Induced-charge electrophoretic motion of ideally polarizable particles

The induced-charge electrophoretic (ICEP) motion of ideally polarizable particles is numerically studied in this paper. A complete three-dimensional multi-physics model is set up to simulate the transient ICEP motion of ideally polarizable, spherical particles in an unbounded liquid. The study shows the nonlinear induced zeta potential on the particle's surface causing a varying slipping (electroosmotic flow) velocity along the particle's surface, and hence producing microvortexes in the liquid. ICEP particle-particle interactions are also studied. The simulations show that a low pressure zone between the two polarizable conducting particles will be induced if the external electric field is applied parallel along the imaginary line connecting the two particles, resulting in an attracting effect between the two particles. Oppositely, a high pressure zone is induced between the two particles if the applied field is perpendicular to the imaginary line connecting the two particles, giving a repelling effect. The ICEP attracting or repelling effects depend on the particles’ separation distance, the electric field strength and the particle size.     

Electrorheological suspensions of two polarizable particles

Bi-disperse Electrorheological (ER) suspensions of two polarizable particles of the same size are investigated to understand the ER behavior of poly-disperse suspensions composed of various polarizable particles. The electrostatic polarization model is employed to describe ER suspensions, and solutions to the equation of motion are obtained by dynamic simulation. Even with the applied electric field, metastable structures and sheared configurations at a shear rate of 0.01 and 10 s^-1 show no inhomogeneous higher polarizable particle distributions (no higher polarizable particle cluster formation) regardless of the ratio of the two types of particles. The shear stress increases with the increase of the higher polarizable particle concentration both in the electrostatic force and hydrodynamic force dominant regions.     

Microscopic aspects of the deposition of nanoparticles from the gas phase

The deposition process in a homogeneous electric field, and the subsequent microscopic arrangement of charged, metallic nanoparticles in the size range below $\text{100}\text{nm}$ on flat substrate surfaces is described. The main aspect of the investigation is the transfer of the particles from a three-dimensional distribution in the gas phase into their arrangement on the substrate surface, in dependence on particle–particle interactions and on Brownian motion. For this purpose, a trajectory model has been developed, which takes into account the flow field above the substrate surface, the electric field, the interactions of incoming particles with the substrate surface and with already deposited particles, as well as Brownian motion. The results from the trajectory calculations are compared with experimental results, obtained by scanning electron microscopy investigations of deposition patterns, created by deposition of indium and gold nanoparticles in an electrostatic precipitator. The particle diameter, the particle charge, the substrate material, the electric field strength and the number of particles deposited per unit area have been varied.     

Features of micro and nano-particles produced by pulsed arc submerged in ethanol

Ni-C powders were synthesized using a pulsed arc between Ni electrodes submerged in pure ethanol. The arc pulses with energies 7.7, 22.7 or 48 mJ and 20 μs duration, and a repetition rate of 100 Hz were applied for 5 min. Powder samples were obtained by extracting liquid from the treatment vessel after a pre-determined sedimentation time, or by collecting the residue after the liquid evaporated. The particles size distribution was studied using a High Resolution Transmission Electron Microscope (HRTEM). The particle diameters after 25 min sedimentation time were in the range of 3-30 nm, and the distribution depended weakly on the discharge energy. Effects of applied electric and magnetic fields on the particle motion in ethanol were also studied. The particles moved in an applied electric field with velocities increasing with the field strength and the particle diameter. The as-produced particles with diameter ∼80 μm had velocities ∼2-2.5 mm/s in an electric field of ∼800 V/cm. The powder had soft ferromagnetic properties at room temperature, and the particles in the liquid moved in the direction of increasing magnetic field.     

Numerical simulation of microscopic motion and deposition of nanoparticles via electrodynamic focusing

A computational model was developed to simulate microscopic motion and deposition of charged aerosols during the nanoparticle patterning process utilizing electrodynamic focusing concept (Kim et al., 2006). Our computational model includes Brownian random force, Coulomb and image forces, fluid drag and van der Waals force for determining Lagrangian particle trajectories after solving electrostatic fields in the deposition chamber. Our results are in agreement with the previous experimental findings. The effects of operation parameters such as surface charge density, applied voltage and particle charges were investigated. It was found that the electric field-induced motion of particles dominated over Brownian random motion of 10 nm nanoparticles near the surface and the inertial motion of charged nanoparticles under high electric field would be important to determine the precise deposition pattern within submicrometer scale structures.     

