Capillary disintegration of a suspended filamentary drop of a viscous magnetic fluid in a longitudinal magnetic field

A differential equation that describes the axisymmetric motion of two immiscible magnetic fluids of the same density and viscosity is derived. It includes in explicit form the contribution of capillary forces localized at the interface between the fluids, which has the form of a weakly distorted cylindrical surface. With this equation, a dispersion relation for the problem of capillary instability of an extended axisymmetric drop placed in a uniform longitudinal magnetic field is obtained. The effect of magnetic forces on the capillary disintegration of the drop for the extreme cases (large and small Ohnesorge numbers) is analyzed.     

Periodic structure of acicular magnetic clusters in a magnetic liquid

The behavior of magnetic clusters in a magnetic liquid placed in a circular capillary is considered. When a uniform magnetic field is applied to the system, acicular clusters grow from the sediment, being aligned with the field. The interaction of the clusters as magnetic dipoles with one another and with an external gradient magnetic field is considered theoretically. In a nonuniform symmetric magnetic field with a peak, the cluster distribution is uniform near the peak. Such a distribution is fairly stable when the magnetic field gradient is varied over certain limits. The ordered (periodic) cluster configuration is realized experimentally, and it is shown that its period can be controlled. As the magnetic field gradient exceeds a certain threshold, the clusters are redistributed, forming a multirow hexagonal array.     

Dynamic rheological properties of viscoelastic magnetic fluids in uniform magnetic fields

The dynamic rheological properties of viscoelastic magnetic fluids in externally applied uniform magnetic fields are investigated by a laboratory-made cone-plate rheometer in this study. In particular, the effects of the magnetic field on the viscoelastic properties (the complex dynamic modulus) of the viscoelastic magnetic fluids are studied. In the investigation, three viscoelastic magnetic fluids are made by mixing a magnetic fluid and a viscoelastic fluid with different mass ratios. As a supplementation to the experimental investigation, a theoretical analysis is also presented. The present study shows that the viscosity and elasticity of the viscoelastic magnetic fluids are significantly influenced by the magnetic field and the concentrations of the magnetic particles in the test fluids. Theoretical analysis qualitatively explains the present findings.     

Distribution of micrometer-size particles in magnetic fluids in the presence of steady uniform magnetic field

In this paper, distribution of suspended micrometer-size particles in magnetic fluids is investigated. Microstructure formation of particles in magnetic fluids is simulated by using the discrete particle method based on the simplified Stokes dynamics. Not only magnetic particles but also nonmagnetic particles are rearranged in the field direction and form chain-like clusters due to the apparent magnetization in magnetic fluids in the presence of magnetic field. When the diameter of nonmagnetic particles is smaller than that of magnetic particles, nonmagnetic particles move into the empty space of microstructure of magnetic particles, and they are rearranged in the field direction. Uniformity of distribution of particles on the plane perpendicular to the field direction is maintained even after microstructure formation.     

Magnetization distribution in magnetic films studied with Larmor-encoding

Additional information about the magnetization distribution in magnetic films is obtained with a 3D-polarimetry set-up. A pilot experiment was performed with the neutron polarization aligned perpendicular to the surface of a Fe-film in a magnetic field parallel to its surface. The Larmor-precession in the magnetic field between two current sheets was used to adjust the neutron polarization perpendicular to the sample surface. This new polarization-magnetization configuration was probed with a Fe-film in specular and off-specular scattering. The off-specular scattering is created by the magnetic domain structure of the Fe-film in remanence. The results of specular and off-specular scattering are reproduced by calculations for the configuration of the incoming neutron polarization parallel to the sample surface and the magnetic field and for the configuration of the incoming neutron polarization perpendicular to the sample surface and the magnetic field.     

Numerical calculation of the magnetic field and magnetic moment of ferromagnetic bodies with a complex spatial configuration

A method for numerical calculation of the magnetic field and magnetic moment of 3D ferromagnetic objects is described using the method of finite volume and absorbing boundary conditions. The magnetic moments and magnetic field of some ferromagnetic systems imitating various objects are calculated. It is shown that the configuration of ferromagnetic structures consisting of various combinations of rods determines their magnetic moments.     

