the density.     

Optimal lattice configurations for interacting spatially extended particles

We investigate lattice energies for radially symmetric, spatially extended particles interacting via a radial potential and arranged on the sites of a two-dimensional Bravais lattice. We show the global minimality of the triangular lattice among Bravais lattices of fixed density in two cases: In the first case, the distribution of mass is sufficiently concentrated around the lattice points, and the mass concentration depends on the density we have fixed. In the second case, both interacting potential and density of the distribution of mass are described by completely monotone functions in which case the optimality holds at any fixed density.     

垂直振动颗粒混合气体的振荡现象研究

运用事件驱动算法研究颗粒混合物在垂直振动容器中的振荡现象.容器被具有一定高度的隔板分成相等大小的两个小室,并采用半径相差一倍的两种颗粒.研究结果表明,颗粒振荡周期随两种颗粒密度比的减少而急剧增加.通过计算在垂直方向上两种颗粒高度之比随颗粒密度的变化关系,说明决定颗粒振荡与否的主要因素并不是"巴西坚果效应"或"反巴西坚果效应".通过计算颗粒温度,发现颗粒振荡取决于颗粒混合气体的小颗粒温度.当小颗粒温度大于一定值时,颗粒混合气体发生颗粒振荡现象.根据Viridi等提出的流体动力学模型,文中对该模型做出相应的修改,加入密度因子,从而可以解释颗粒振荡周期与颗粒密度比的关系.     

On the dynamics of cosmological particle production

The time evolution of the particle number density and spectrum of massive scalar particles, coupled conformally to a classical Friedman-Robertson-Walker gravitational field is followed numerically. Not only for a pure radiation cosmos, but also for one with an inflationary interlude, the density of particles produced remains constant in time. This results in a constant equivalent temperature scale up to the Compton time of the massive particles, and opens the possibility that during a sufficiently long inflation the energy density of the particles produced can exceed that of the radiation background.     

On Possible Sources of Heavy Neutral Particles in Cosmic Rays

We explore some possible mechanisms by which the heavy neutral cosmic particles are accelerated to gain sufficiently high kinetic energies and cause observable effects at detectors on the earth. Our results indicate that all the known sources which are based on the established theories and observations fail to give rise to very high energy with sizable flux intensity. If the heavy neutral particles of kinetic energies of GeV order indeed exist in cosmic rays, there must be certain mechanisms beyond our present knowledge of dark matter and universe evolution.     

Co–CoO nanoparticles prepared by reactive gas-phase aggregation

The technique of gas-phase aggregation has been used to prepare partially oxidized Co nanoparticles films by allowing a controlled flow of oxygen gas into the aggregation zone. This method differs from those previously reported, that is, the passivation of a beam of preformed particles in a secondary chamber and the conventional (low Ar pressure) reactive sputtering of Co to produce Co–CoO composite films. Transmission electron microscopy shows that the mean size of the particles is about 6 nm. For sufficiently high oxygen pressures, the nanoparticles films become super-paramagnetic at room temperature. X-ray diffraction patterns display reflections corresponding to fcc Co and fcc CoO phases, with an increasing dominance of the latter upon increasing the oxygen pressure in the aggregation zone, which is consistent with the observed reduction in saturation magnetization. The cluster films assembled with particles grown under oxygen in the condensation zone exhibit exchange-bias fields (about 8 kOe at 20 K) systematically higher than those measured for Co–CoO core-shell nanoparticles prepared by oxidizing preformed particles in the deposition chamber, which we attribute, in the light of results from annealing experiments, to a higher ferromagnetic–antiferromagnetic (Co–CoO) interface density.     

少量氧气对大气压氩氧混合气体放电特性的影响

基于一维流体模型,采用数值模拟的方法研究了大气压氩氧混合气体放电中少量氧气含量对放电特性的影响。根据模型方程进行了数值模拟分析,计算结果表明：随着氧气含量由0.1%增加到0.6%,电子密度减小,O^-离子密度增加,总的负粒子密度增加,鞘层区电子温度增加。随着放电时间的增加,O^-离子密度增加,电子密度的变化分为三个阶段：电子密度快速增长阶段、电子密度下降阶段和电子密度稳定阶段。当氧气含量小于1%时,电子密度随着氧气含量的增加迅速减小,氧原子密度快速增加;氧气含量大于1%小于4%时,电子密度随氧气含量的增加而缓慢减少,氧原子密度缓慢增加直至不变。     

Surface treatment of CaCO3 with a mixture of amino- and mercapto-functional silane coupling agents and tensile properties of the rubber composites

