Thermal behaviour and particle size evaluation of primary clusters in a water-based magnetic fluid

This paper reports the experimental research on thermal behaviour and particle size evaluation of primary clusters of ferromagnetic nano-particles in a water-based magnetic fluid. The magnetic fluids are suspensions of ultra fine particles coated with a molecular layer of dispersant in a liquid carrier such as water or kerosene. The particles are coated with single- or double-layer of surfactant to achieve stable dispersion. Numerous experimental studies have indicated the existence of the primary cluster of ferromagnetic nano-particles in a water-based magnetic fluid. The purpose of this research is to evaluate the particle size of the primary clusters by applying the Einstein's equation for Brownian motion assuming that the primary cluster has a spherical-shape. The thermal behaviour of ferromagnetic nano-particles in magnetic fluids is investigated through the micro visualization using the optical darkfield microscope system and particle tracking velocimetry data processing system. Real-time visualization of the Brownian motion of primary clusters in a water-based magnetic fluid was carried out. The experimental results clarified that the primary cluster size depends upon the concentration of the ferromagnetic nano-particle in the magnetic fluid.     

Thermal behaviour and particle size evaluation of primary clusters in a water-based magnetic fluid

This paper reports the experimental research on thermal behaviour and particle size evaluation of primary clusters of ferromagnetic nano-particles in a water-based magnetic fluid. The magnetic fluids are suspensions of ultra fine particles coated with a molecular layer of dispersant in a liquid carrier such as water or kerosene. The particles are coated with single- or double-layer of surfactant to achieve stable dispersion. Numerous experimental studies have indicated the existence of the primary cluster of ferromagnetic nano-particles in a water-based magnetic fluid. The purpose of this research is to evaluate the particle size of the primary clusters by applying the Einstein’s equation for Brownian motion assuming that the primary cluster has a spherical-shape. The thermal behaviour of ferromagnetic nano-particles in magnetic fluids is investigated through the micro visualization using the optical darkfield microscope system and particle tracking velocimetry data processing system. Real-time visualization of the Brownian motion of primary clusters in a water-based magnetic fluid was carried out. The experimental results clarified that the primary cluster size depends upon the concentration of the ferromagnetic nano-particle in the magnetic fluid.     

Analysis of particles in magnetorheological polishing fluid for finishing of ferromagnetic cylindrical workpiece

Magnetorheological honing process is developed for nanofinishing of internal surfaces of ferromagnetic and non-ferromagnetic cylindrical objects. The process makes use of smart fluid called magnetorheological (MR) polishing fluid for finishing which has a property to become stiff in the existence of magnetic field. The smart MR polishing fluid is made with the ingredients of carbonyl iron (CI) particles, abrasive particles, and base fluid. Direct current given to the electromagnet coil engenders magnetic field on finishing tool surface. Magnetic force acts on magnetic CI particles which further exert the repulsive force on nonmagnetic silicon carbide (SiC) abrasive particles and performs finishing when tool rotates as well as reciprocates inside the cylindrical workpiece. The CI and SiC particles present in MR polishing fluid are magnetically simulated and analyzed using finite element (FE) analysis. The distribution of magnetic flux density and magnitude of magnetic force acting on CI particles are analyzed through FE analysis. It is found that the CI particles which are available adjacent to the active abrasives are major responsible for indenting the active abrasive particles into workpiece surface. Also, the effect of finishing tool surface areas and particles size on the strength of chains of CI particles in MR polishing fluid have been analyzed.     

Orientation process of acicular oxides in the magnetic tape layer

The orientation of acicular particles in the active layer of a magnetic tape is presently a usual technological process applied in magnetic tape production. The theory of this process is dealt with. A mathematical model is deduced, describing the behavior of an isolated agglomerate of acicular particles in an external magnetic field and the change of its microstructure, i.e., the orientation of acicular particles in the agglomerate. The term for the density of probability ω(α) of the angle distribution α of the longer particle axes in the orientated agglomerate is also deduced. Also the approximate relation between the orientation degree, defined by the mean quadratic deviationsof the angles α of the particular particles and by the bevel values γ of the deformed agglomerate, is shown. It appears that, besides the orientation by means of the external magnetic field, it would also be possible to orientate the magnetic suspension by other methods.     