磁性液体在磁场中产生光的双折射效应机理

磁性液体是一种特殊的高分子稳定胶体,在磁场中会产生光的双折射效应,对磁性液体在胶体学科方面展开研究,发现磁性液体在磁场中的弱絮凝行为表现异常明显,显示出特有的方向性,且又不至胶体系统失稳,证明了磁性液体中的磁性微粒在磁场中聚集成方向性的链状而又不失稳的临界状态存在。从而揭示了方向性弱絮凝是磁性液体在磁场中产生光的双折射效应的机理。     

Pair correlations in magnetic nanodispersed fluids

The pair distribution function of a monodisperse magnetic fluid simulated by a liquid made of dipolar hard spheres with constant magnetic moments is calculated. The anisotropy of the pair distribution function and the related structure factor of scattering in a dc uniform magnetic field are studied. The calculation is performed by diagrammatic expansion in the volume concentration of particles and the interparticle magnetic-dipole interaction intensity using a thermodynamic perturbation theory. Limitation by three-particle diagrams makes it possible to apply the results obtained to magnetic fluids with a moderate concentration. Even for low-concentration and weakly nonideal magnetic fluids, the anisotropic interparticle magnetic-dipole correlations in a magnetic field lead to the repulsion of particles in the direction normal to the field and to the formation of particle dimers along the field.     

Dimensional analysis of aqueous magnetic fluids

A comparison of the synthesis and characterization of three aqueous magnetic fluids intended for biomedical applications is presented. Stable colloidal suspensions of iron oxide nanoparticles were prepared by a co-precipitation method with the magnetite cores being coated with β-cyclodextrin, tetramethylammonium hydroxide and citric acid. Rheological properties of the fluids were investigated, i.e. viscosity (capillary method) and surface tension (stalagmometric method) in correlation with their density (picnometric method). The dimensional distributions of the ferrophase particles physical diameter of these three magnetic fluids – revealed on the basis of transmission electron microscopy (TEM) data – as well as the diameter distributions of some other magnetic fluids presented in the literature, were comparatively analyzed using the box-plot statistical method. In order to extract complementary data on the magnetic diameter of an iron oxide core, magnetization measurements as well as X-ray diffraction pattern analysis were carried out. Interpretation of all the measurement data was accomplished by assessing the suitability of the three magnetic fluid samples from the viewpoint of their stability and biocompatibility. PACS 75.50.Mm; 61.46.Df; 68.37.Lp; 96.15.Pf; 75.75.+a     

Effect of temperature on rotational viscosity in magnetic nano fluids

Flow behavior of magnetic nano fluids with simultaneous effect of magnetic field and temperature is important for its application for cooling devices such as transformer, loud speakers, electronic cooling and for its efficiency in targeted drug delivery and hyperthermia treatment. Using a specially designed horizontal capillary viscometer, temperature-sensitive and non-temperature-sensitive magnetic nano fluids are studied. In both these case the temperature-dependent rotational viscosity decreases, but follows a quite different mechanism. For temperature-sensitive magnetic nano fluids, the reduction in rotational viscosity is due to the temperature dependence of magnetization. Curie temperature (^T _c ) and pyromagnetic coefficient are extracted from the study. A fluid with low T _c and high pyromagnetic coefficient is useful for thermo-sensitive cooling devices and magnetic hyperthermia. For non-temperature-sensitive magnetic nano fluids, reduction in rotational viscosity is due to removal of physisorbed secondary surfactant on the particle because of thermal and frictional effects. This can be a good analogy for removal of drug from the magnetic particles in the case of targeted drug delivery.      

The influence of magnetic field swept rate on the ultrasonic propagation velocity of magnetorheological fluids

Structure of magnetorheological (MR) fluids depends on the strength of the magnetic field applied and on the mode of its application. The ultrasonic wave propagation velocity changes under the effect of an external magnetic field as a result of formation of clusters arranged along the direction of the field in the MR fluids. Therefore, we propose a qualitative analysis of these clustering structures by measuring properties of ultrasonic propagation. Since the MR fluids are opaque, the non-contact inspection using this ultrasonic technique can be very useful. In this study, we measured ultrasonic propagation velocity in MR fluid influenced by an external magnetic field for different swept rate precisely. With increasing magnetic field intensity, the changes of the ultrasonic wave velocity are more pronounced. Sedimentation effect takes place in certain time for different swept rate due to magnetic particle size and it follows linear relationship in log scale. Significant differences of the ultrasonic wave velocity are established between the case when the field is swept at a constant rate and the case when it is stepped up.     

The effect of magnetic field on the electrodeposition of CoFe alloys

The superimposition of a homogenous magnetic field during an electrodeposition of CoFe alloy was investigated. A magnetic field superimposed parallel to the electrode surface increases the limiting current density and the deposition rate due to the magnetohydrodynamic (MHD) effect. The deposits obtained in this field configuration appear smoother and more homogenous than the ones obtained without the magnetic field. On the contrary, a magnetic field superimposed perpendicular to the electrode surface does not influence significantly the electrochemical reaction but the morphology of the deposited layers is strongly affected. The roughness is strongly increased in this field configuration and grains grow as separated columns aligned perpendicular to the electrode surface, in the field direction. A magnetic field applied during the deposition affects the magnetic properties of the deposited layers as well. These changes are discussed with respect to the surface roughness and the internal stress state of the layer.     