A phase field model of pressure-assisted sintering

The incorporation of an efficient contact mechanics algorithm into a phase field sintering model is presented. Contact stresses on the surface of arbitrarily shaped interacting bodies are evaluated and built into the model as an elastic strain energy field. Energy relaxation through deformation is achieved by diffusive fluxes along stress gradients and rigid body motion of the deforming particles maintain contact between the particles. The proposed model is suitable for diffusion deformation mechanisms occurring at stresses below the yield strength of a defect-free material; this includes Nabarro-Herring creep, Coble creep and pressure-solution. The effect of applied pressure on the high pressure-high temperature (HPHT) liquid phase sintering of diamond particles was investigated. Changes in neck size, particle coordination and contact flattening were observed. Densification rates due to the externally applied loads were found to be in good agreement with a new theory which implicitly incorporates the effect of applied external pressure.     

Combination of the direct-forcing fictitious domain method and the sharp interface method for the three-dimensional dielectrophoresis of particles

In this paper, we combine the direct-forcing fictitious domain (DF/FD) method and the sharp interface method (SI) to resolve the problem of particle dielectrophoresis in three dimensions. The flow field is solved with the DF/FD method. The electric field governed by a Laplace equation with a jump coefficient across the particle surface is solved with the sharp interface (SI) method, and the dielectrophoretic force on particles is then calculated with the Maxwell stress tensor (MST) method. The main feature of our method is that both hydrodynamic and dielectrophoretic forces on particles are calculated with the interface-resolved methods instead of the point-particle model. The accuracy of the SI/MST method for the dielectrophoretic force without the consideration of the flow is validated via two problems: the electrostatic force on the particle in a non-uniform electric field, and the electrostatic force between two particles. The capability of the proposed DF/FD-SI/MST method is demonstrated with two numerical examples: the aggregation of particles due to the conventional dielectrophoretic force, and the motion of particles due to both conventional and traveling wave dielectrophoretic forces.     

Multiscale Lattice–Boltzmann Approach for Electrophoretic Particle Deposition

In this work, a gas-particle flow over a structured sensor surface is numerically investigated. A system of parallel electrodes with an applied voltage is distributed on top of a nonconducting flat surface. The considered submicron particles (size range 25–200 nm) are electrically charged. The simulation takes into account the interaction between particle motion, fluid flow and electrical field causing the particles to deposit on the surface. As a result, dendrite microstructures of particles start growing on the electrode surface. To model these effects in detail the numerical simulations are carried out on a mesh with very high resolution of up to Δx = 0.5 μm. The fluid-flow is calculated with the Lattice–Boltzmann method incorporating automatic local grid refinement. The Laplacian equation describing the electrical field is solved by a finite-difference-scheme. The particle movement is calculated by the Lagrangian point-particle approach, accounting for drag force, Brownian motion, and Coulomb forces. Results of particle transport and dynamics of particle deposition are presented for different applied voltage, electrode configurations, flow velocities, and particle sizes.     

Bipolar Electrochemistry,a Focal Point of Future Research

Bipolar electrochemistry is a conventional method based on the polarization of an isolated substrate under an applied electric field. This technique has been applied to electrolysis, corrosion, and other areas of chemical engineering since the twentieth century. However, it has been recognized as a powerful tool in many modern domains after water splitting has been demonstrated to be possible using micrometer-sized bipolar electrodes. Modifying inorganic objects in novel ways, such as creating electrical contacts between metal particles using directed electrochemical growth or shaping and exploring the micro- and nanoworld are some of the new applications in this field. Fabrication of electronic devices, electroanalytical purposes, generation of molecular and material gradients, functionalization of single micro- and nanopores, synthesis of Janus particles, design of swimmers, and asymmetric modification of nanoparticles will be discussed in this article as a focal point of future research in bipolar electrochemistry.     

A Numerical Simulation of the Boycott Effect

The lattice Boltzmann method has been used to simulate the velocity field induced and the motion of an ensemble of particles during the sedimentation process in inclined tubes. The simulations show the trajectories and flow behavior of individual particles and particle-particle and particle-wall interactions as well as the formation of particle clusters. The global convection motion that was experimentally observed during such processes and tends to enhance the sedimentation process is also reproduced numerically. In addition we have found that smaller intermittent vortices, formed from the wakes of groups of settling particles, also play an important role in the sedimentation process and the final distribution of particles.     