High gradient magnetic particle separation in viscous flows by 3D BEM

The boundary element method was applied to study the motion of magnetic particles in fluid flow under the action of external nonuniform magnetic field. The derived formulation combines the velocity-vorticity resolved Navier–Stokes equations with the Lagrange based particle tracking model, where the one-way coupling with fluid phase was considered. The derived algorithm was used to test a possible design of high gradient magnetic separation in a narrow channel by computing particles trajectories in channel flow under the influence of hydrodynamic and magnetic forces. Magnetic field gradient was obtained by magnetization wires placed outside of the channel. Simulations with varying external magnetic field and flow rate were preformed in order to asses the collection efficiency of the proposed device. We found that the collection efficiency decreases linearly with increasing flow rate. Also, the collection efficiency was found to increase with magnetic field strength only up a saturation point. Furthermore, we found that high collection efficiently is not feasible at high flow velocity and/or at weak magnetic field. Recommendation for optimal choice of external magnetic field and flow rate is discussed.     

Magnetic multicore nanoparticles for hyperthermia--influence of particle immobilization in tumour tissue on magnetic properties

When using magnetic nanoparticles as a heating source for magnetic particle hyperthermia it is of particular interest to know if the particles are free to move in the interstitial fluid or are fixed to the tumour tissue. The immobilization state determines the relaxation behaviour of the administered particles and thus their specific heating power. To investigate this behaviour, magnetic multicore nanoparticles were injected into experimentally grown tumours in mice and magnetic heating treatment was carried out in an alternating magnetic field (H = 25 kA m(-1), f = 400 kHz). The tested particles were well suited for magnetic heating treatment as they heated a tumour of about 100 mg by about 22 K within the first 60 s. Upon sacrifice, histological tumour examination showed that the particles form spots in the tissue with a mainly homogeneous particle distribution in these spots. The magnetic ex vivo characterization of the removed tumour tissue gave clear evidence for the immobilization of the particles in the tumour tissue because the particles in the tumour showed the same magnetic behaviour as immobilized particles. Therefore, the particles are not able to rotate and a temperature increase due to Brown relaxation can be neglected. To accurately estimate the heating potential of magnetic materials, the respective environments influencing the nanoparticle mobility status have to be taken into account.     

磁流变抛光中的磁场与磁流变液缎带成型分析

利用标量磁位对磁流变抛光中的磁场进行了分析计算,并根据磁偶极子在磁场空间受力的模型,对磁流变液中的磁性微粉颗粒的受力进行理论推导,进一步分析磁流变液形成单一稳定缎带凸起的条件。理论分析经过试验验证,证明分析是正确的。     

考虑磁性颗粒不均匀分布的磁流变液修正微观力学模型及试验验证

为了研究磁性颗粒在磁场作用下的不均匀分布对磁流变液力学性能的影响,通过卡方分布来模拟磁性颗粒的间距分布,对现有的磁流变液微观力学模型进行修正,并通过磁流变阻尼器的力学性能试验验证了模型的有效性。在磁流变液双链微观力学模型的基础上,修正相邻磁性颗粒的间距完全相等且不随磁感应强度而变化的假设,采用卡方分布来表征磁性颗粒间距的不均匀分布,并引入分布参数来描述磁性颗粒间距随磁感应强度的变化关系,推导了考虑磁性颗粒不均匀分布的磁流变液修正微观力学模型;基于修正的微观力学模型,分析了分布参数对磁流变液剪切屈服应力的影响;将该文提出的磁流变液修正微观力学模型带入到磁流变阻尼器的准静态模型中,可以得到不同电流下的阻尼器最大出力,并与磁流变阻尼器力学性能试验数据进行对比来验证所提模型的有效性。结果表明,考虑了磁性颗粒不均匀分布的磁流变液修正微观力学模型可以更加精确地预测磁流变液在不同磁感应强度下的剪切屈服应力,尤其是在低磁感应强度情况下可以改善现有微观力学模型放大了磁流变液剪切屈服应力的缺点。     

