Nonequilibrium accumulation of surface species and triboelectric charging in single component particulate systems

Triboelectric charging occurs in granular systems composed of chemically identical particles even though there is no apparent driving force for charge transfer. We show that such charging can result from nonequilibrium dynamics in which collision-induced electron transfer generates electron accumulation on a particle-size-dependent subset of the system. This idea rationalizes experimental results that suggest that smaller particles charge negatively while the large ones charge positively. This effect occurs generally when there are high energy electrons on a surface that cannot equilibrate to lower energy states on the same surface, but can transfer to lower energy states on other particles during collisions.     

Effect of particle size distribution on the polarity of triboelectric charging in granular insulator systems

Triboelectric charging occurs in granular insulating systems even when all particles are composed of identical material. A simple model is used here to address triboelectric charging in such systems. The basis of the model is the existence of electrons trapped in high-energy states, which can be released during collisions with another particle and transferred to the other particle. This model shows that triboelectric charging in insulator systems composed of particles of identical material can be attributed to a distribution of particle sizes, such that smaller particles tend to charge negatively and larger particles tend to charge positively. This polarity of charging has been observed in field studies of sand storms, dust devils and volcanic plumes, and most laboratory experiments on triboelectric charging in granular systems.     

Simulating self-organized molecular patterns using interaction-site models

The triboelectric charging of collision particles is essential to understand sand electrification in wind-blown sand fluxes. The physical model of electron trapped in high-energy states has been proposed to explain the triboelectric charging between identical insulating granular materials. In this study we propose an improved triboelectric charging model which combines the soft sphere model and the trapped electron model to calculate the net charge transfer during particles' collisions. Based on our charging model, we investigate the sand electrification of wind-blown sand, such as the charge flux varying with height, the charge-to-mass ratio of wind-blown sand, and the equilibrium time that the charge takes to approach a stable state. Numerical simulation results of the averaged charge-to-mass ratio in wind-blown sand fluxes are in good agreement with the experimental data.     

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.     

工业中粉体颗粒的荷电机理及数值模拟方法

工业过程中粉体颗粒不可避免地会相互摩擦碰撞而荷电. 荷电颗粒的存在可能会危害正常的工业生产过程, 也可能对工业过程起促进作用. 因此, 荷电粉体颗粒及其特性受到了广泛的关注, 但目前对粉体颗粒的荷电机理依然缺乏透彻的了解, 尤其是在气固两相流动中的粉体颗粒荷电现象. 事实上, 工业中存在的粉体颗粒的运动都受到流体的影响, 是典型的气固两相流系统, 流体对粉体颗粒的作用使粉体颗粒接触的荷电现象变得更为复杂, 因此从两相流动的观点来研究粉体颗粒荷电的物理本质就显得越来越重要. 本文介绍了工业过程中的几种不同类型的粉体颗粒荷电行为, 回顾了颗粒的荷电机理与描述颗粒荷电的数学模型. 对于工业过程中颗粒的荷电现象及颗粒在多相流体中的动力学行为, 介绍了研究颗粒受流体影响时荷电特性的数值模拟方法. 本文旨在对粉体颗粒的荷电机理、应用以及研究方法进行梳理与探讨, 为正确认识工业过程中粉体颗粒的荷电现象并加以控制利用提供理论借鉴.     

Methodology for studying particle–particle triboelectrification in granular materials

A critical challenge for experimental studies of triboelectric charging is to generate reproducible and unambiguous data that can be linked to theoretical concepts. We have developed a methodology to investigate the triboelectric charging of granular materials due solely to particle–particle interactions (i.e. no particle–wall interactions). The methodology is based on a particle flow apparatus that generates a fountain-like flow in which the particles contact only other particles, but no equipment surfaces. Non-contact methods of measuring charge and separating particles by charge are employed so that probe-particle charging does not occur.     

