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.     

Separation of minerals in electric field by ponderomotive forces (classification in high-voltage vertical capacitor)

A three-electrode capacitor, designed for an alternating electric field, was used to effect the dry separation of materials according to permittivity.A stable fall regime was secured through a isodynamic field of the ponderomotive forces. The solid fractions were arranged in concentric circles about the capacitor axis.Numerical examples were worked out for two-component mixtures of gold and quartz, copper and aluminum, and glass and aluminum.The required voltage on the capacitor plates and the fall distance were determined.     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

HEAT TRANSFER IN A THIN LIQUID FILM IN THE PRESENCE OF AN ELECTRIC FIELD

Heat transfer enhancement in an evaporating thin liquid film utilizing a electric field under isothermal interfacial condition is presented. A new mathematical model subjected to van der Waals attractive forces, capillary pressure, and an electric field is developed to describe the heat transfer enhancement in the evaporating thin liquid film. The effect of the electrostatic field on the curvature of the thin film, evaporative flux, pressure gradient distribution, heat flux, and heat transfer coefficient in the thin film is presented. The results show that applying an electric field can enhance heat transfer in a thin liquid film significantly. In addition, utilizing electric fields on the evaporating film will be a way to expand the extended meniscus region to attain high heat transfer coefficients and high rates of heat flux.     

The anomalous increase in tunability in Ba0.55Sr0.45TiO3-MgO-Mg2SiO4 composite ceramics

The Ba_0.55Sr_0.45TiO₃–Mg₂SiO₄–MgO composite ceramics were prepared via solid-stated method and the dielectric tunable properties of composite ceramics were investigated. With the increase in dielectrics content, the tunability of composites ceramics increased abnormally first and then decreased, while the anharmonic coefficient increased monotonously. A simple 3D Finite Element model was developed to simulate the dielectric response and an anomalous increase in tunability when increasing the dielectrics content in composites was obtained. The simulation indicated that adding dielectrics into ferroelectrics causes the redistribution of the electric field. Increasing dielectrics content, the average electric field strength in the enhanced area of ferroelectrics increases, which lead to the increase in the tunability. The abnormal increase in tunability originates from the enhancement of the electric field on a fraction of the ferroelectrics around dielectrics.     

Non-linear electrohydrodynamics in microfluidic devices

Since the inception of microfluidics, the electric force has been exploited as one of the leading mechanisms for driving and controlling the movement of the operating fluid and the charged suspensions. Electric force has an intrinsic advantage in miniaturized devices. Because the electrodes are placed over a small distance, from sub-millimeter to a few microns, a very high electric field is easy to obtain. The electric force can be highly localized as its strength rapidly decays away from the peak. This makes the electric force an ideal candidate for precise spatial control. The geometry and placement of the electrodes can be used to design electric fields of varying distributions, which can be readily realized by Micro-Electro-Mechanical Systems (MEMS) fabrication methods. In this paper, we examine several electrically driven liquid handling operations. The emphasis is given to non-linear electrohydrodynamic effects. We discuss the theoretical treatment and related numerical methods. Modeling and simulations are used to unveil the associated electrohydrodynamic phenomena. The modeling based investigation is interwoven with examples of microfluidic devices to illustrate the applications.     

Hydrostatic Levitation of Particles in Electric and Magnetic Fields

Abstract

Nonhomogeneous magnetic and electric fields generate levitational forces in magnetic and dielectric liquids whereby particles are separated according to their specific gravity and magnetic and electric properties, respectively. The corresponding systems are called magnetohydrostatic separation and wet dielectric separation. These forces depend on the field intensity and gradient, and on properties of the liquid and the particles to be levitated. This paper deals with the potential application of electric fields as compared with that of the magnetic field. It is shown that unless special measures are taken, the electric field is inferior to such an extent that its application is not promising. In this context the influences of particle size and shape in both methods are discussed.     

Electrically and magnetically induced transfer of solvated ions in an isolated solution of salt in a polar dielectric

Solutions of metal salts in liquid polar dielectrics were considered in the approximation of a single ion placed in a system of liquid polar dielectric molecules and a system of interacting charged particles, such as cations, anions, and polarized liquid polar dielectric molecules, arranged around them. The sizes of the solvated ion clusters were estimated in the approximation of a single ion and in the approximation of the existence of a self-consistent field in the solution volume. The characteristic frequencies of the external electric field were determined so that, being imposed on an aqueous salt solution, it produces the effect of the induced transfer of solvated ions. The behavior of the solutions under the action of external asymmetric electrical and alternating magnetic fields was analyzed.     