Structuralization of Magnetic Nanoparticles Induced by Laser Heating in Magnetic Fluids

The structuralization of magnetic particles in magnetic fluids due to the thermodiffusion induced by laser light illumination was experimentally observed in two types of magnetic fluids: one based on a mineral oil with magnetite particles covered by a monolayer of oleic acid as a surfactant and the other a kerosene-based magnetic fluid sterically stabilized by a double layer consisting of oleic acid and dodecylbenzenesulphonic acid (DBS). Forced Rayleigh scattering (FRS) showed different behaviors of magnetic particle structuralization in the observed magnetic fluids. While for the case of mineral oil-based magnetic fluids, there was observed a positive thermodiffusion (S > 0), an indication of negative thermodiffusion (S < 0) was observed in magnetic fluids based on kerosene. This was also confirmed by the time-dependent decay of a grating of magnetic particles. Numerical simulation of aggregation for the case of negative thermodiffusion was confirmed by the observed aggregation after laser illumination in kerosene-based magnetic fluids and enabled an estimated value of the negative Soret constant in the magnetic fluid studied (S ≈ −10⁻² K ⁻¹).     

铁粒子多分散系统磁流变液的制备和性能研究

磁流变液是铁磁性颗粒分散到低粘度的油中形成的稳定的悬浊液。其扩散特性主要由颗粒本身的物性和颗粒间的相互作用而决定。为了更好的控制颗粒间的相互作用,我们加入同样物性,但尺寸更小的颗粒,以调控悬浊液的流动特性。研究显示,在高体积百分比的磁流变液中加入亚微米尺寸的铁颗粒可以显著改善流体在没有外加磁场情况下的流动特性。受范得华力影响,加入的小颗粒首先附着在大颗粒周围,形成组装体,组装体间范得华力较小,从而有效降低了流体粘度。流体的粘度随着小颗粒在大颗粒上附着的比例不同而变化。随着小颗粒的增加,流体中铁颗粒的体积百分比增加,从而提高了磁流变液在磁场中的屈服应力,增强了磁流变液的磁流变效果。     

尖劈状磁场中磁流体磁性微粒的受力分析

通过有限元方法对尖劈形磁场中磁流体磁性微粒所受的磁场力进行了数值分析,分别给出了磁场气隙处磁通量和磁场强度随不同气隙宽度和夹角的变化规律.在此基础上,利用虚位移法和麦克斯韦应力张量法计算磁流体磁性微粒在气隙上方所受的磁场力.结果表明,随着气隙宽度的减小,磁通量、磁场强度和磁场力明显增加;随着气隙夹角的增加,磁通量、磁场...     

不同形貌纳米Fe3O4颗粒磁流体的传动性能

选用无规则、正八面体和六方片状形貌的纳米Fe3O4磁性颗粒制备的磁流体,通过设计、组装磁流体传动性能测试仪,探讨了磁流体传动性能与传动盘间距、传动盘之间的转速差的关系,并研究了磁流体中纳米磁性颗粒的形貌对磁流体传动性能的影响。结果表明,在传动盘间隙一定时,磁流体传递扭矩的大小在磁性粒子未达到其饱和磁化强度时,传递扭矩大小随感应磁场强度增大而迅速增大,但随着磁感应强度的进一步加大,磁性粒子逐步达到其饱和磁化强度,磁流体传递扭矩大小的增长减缓,最后几乎不再增大;传动盘之间的间隙对磁流体传递扭矩的大小影响很大,间隙越大,传递的扭矩越小;传动盘之间的转速差对磁流体传递扭矩的大小影响较小,在低转速差下传递的扭矩随转速差的增加而有所增加,但超过一定的转速差后,由于磁流体的剪切稀化效应,传递的扭矩将有所减小。另外,磁流体中磁性粒子的形貌对磁流体传递扭矩的大小有一定的影响,正八面体形貌的磁性粒子相对于无规则和六方片状形貌的磁性粒子,其磁流体能够传递更大的扭矩。     