Experimental determination of dust-particle charge in a discharge plasma at elevated pressures

The charge of dust particles is determined experimentally in a bulk dc discharge plasma in the pressure range 20-100 Pa. The charge is obtained by two independent methods: one based on an analysis of the particle motion in a stable particle flow and another on an analysis of the transition of the flow to an unstable regime. Molecular-dynamics simulations of the particle charging for conditions similar to those of the experiment are also performed. The results of both experimental methods and the simulations demonstrate good agreement. The charge obtained is several times smaller than predicted by the collisionless orbital motion theory, and thus the results serve as an experimental indication that ion-neutral collisions significantly affect particle charging.     

Effect of finite charging time on particulates in RF discharges

The effect of the finite charging time on spherical and cylindrical particulates in an RF discharge is considered. Using probe theory, analytical expressions for the charging rate under various conditions are derived and compared with numerical solutions. Scaling of the charging time with discharge parameters, in particular the electron temperature, is demonstrated. Using a one-dimensional fluid model for an rf discharge, the equilibrium electric and ion drag forces are compared for spherical and cylindrical particulates. The effect of the finite charging time on the dynamics of particulates of various sizes in the model discharge are discussed. Overall, a long cylindrical particle with the same mass as a spherical particle charges up more slowly to a larger net (negative) charge and has a significantly larger polarization. As a consequence, small cylindrical particles introduced near the electrodes are more likely to escape from the sheath region before becoming fully charged     

The collision of particles in granular systems

Collisions between granular particles are irreversible processes which cause dissipation of mechanical energy by fragmentation or heating of the colliders. The knowledge of these phenomena is essential for the understanding of the behaviour of complex systems of granular particles. We have developed a model for inelastic collisions of granular particles and calculated the velocity restitution coefficients, which describe all possible collisions in the system. The knowledge of these coefficients allows for event-driven many-particle simulations which cannot be performed in the frame of molecular dynamics. This approach has the advantage that very large particle numbers can be treated which are necessary for the understanding of intrinsic large-scale phenomena in granular systems.     

Charging mechanisms of trapped element-selectively excited nanoparticles exposed to soft x rays

Charging mechanisms of trapped, element-selectively excited free SiO2 nanoparticles by soft x rays are reported. The absolute charge state of the particles is measured and the electron emission probability is derived. Changes in electron emission processes as a function of photon energy and particle charge are obtained from the charging current. This allows us to distinguish contributions from primary photoelectrons, Auger electrons, and secondary electrons. Processes leading to no change in charge state after absorption of x-ray photons are identified. O 1s-excited SiO2 particles of low charge state indicate that the charging current follows the inner-shell absorption. In contrast, highly charged SiO2 nanoparticles are efficiently charged by resonant Auger processes, whereas direct photoemission and normal Auger processes do not contribute to changes in particle charge. These results are discussed in terms of an electrostatic model.     

Effect of particle size in fluidization of polyethylene particle mixtures on the extent of bed electrification and wall coating

In this work the effects of polyethylene fluidizing particle size (smaller than 400 μm) on the degree of fluidized bed electrification and wall coating formation was studied. Experiments were conducted in a stainless steel, 0.15 m diameter column, under ambient conditions. Polyethylene resin as received (20–1500 μm) as well as mono-sized and binary mixture of large (600–710 μm) and small (212–300 & 300–425 μm) polyethylene particles were fluidized while their mass, net specific charge and size distribution in the bulk of the bed and the wall coating were measured. For the binary mixture the fraction of the small particles examined were 5%–10% and 20%. The extent of wall coating varied between different cases tested with the mono-sized large particles resulting in the most amount coating. It was found that as the fraction of the small particles in the binary mixture was increased, these particles formed majority of the wall coating. At the mass fraction of 20%, the extent of wall coating and its net specific charge were similar to that of resin as received. Overall results implied that the magnitude of the smaller sized particles within the resin played an important role in the degree of particles electrostatic charging and the extent of the particles adhesion to the column wall. Small particles were found to generate a much larger net specific charge which although resulted in them coating the column wall but prevented the coating layer growth.     