HYDRODYNAMIC FORCES IN BANDS OF MAGNETOTACTIC BACTERIA, REVISITED: FORMATION, STABILITY AND STRUCTURE

This paper reports the results of simulations of the motion of dense suspensions of magnetotactic bacteria, bacteria that contain small magnetic dipoles. These simulations build on a previously described mechanism in which pair-wise, far-field, hydrodynamic interactions between swimming bacteria are proposed to be responsible for the band structure that appears when such bacteria are oriented in a given direction by an external magnetic field. Previous work has shown that these hydrodynamic forces act in a direction and with a magnitude that are fully consistent with the observed behavior. Applying this pair-wise, hydrodynamic force mechanism to hundreds of bacteria in a numerical simulation shows that band formation and stability is predicted by this simple model, while the precise band shape appears to depend on effects not included in the simulation.     

THE TAYLOR PUMP: VISCOUS-FREE SURFACE FLOW DRIVEN BY ELECTRIC SHEAR STRESS

The pump invented by G.I. Taylor consists of low conductivity liquid in an insulated tank with electrodes at each end, one of which has an extension canted over the liquid/gas interface. When a potential difference is imposed between the electrodes, an electric shear stress acts on the interface and drives a Marangoni-like flow. The steady, two-dimensional flow of an incompressible, Newtonian, ohmic liquid in the tank is analyzed by solving simultaneously the Navier-Stokes equations for velocity and pressure in the tank and the Laplace equation for electric field in the liquid and the gas. The results show that an egg-shaped vortex develops in the liquid as the relative importance of inertial to viscous forces increases, in accordance with the experiments of Melcher and Taylor (1969). The methods developed here can be readily extended to analyze drop deformation and enhancement of mass transfer in two-phase systems due to an imposed electric field.     

Growth rate and oscillation frequency of electrified jet and droplet: Effects of charge and electric field

Electrified jets are applied industrially in agriculture, automobiles, targeted drug delivery systems, spacecraft propulsion units, liquid metal sprayers, ion sources, emulsifiers, dust scavenging systems, and ink-jet printers. Electrified columnar jets experience instability caused by electrohydrodynamic interactions of the charged liquid surfaces with electric fields. Electrostatic and surface tension forces competing along the liquid surface create surface pressure differences. The temporal rise and fall of the surface pressure induce oscillations of jets and droplet. A linear theory was derived to yield a dispersion equation determining the most dominant wavelength of oscillation for a given charge level and electric field; this enabled the estimation of the diameter of an atomized droplet. In addition, the frequency of oscillation was derived for a cylindrical jet and spherical droplet. Parametric studies were performed for various charging levels and electric field strengths.

© 2018 American Association for Aerosol Research     

Wet dielectric separation

We have studied the possibility of using electrical phenomena in order to selectively separate minerals with various densities and dielectric constants in non-homogeneous electric fields. This technique is based on levitation forces acting on a solid particle immersed in dielectric fluid.When the gradient of the electric field is directed downwards, the levitation force in the dielectric fluid (pure liquid or electrical colloids) will be in an upwards direction to float sinking minerals by counterbalancing their density mismatch with the liquid medium.The separation process can be made by using two possible modes: either by direct application of electrical forces or by change in the density and dielectric constant of a liquid medium in which the mineral particles are placed. We have investigated several electrode configurations in order to produce electrical fields which are strongly non-homogeneous. Further investigations on the dielectric properties of liquids have been performed.     

Electrorheological characterization of semiconducting polyaniline suspension

Electrorheological (ER) properties of polyaniline suspensions in silicone oil with various experimental conditions were investigated. These suspensions exhibit dramatic changes in their rheological properties which are caused by the formation of particle chain structures induced by an applied electric field. Polyaniline was synthesized by the chemical oxidation of aniline with ammonium peroxysulfate. This study examined the effects of electric field, volume fraction, polymerization temperature and operation temperature on the ER properties of polyaniline suspensions in silicone oil. Rheological measurements were carried out by using a rotational rheometer with a high voltage generator in both controlled shear stress and shear rate modes. Experimental results showed that the ER properties were enhanced by increasing the volume fraction and electric field and were independent of the operating tempertures. The best ER properties were also obtained by using polyaniline particles synthesized at -10°C, and these had a conductivity of ? 10^?10 S/cm.     

气泡聚并机理模型

对气液二相体系中气泡间凸面状薄液膜内以及气液界面上动量和质量传递行为进行了分析,建立了气泡聚并时间模型,考虑了范德华作用力以及双电层作用力对液膜变薄过程产生的影响.模型值与文献实验值吻合较好.     

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.     