Detection of C-reactive protein based on immunoassay using antibody-conjugated magnetic nanoparticles

We report a detection method for C-reactive protein (CRP) based on competitive immunoassay using magnetic nanoparticles under magnetic fields. Functional magnetic nanoparticles were prepared and conjugated with anti-CRP for immunoassay. Magnetic nanoparticles labeled with anti-CRP were flowed through a separation channel to form depositions for selective capture of CRP under magnetic fields. Free CRP and a fixed number of CRP-labeled particles were used to compete for a limited number of anti-CRP binding sites on the magnetic nanoparticles. The deposited percentages of CRP-labeled particles at various concentrations of free CRP were determined and used as a reference plot. The determination of CRP in the unknown sample was deduced from the reference plot using the deposited percentages. The running time was less than 10 min. The CRP concentration of serum sample was linearly over the range of 1.2-310 microg/mL for deposited percentages of CRP-labeled particles. The detection limit of this method was 0.12 microg/mL which was approximately 8-fold lower than the typical clinical cutoff concentration (1 microug/mL). This method can provide a fast, simple, and sensitive way for protein detection based on competitive immunoassay using magnetic nanoparticles under magnetic fields.     

An NMR method of measuring the mass of magnetic beads in a lowdensity fluid [biomedical measurement]

A nuclear magnetic resonance (NMR) method of determining the mass of the magnetic particles present in a fluid by observing the relative frequency shift of an NMR signal is proposed. Experimental tests have been carried out to find the ratio of the mass of the beads to the total mass of the liquid with various densities by using this method. The method can be used to measure masses in a fluid containing a low concentration of magnetic particles     

Dynamic capture and accumulation of multiple types of magnetic particles based on fully coupled multiphysics model in multiwire matrix for high-gradient magnetic separation

High-gradient magnetic separation (HGMS) effectively separates fine weakly magnetic minerals using a magnetic matrix. The basic principle of single-wire capture of magnetic particles in HGMS has received considerable attention. In practice, however, a real matrix is made of numerous magnetic wires. Transport of magnetic particles inside a multiwire matrix under various operating conditions has not been sufficiently investigated, and it is not clear whether single-wire and multiwire matrices differ significantly. A fully coupled multiphysics model based on the idealized capture model was developed to investigate the 2D capture and accumulation of multiple types of particles in single-wire and multiwire matrices. In this model, the properties of multiple types of particles were defined. Then, particle tracing via the fluid flow model was used to calculate the dynamic capture and accumulation of particles under the determined magnetic and flow fields. The time-dependent dynamic capture mode used in this study can reveal the details of particle capture and accumulation in single-wire and multiwire matrices. All the calculations and analyses indicate that single-wire and multiwire matrices both exhibit basically the similar capture tendency as the particle size, slurry feed velocity, and magnetic induction are gradually increased, and a single-wire matrix always has a much higher capture selectivity than a multiwire matrix. This difference in selectivity between the single-wire and multiwire matrices results mainly from magnetic coupling between magnetic wires in the multiwire matrix, where the fluid flow is also quite complicated. In addition, adjacent columns of wires are staggered vertically, increasing the probability of collisions between the particles and the wires; thus, intergrowth particles that are not captured by the upstream wires are more easily captured by the downstream wires. By comparing the experimental results with the simulation results, the correctness of the HGMS recovery and grade prediction results was verified.     