Effects of Continuous Size Distributions on Pressures of Granular Gases

Direct Monte Carlo simulations are employed to investigate the granular pressures in granular materials with a power-law particle size distribution. Specifically, smooth circular discs of uniform material density are engaged in a two-dimensional rectangular box, colliding inelastically with each other and driven by a homogeneous heat bath at zero gravity. The resulting pressures are found to decrease as the widths of particle size distribution are increased. Moreover, the granular pressures in power-law systems are found to be unequally distributed among the various sizes of particles, with large particles possessing more pressure than their smaller counterparts. The width-dependent nature of the total pressures is induced by the more dispersion of smaller particles in the system as the particle size distribution is widened.     

A Novel Method for Inhalation Control of Workplace Anthropogenic Pollutant Particles

This in vitro study investigated electrically charging effect on the deposition of inhaled workplace anthropogenic pollutant particles (APP) in a hollow throat cast model. Many occupational lung diseases are associated with exposure to workplace dust particles and other pollutants. Since the human throat is an effective filter, this study devised a novel idea of charging particles, and studying their deposition in the throat. Simulated workplace aerosol particles were generated from a commercially available nebulizer, and charged by a corona charger. Charged and uncharged particles were allowed to pass through a polyester resin cast of cadaver based throat, a replicate of a human oropharyngeal region. The aerosol particles' size and charge distribution were characterized by an Electronic Single Particle Aerodynamic Relaxation Time (ESPART) analyzer before and after passing the throat cast. The ESPART operates on the principle of Laser Doppler Velocimetry to measure simultaneously aerodynamic diameter and electrostatic charge on a single particle basis and in real time. The study results revealed that electrically charging increased agglomeration of smaller particles and increased deposition. Deposition of charged particles increased with increasing particle size which can be explained as the effect of inertial impaction.     

Event-driven simulations of insulating grains in an external electric field

In this work we employ event-driven particle dynamics simulations for a system of spherical insulating grains interacting with an external electric field. This system resembles the electrostatic particle separation present on some industrial processes. Here, the particles collide inelastically with each other and with the container walls, for a constant normal and tangential restitution coefficients. During the collisions, the grains can acquire electric charge due to triboelectric contact charging, since two different species of insulating particles are mixed. Particle-particle electric interactions are not considered. Grains are also subjected to the gravitational field and rotation, and are confined in a cubic box with thermal walls in order to prevent the static equilibrium state. We calculate the mass and charge density profile, and the particle charge distribution for different values of the electric field and temperature of the walls. The particle charge distribution and the effect of particle sizes on the separation process were also investigated.     

Coagulation of charged particles in a dusty plasma

An investigation is made of characteristic features in the behavior of small particles in a dusty plasma attributable partly to the suppression of coagulation as a result of monopolar charging for particle sizes smaller than the Debye shielding length and partly to the reduction in the effect of charging for larger particles. Similarity relations linking the plasma composition and particle charge with the parameters of the dust component are used to determine the range of parameters for which the linear approximation of the particle charge as a function of their sizes holds. A modified classical theory of coagulation in the diffusion approximation is used to study some anomalies in the behavior of the particle size distribution. It is established that unlike an ordinary aerosol, in a dusty plasma the dispersion of the distribution and the average particle size may decrease with time. It is shown for the first time that a long-lived “quasi-liquid” state of a dusty plasma may be established as a result of the anomalous behavior of the size distribution function of coagulating charged particles.     