Suspension of flexible cylinders in laminar liquid flow

A methodology for particle-resolved simulation of dense suspensions of flexible cylindrical particles in Newtonian liquid flow is described. It is based on the Lattice–Boltzmann method for solving the liquid flow and an immersed boundary method for imposing no-slip at the particle surfaces and providing the distribution of liquid–solid interaction forces over the particle surfaces. These forces—along with contact forces—translate, rotate as well as bend the cylindrical particles. Verification tests have been performed for a single cylinder settling and deforming under gravity at a low Reynolds number. The method has been applied to a clamped flexible cylinder in microchannel flow for which experimental data are available. It then is used to investigate the behavior of hundreds of flexible cylinders with length over diameter aspect ratios of 4 and 6 in a container agitated by an impeller at a Reynolds number of 87 which implies laminar flow. The overall solids volume fraction is 15%. We study the effect of the bending stiffness of the particles on the solids suspension process, on the extent of particle deformation as well as on the torque required to spin the impeller.     

交变电场下离子交换色谱分离过程

以牛血清白蛋白在阴离子交换剂DEAE -SepharoseFastFlow上的吸附和解吸过程为例 ,考察了交变电场下离子交换色谱过程的吸附和洗脱过程及其传质特性 .结果表明 :在本文实验条件下 ,交变电场电流强度的变化对离子交换吸附平衡无显著影响 ,而外加电场在介质孔内所产生的电渗流可加速待分离组分进出固相介质的传递过程 ,显著提高色谱填充床的动态吸附容量 ,改善洗脱峰的特性     

TAYLOR CONE ELECTROHYDRODYNAMICS. THE MINIMUM AND MAXIMUM FLOW RATES IN ELECTROSPRAYING

The surface charge at the liquid–gas interface in cone-jet electrospraying, almost relaxed from an electrochemical point of view, is driven by the radial electric field created to supply the current to the cone tip that the microjet withdraws. The electric stress applied on the liquid surface provokes a low or high Reynolds number motions in the electrified meniscus depending on a dimensionless parameter which relates the liquid viscosity and its electrical conductivity. The analysis of the surface motion is essential to quantify the surface current convected to the cone’s tip, which is shown to be negligible compared to the one driven by bulk conduction. In the case of high Reynolds number motions, we show mathematically, and also experimentally, the emergence of an interesting self-rotation phenomenon.In addition, an analysis of the equations governing the electrohydrodynamics of the charged liquid ligament issuing from the tip of an electrified meniscus in a steady cone-jet suggests the mechanisms which set the stability limits of this steady regime. It is shown that for low and moderate liquid polarities (less than 40 times the vacuum permittivity), the minimum liquid flow rate that can be electrosprayed in a steady cone-jet is reached when the surface tension stress at the cusp from which the jet issues, which provokes a resulting pressure gradient against the flow, overcomes the electrostatic “suction” effect. To show the role of the different forces involved, we have carried the calculation of the intervening ones in the momentum equation using the digitized shape of a cone-jet close to the minimum flow rate in the case of a permittivity 6.5 times larger than the vacuum one. For larger polarities, which impose large electrical conductivities as well, the role of viscous forces, polarization forces, and charge relaxation effects is discussed. In addition, we have carried out experimental measurements of the minimum flow rate using several different liquids. These results are discussed and compared with the experimental data from different authors, as well as with other previously given scaling laws and estimations of the minimum flow rate in cone-jet electrospraying.     

Design strategies of perovskite energy-storage dielectrics for next-generation capacitors

The next-generation capacitors have placed higher requirements on energy-storage dielectrics, such as high temperature, high frequency and high voltage. Perovskite dielectrics possess various kinds of polar structures, such as ferroelectric domains, polar nano-regions (PNRs), and anti-polar structure as well, which exhibit various responses to external stimulations (temperature, electric field, mechanical loadings). Its design inspires development strategies to improve their energy-storage properties for capacitors involving chemical composition, fabrication process, computer simulation, and even measurement strategies for validation. In this article, we reviewed the recent design strategies and the perovskite dielectrics (covering linear, ferroelectric, relaxor ferroelectric, anti-ferroelectric, even some composite materials, such as glass-ceramics and polymer). This review spans from the atomic to millimeter-scale strategies of property optimization to provide accurate and comprehensive information for researchers in this field. Some novel proposals for overcoming the barriers between materials and properties are presented to accelerate the applications of the next-generation capacitors. Therefore, this review should help to identify the best approach for transferring the new perovskite dielectrics into next-generation capacitor applications.     

Elektrostatisches Zerstäuben von Flüssigkeiten

The electrostatic spraying of liquids is based on electrostatic forces that result from an electrical field and are capable to overcome the capillary forces. In contrast to gravitational and centrifugal fields the value as well as the direction of electrostatic fields depend not only on the setup geometry but additionally on the shape of the liquid surface. This results in a complex interaction between geometries, material properties, and mass and charge currents with respect to the formation of droplets. A capillary tip facing a wide flat counterelectrode has to be considered as the basic geometry. The electrical field needed for droplet production is maintained by a high voltage supply. One has to distinguish between droplet production solely by charge induction and droplet formation influenced by a corona discharge together with an electric wind. Characteristic time constants serve to differentiate between the different spraying modes.