A Numerical Simulation of the Boycott Effect

The lattice Boltzmann method has been used to simulate the velocity field induced and the motion of an ensemble of particles during the sedimentation process in inclined tubes. The simulations show the trajectories and flow behavior of individual particles and particle-particle and particle-wall interactions as well as the formation of particle clusters. The global convection motion that was experimentally observed during such processes and tends to enhance the sedimentation process is also reproduced numerically. In addition we have found that smaller intermittent vortices, formed from the wakes of groups of settling particles, also play an important role in the sedimentation process and the final distribution of particles.     

Interactions between Janus particles and membranes

Understanding how nanoparticles interact with cell membranes is of great importance in drug/gene delivery. In this paper, we investigate the interactions between Janus particles and membranes by using dissipative particle dynamics, and find that there exist two different modes (i.e., insertion and engulfment) in the Janus particle-membrane interactions. The initial orientation and properties of Janus particles have an important impact on the interactions. When the hydrophilic part of the particle is close to the membrane or the particle has a larger section area and higher hydrophilic coverage, the particle is more likely to be engulfed by the membrane. We also provide insights into the interactions between Janus particles and membranes containing lipid rafts, and find that a Janus particle could easily detach from a membrane after it is engulfed by the raft. The present study suggests a potential way to translocate Janus particles through membranes, which may give some significant suggestions on future nanoparticle design for drug delivery.     

Detachment of rough particles with electrostatic attraction from surfaces in turbulent flows

The detachment of particles with coarse and fine roughnesses from surfaces in a turbulent boundary layer flow including electrostatic effects is studied. It is assumed that the real area of contact is determined by elastic deformation of asperities, and the effect of topographic properties of surfaces is included. The Johnson-Kendall-Roberts (JKR) adhesion model is used for analyzing the behavior of individual asperities. For an average Boltzmann charge distribution, the saturation charge condition as well as a fixed charge per unit mass, the Coulomb, the image, the dielectrophoretic, and the polarization forces acting on the particle in the presence of an imposed electric field are evaluated. The theories of rolling and sliding detachment are used to study the onset of removal of bumpy particles and those with fine roughness from plane surfaces. The hydrodynamic forces and torques acting on the particle attached to a wall, along with the adhesion force for the particle, are used in the model development. The minimum critical shear velocities needed to detach particles of different sizes from plane surfaces in the presence of an applied electric field are evaluated and discussed. The electric detachment of the particles is also studied and the field strength needed for particle removal is determined. It is shown that the surface charge distribution significantly affects the removal of particles from surfaces.     

Charged Particle Trajectory Statistics and Deposition in a Turbulent Channel Flow

The statistical properties of charged particles and their wall deposition in a turbulent channel flow in the presence of an electrostatic field is studied in this paper. For a dilute concentration, the influence of small particles on the fluid motion is neglected. The instantaneous velocity field is generated by a direct numerical simulation of the Navier-Stokes equation via a pseudospectral method. The case in which each particle carries a single unit of charge and the case in which the particles have a saturation charge distribution are analyzed. Ensembles of 8192 particle trajectories are used for evaluating various statistics. Effects of size and electric field intensity on particle trajectory statistics and wall deposition rate are studied. RMS particle velocities and particle concentrations at different distances from the wall are evaluated and discussed. The results for deposition rates are compared with those obtained from empirical equations.     

Simulation of particle deposition on filter fiber in an external electric field

Particle deposition onto a filter fiber was numerically simulated when a uniform external electric field was applied. The effects of electric field strength, particle inertia, and electrical conductivity of particles on particle deposition characteristics such as particle loading patterns and collection efficiency were qualitatively investigated. As a result, the electrostatic forces between a newly introduced particle and the already captured particles on the fiber were found to have a great influence on the particle deposition patterns compared with the results where the electrostatic forces were neglected. Conductive particles and filter fibers lead to higher collection efficiency and more linear structure of particle deposits than those of dielectrics, and the particle inertia could also be more important to the collection efficiency of a fibrous filter when electric fields are present. The simulated particle deposits obtained from this work agreed well with the existing experimental results, in which the photographs of particle loaded fibers, within an external electric field, were reported.     