Measurement of the Viscosity of Dilute Magnetic Fluids

The capillary tube viscometer is used to measure the viscosity of aqueous magnetic fluids under the influence of parallel and perpendicular magnetic fields. The effects of the volume fraction of the suspended magnetic particles, the concentration of surfactants, and the external magnetic field strength, as well as the orientation, on the viscosity of the magnetic fluid are analyzed. The experimental results show that the viscosity of the sample magnetic fluids increases with increases in the concentrations of suspended magnetic particles and surfactants. The external magnetic field is also an important factor that affects the viscosity of the magnetic fluid. The viscosity first increases with the magnetic field and finally approaches a constant as the magnetization attains a saturation state. For the same magnetic fluid, the viscosity in a perpendicular magnetic field is larger than that in a parallel magnetic field for the same magnetic field.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China     

Three‐dimensional modelling and simulation of magnetorheological fluids

A magnetorheological fluid (MR fluid) is a type of smart fluid composed of micrometer‐sized magnetizable particles suspended in a carrier fluid. The rheological properties of an MR fluid can be greatly altered upon application of an external magnetic field. This paper presents a computational framework for the numerical study of MR fluids, in which a two‐stage modelling and simulation strategy is proposed to achieve reasonable accuracy and computational efficiency. At the first stage for simulating the particle chain formation, the particle dynamics plays a major role whereas the hydrodynamics of the fluid flow is of secondary importance. Thus an MR fluid is modelled in the context of the discrete element method and the simple Stokes formula is adopted for the hydrodynamic interaction. At the second stage, the formulated particle chains are applied as the initial configuration for simulating the rheological properties of the fluid under different shear loading conditions. A combined lattice Boltzmann and discrete element approach is employed to fully resolve the fluid field and the hydrodynamic interactions between the fluid and the particles. Some relevant magnetic models are comprehensively reviewed and the mutual dipole model is employed in this work to account for the magnetic interactions between the particles. The proposed solution procedure is illustrated via a set of numerical simulations for a representative volume element of an MR fluid. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermomagnetic Convection as a Tool for Heat and Mass Transfer Control in Nanosize Materials Under Microgravity Conditions

Magnetic colloids are relatively new man-made nanomaterials whose magnetic susceptibility is several orders of magnitude larger than that of natural substances. Experiments conducted in magnetic fluids show that strengthening or weakening of thermal convection in colloids is dictated by a competition between the gravitational and magnetic mechanisms as well as by the effect of the fluid density stratification due to gravitational sedimentation of magnetic particles and their aggregates. Therefore experiments in microgravity conditions are required to eliminate gravitational sedimentation. This will enable an accurate investigation of convection in magnetic fluids and unambiguous study of the interaction of a magnetic field with a magnetopolarized medium. Such experiments are also needed to perform an accurate measurement of fluid’s transport coefficients.     

Numerical simulation of chainlike cluster movement of feeble magnetic particles by induced magnetic dipole moment under high magnetic fields

In this paper, the motion of a chainlike cluster of feeble magnetic particles induced by high magnetic field is discussed on the basis of the results of numerical simulations. The simulations were performed on glass particles with a diameter of 0.8 mm; and the viscosity, applied magnetic field and magnetic properties of the surrounding medium were changed. In addition to the magnetic field and the difference in magnetic susceptibility between the particles and the surrounding medium, the obtained results indicate that the viscosity is an essential factor for the formation of the chainlike alignment of feeble magnetic particles. We also carried out simulations using glass particles with a smaller diameter of 0.1 mm. Chainlike clusters were produced similar to those of ferromagnetic particles formed in a ferromagnetic fluid.     

SUBMICRON MAGNETIC PARTICLE SEPARATION

The control, collection or separation of particles on the basis of Their magnetic moment relative Co the carrier fluid has been demonstrated in many applications. Usually the particle sizes are larger than one micron and the magnetic susceptibility at least moderately paramagnetic. Recently, particle separation techniques have been developed for both diamagnetic and submicron particles. These techniques have found application in mineral beneficiation, nuclear reactor coolants, biology and medicine. Such developments require an understanding of flow forces in liquids and gases, diffusion and Brownian motion, and of magnetic properties which range from the strong magnetic moments of ferromagnetic and superparamagnetic particles down orders of magnitude to those of diamagnetism.