磁流体粘度的实验研究

采用毛细法粘度计测量了水基Fe磁流体的粘度,分析了磁性粒子份额、表面活性剂含量以及外加磁场强度和方向对粘度的影响。实验结果表明,磁流体粘度随着磁性粒子和表面活性剂浓度的增加而增加;随着外加磁场强度的增大而增大,对于相同的磁流体,在外加磁场方向垂直于流动方向时的粘度大于外加磁场方向平行于流动方向时的粘度;表面活性剂含量的增大将减弱外加磁场对磁流体粘度的影响。     

Rayleigh-taylor instability of viscoelastic fluids with suspended particles in porous medium in hydromagnetics

The instability of the plane interface between two viscoelastic (Oldroydian) superposed conducting fluids permeated with suspended particles in porous medium is studied when the whole system is immersed in a uniform magnetic field. The dispersion relation for the Oldroydian viscoelastic fluid is obtained which also yields dispersion relations for Maxwellian and Newtonian fluids in special cases, in the presence of suspended particles in porous medium in hydromagnelics. The system is found to be stable for potentially stable case. The presence of magnetic field stabilizes certain wave number band whereas the system was unstable for all wave numbers in the absence of magnetic field, for the potentially unstable configuration. The growth rates increase (for certain wave numbers) and decrease (for other wave numbers) with the increase in stress relaxation time, strain retardation time, suspended particles number density and medium permeability.     

Formation and focusing of electron beams in an electron-optical system with a plasma emitter placed in a magnetic field

The configuration and strength of a magnetic field are calculated in the regions of electron generation, acceleration, and transport in the electron-optical system of the plasma electron source. A magnetic field necessary for discharge initiation and maintenance is generated with a permanent magnet placed in a discharge chamber. It is shown that the magnetic field strength and configuration in these regions can be considerably varied by appropriately choosing the materials of electrodes forming the magnetic circuit. It is found that the beam focusing can be significantly improved by producing a quasi-uniform magnetic field in the electron-optical system of the plasma electron source.     

Instability of the origami of a ferrofluid drop in a magnetic field

Capillary origami is the wrapping of a usual fluid drop by a planar elastic membrane due to the interplay between capillary and elastic forces. Here, we use a drop of magnetic fluid whose shape is known to strongly depend on an applied magnetic field. We study the quasistatic and dynamical behaviors of such a magnetic capillary origami. We report the observation of an overturning instability that the origami undergoes at a critical magnetic field. This instability is triggered by an interplay between magnetic and gravitational energies in agreement with the theory presented here. Additional effects of elasticity and capillarity on this instability are also discussed.     

Effects of sweep rates of external magnetic fields on the labyrinthine instabilities of miscible magnetic fluids

The interfacial instability of miscible magnetic fluids in a Hele-Shaw Cell is studied experimentally, with different magnitudes and sweep rates of the external magnetic field. The initial circular oil-based magnetic fluid drop is surrounded by the miscible fluid, diesel. The external uniform magnetic fields induce small fingerings around the initial circular interface, so call labyrinthine fingering instability, and secondary waves. When the magnetic field is applied at a given sweep rate, the interfacial length grows significantly at the early stage. It then decreases when the magnetic field reaches the preset values, and finally approaches a certain asymptotic value. In addition, a dimensionless parameter, Pe, which includes the factors of diffusion and sweep rate of the external magnetic field, is found to correlate the experimental data. It is shown that the initial growth rate of the interfacial length is linearly proportional to Pe for the current experimental parameter range and is proportional to the square root of the sweep rate at the onset of labyrinthine instability.     

Magnetic properties of undecane-based magnetic fluids

The magnetic properties of magnetic fluids based on n-undecane have been presented. The temperature dependences of the magnetic susceptibility in the absence and presence of a bias field have been analyzed. A step change in the specific heat of the medium, which is characteristic of magnetic phase transitions, has been discovered.     

磁流体中Helmholtz和Kelvin力的界定

磁流体磁彻体力的两种简化形式Helmholtz力和Kelvin力具有一定的适用范围.在推导磁流体中的磁彻体力表达式基础上,分析Helmholtz力和Kelvin力在磁流体中的起源,得出两种形式的成立条件.计算结果表明:当磁流体磁导率与外磁场强度无关时,磁流体磁彻体力可由Helmholtz力表示;当磁流体中磁性颗粒的平均磁矩与磁流体比体积无关时,Kelvin力为磁彻体力的简化形式;在磁流体磁化系数与其密度成正比情况下,Helmholtz力可转换为Kelvin力. 关键词： 磁流体 磁彻体力 Helmholtz力 Kelvin力