Parametric channeling and collapse of beams of charged particles with internal structure in crystals: 2. Parametric processes during channeling of atomic ions, nuclei, and relativistic electrons in crystals

Features of parametric effects during channeling of atomic ions, nuclei, and relativistic electrons (positrons) in crystals were considered. It was shown that parametric coupling between ion channeling states in the field of crystal axes and planes and electronic states in the ion volume leads to the possibility of “parametric collapse” of the beam, i.e., a decrease in the oscillation amplitude of the atomic ion in the channel due to periodic transfer of the ion oscillation energy to the inner electron of the atom. The same effect can be used to cool beams due to energy transfer to intrinsic nuclear states with low energy levels. It was shown that parametric cooling of beams with a decrease in the transverse energy can also occur during axial channeling of relativistic electron beams. This process results from the parametric coupling between channeling states, which are caused by the particle charge and electron spin states in an effective magnetic field induced in the moving coordinate system.     

Comparison of influence of fluidization time on electrostatic charge build-up in the bubbling vs. slugging flow regimes in gas–solid fluidized beds

Electrostatic charge generation poses significant problems in some commercial gas–solid fluidized bed reactors such as those in gas-phase polyolefin production. Understanding the contributing factors to charge generation is important in determining the charge generation mechanisms, leading to the development of methods to reduce or prevent this phenomenon. This work focused on determining the effect of fluidization time on particle charging and the amount of particle adhesion on the fluidization column wall in both the bubbling and slugging flow regimes. The charging effect was investigated for particles in three regions of the fluidized bed: elutriated fines, bulk particles inside the bed, and particles adhered to the column wall. The particles size distribution, mass and charge were measured for all three regions. Fluidization was carried out with polyethylene resins from an industrial reactor; times of 15, 30, 60, 120, 180, and 360 min were evaluated. Increased fluidization time decreased the amount of particles mass collected in the bulk region and increased those adhered to the column wall during the velocities tested in the bubbling flow regime. Whereas the quantity of particles in each region was not affected by fluidization time for the velocities examined in the slugging flow regime. Bipolar charging was observed with relatively smaller particles becoming predominately positively charged and larger particles becoming predominately negatively charged. Each region of the bed affected the magnitude of net q/m, with elutriated fines having the largest magnitude, followed by those adhered to the column wall, and finally those in the bulk of the bed. Charge saturation was attained for fluidization times greater than 60 min for particles in the bulk and along the column wall for all gas velocities. However, extended fluidization times were required with the entrained fines in bubbling flow; whereas charge saturation of fines in slugging flow occurred shortly after the onset of fluidization. Mean particle diameter for each measurement region was not impacted by the fluidization time for any of the gas velocities tested. The bed hydrodynamics was found to definitely have an impact on the particle–wall fouling where the particle layer continued to develop on the inner column wall as fluidization time increased for those velocities in the bubbling regime while comparatively less impact on particle layer growth was observed in the slugging flow regime. In addition, the bubbling flow regime resulted in particle layers formed on the column wall to be longer and thinner whereas those formed in the slugging flow regime were shorter and thicker.     

Spacecraft charging potential during electron-beam injections intospace plasmas

Injections of nonrelativistic electron beams from an infinite conductor have been simulated by using a two-dimensional electrostatic particle code to study the spacecraft charging potential. The simulations show that the conductor charging potential at the end of simulations does not vary with the beam density when the beam density exceeds four times the ambient density. The reflection coefficient, which determines a percentage of incident electrons reflected by the conductor, increases the charging potential. To charge the conductor to the beam energy, the reflection coefficient needs to be about 0.5. The results are applied to explain the spacecraft charging potential measured during the SEPAC (Space Experiments with Particle Accelerators) experiments from Spacelab 1     

Effect of Cluster Coulomb Fields on Electron Acceleration in Laser-Cluster Interaction

Single particle simulations are used to investigate electron acceleration in the laser-cluster interaction, taking into account the Coulomb fields around individual clusters. These Coulomb fields are induced from the cluster cores with positive charge when electrons escape from the cluster cores through ponderomotive push from the laser field. These Coulomb fields enable some stripped electrons to be stochastically in phases with the laser fields so that they can gain net energy from the laser efficiently. In this heating mechanism, circularly polarized lasers, larger cluster size and higher cluster densities make the acceleration more efficient.