电场控制火焰中细颗粒生成及分布的研究进展

以碳黑为主的燃烧源可吸入颗粒物的形成与控制是关系地球环境及人类健康的重要问题.目前对该课题的研究主要集中在颗粒生成后的控制方法上,而对如何在燃烧源内部控制颗粒物的生成尚处在初步探索阶段.介绍了电场对火焰中细颗粒控制的影响因素,对国内外研究现状进行了评述,并从生成、团聚和污染物协同控制三个方面总结了以往实验研究的成果.研究表明,在电场作用下,碳黑颗粒的质量浓度最高可减小90%;而在电场和催化金属的作用下,则可生成结构紧凑的碳纳米管束.最后提出了进一步研究的重点和方向.     

UNSTEADY MOTION OF A SPHERE IN A NEWTONIAN FLUID: II. FORCE BOUNDARY CONDITIONS

A general framework is developed to determine the time-dependent particle velocity, velocity field, and pressure field of an incompressible Newtonian fluid in the creeping flow limit, when a force is applied to a rigid particle. The velocity and pressure fields far from the particle can be fully three-dimensional and the time-dependent particle motion need not be rectilinear. When the force on the particle's surface is specified, an integral boundary condition in the velocity results, requiring a different solution procedure than that used when the velocity on the particle's surface is specified. Detailed eigenfunction expansions are determined for rigid spherical particles in several physically interesting flow situations.     

固-液-液三相水力旋流器内部固相颗粒受力分析研究

在进料浓度较低时,固相颗粒之间距离较大,相互之间的作用以及影响极微,颗粒的分离可以通过对其施加一个外力,将其往器壁方向推,这样就完成了固-液-液三相的分离。这个外即惯性离心力的大小很容易得到,但颗粒所受流体介质阻力则很难得到。文章文通过研究对对旋流器内部固相颗粒进行了受力分析并从理论上对细颗粒易受参数影响的原因进行了阐述。     

Study on the Dynamics of Tribocharged Coal and Mineral Particles in Free-Fall Triboelectric Separator

The dynamics of 2.0?0.8 mm or 0.8?0.5 mm size fraction of tribocharged organic coal, pyrite, and calcite particles were studied under the electric field using the high-speed dynamic camera combined with high-speed motion analysis system. Motion images of these particles were obtained and used to analyze their dynamic parameters. Organic coal particles tribocharged positively move to the negative plate, while pyrite and calcite particles reach the positive plate under the influence of electric force. These results indicate that the trajectories of all 2.0?0.8 mm particles are similar to parabolic curves. For 0.8?0.5 mm particles, the trajectories are approximate straight line, except for the calcite. The vertical velocities of all particles increase with a fluctuant acceleration as a result of gravity and drag force. The horizontal velocities of all particles vary slightly. The dynamics of 0.8?0.5 mm particles prove that size is very important for the triboelectric separation. The actions of electric force and drag force are increased with the decrease of particle size.     

Assembly of alumina particles in aqueous suspensions induced by high-frequency AC electric field

The role of high-frequency alternating current (AC) electric field in the assembly of alumina particles in aqueous media was investigated. Field–particle interactions were in situ investigated for coarse and fine powder particles in very dilute suspensions. For both coarse and fine particles, AC field-induced assembly led to the formation of chains of particles within a minute, which were aligned in the field direction. However, a much finer network of particle chains evolved in fine particle suspensions. Threshold field strength for chain formation was also lower for fine particles (28 V/mm) than for coarse particles (50 V/mm), suggesting stronger interactions for finer particles. Chain length increased with both field strength and field duration. Chain formation was attributed to mutual dielectrophoretic (DEP) interaction forces. Increase in DEP forces with field strength resulted in enhanced interactions. For finer particles, decreasing interparticle distance might have favored stronger interactions. Suspension microstructure was disrupted as soon as the field was removed. However, higher field duration was associated with an improved pattern stability and retention following the field removal. Finally, particle motion was studied in deliberately applied spatially nonuniform AC field, which revealed different mechanisms of chain formation for coarse (negative-DEP) and fine (positive-DEP) particles.