In order to reinforce the composite consisting of isoprene rubber (IR) and calcium carbonate (CaCO₃) particles, the surface treatment of CaCO₃ particles with a mixture of amino- and mercapto-functional silane coupling agents was investigated. The quantity of chemisorbed silanes in treated CaCO₃ measured using thermogravimetry was greater for amino- than for mercapto-silane and for the tri- than for the dialkoxy structure. Second, the molecular mobility of polycondensate of the mixtures with the trialkoxy structure measured using ¹H pulse nuclear magnetic resonance had the least molecular mobility, i.e., formed the highest density network. The greater values of stress at 500% strain, fracture stress, and elongation at break were determined for the treatment with amino- and mercapto-functional silanes having a trialkoxy structure from the stress-strain curves of composite. The mixture treatment with dialkoxy structure and with amino- or mercapto-functional silane only did not improve the mechanical properties sufficiently. Interactions between the amino group and the CaCO₃ surface, covalent bonding between the mercapto group and the IR, and high density network formation of trialkoxy silane were important for improving the mechanical properties of the composite.     

Flame structure and burning velocity of flames propagating in binary iron aerosols

A numerical study was performed for binary dispersed iron aerosols in air using different particle sizes with constant average particle size. The effects of particle size and density of the two aerosols on flame structure and speed are systematically investigated. Varying the amount of small and big particles results in separated and overlapped flame fronts. For higher values of particle size ratio (ratio between the size of big and small particles) and density of small particles, flame fronts are observed to overlap. The flame speed of the binary mixture is compared with the mono-dispersed case and the difference is analyzed for different particle size ratios. The addition of a small fraction of small particles in the binary mixture is found to result in a substantial increase in the flame speed if the particle size ratio is large. Detailed analyses on the variation of the total amount of fuel shows the particle size ratio determines the equivalence ratio at which the maximum flame speed occurs. The maximum flame speed as a function of equivalence ratio was observed to move from the lean to the rich side for particle size ratio sufficiently large enough.     

Spin and torsion in the very early universe

In the very early universe with temperature T between 10²⁴ K and 10³² K the gravitational effect of torsion is dominant if particles with spin are sufficiently polarized. The source of the torsion is the spin density and the latter is usually described by a classical theory of Weyssenhoff and Raabe. In this article the spinning particles are described quantum mechanically, i.e. with a Dirac field and the spin density is defined as the source of the torsion. The macroscopic average of the spin density is obtained by the relativistic Wigner function formalism. The expression of the spin density, as derived in this article, is different from the classical one, except when both are zero.     

Synthesis of single-walled carbon nanotubes in oxy-fuel inverse diffusion flames with online diagnostics

A novel technique for synthesis of single-walled carbon nanotubes (SWNTs) in diffusion flames is presented, as is a diagnostic tool that can provide online information about nanotube size, number density, and purity. An inverse diffusion flame with a high stoichiometric mixture fraction (Z_st) is used to produce SWNTs with an average length of 1 μm. The high Z_st flame allows nanotubes to be produced in a fuel-rich region that is void of soot and polycyclic aromatic hydrocarbons (PAH). In addition, by operating as an inverse diffusion flame the carbon nanotubes (CNTs) are not exposed to oxygen and thus, can be collected downstream. Consequently, this flame provides a potential approach to large-scale synthesis of pure SWNTs. In addition, a differential mobility analyzer (DMA) is employed as an online diagnostic tool. The DMA can distinguish between excess catalyst particles and CNTs due to the differences in their electrical mobilities. Thus, the presence of CNTs as well as their size, number density, and purity relative to excess catalyst particles can be identified from the size distribution of the aerosol sampled downstream of the flame. This tool allows for rapid identification of the effect of changing process variables on nanotube growth and thus, the production process can be quickly optimized.     

High temperature superfluidity with abnormal fermionic occupancy

We examine a Lipkin based two-level pairing model at finite temperature and in the thermodynamic limit. Whereas at T=0 the model exhibits a superconducting ground state for sufficiently high values of the coupling constant, a partially superconducting phase in whichsome of the particles are paired, is found to survive at high temperatures in a special treatment. This phase is a mixture of “abnormally-occupied” eigenstates, which lie at higher energy, of the interactionless model Hamiltonian.     

Energy concentration in electric discharges between particles of semiconducting sulfide minerals under the action of high-power nanosecond pulses

A model for development of electric discharges between particles of sulfide minerals (pyrite) under the action of high-voltage nanosecond pulses is proposed. It is shown that through discharges in a layer of pyrite particles lead to energy concentration in small contact regions between particles; the concentrated energy is sufficiently high for local decomposition (disintegration) of mineral complexes.