Two-dimensional Monte Carlo simulations of a suspension comprised of magnetic and nonmagnetic particles in gradient magnetic fields

Distributions of particles in a suspension comprised of magnetic particles (MPs) and nonmagnetic particles (NPs) under gradient magnetic fields are vitally important for the preparation of magnetic-nonmagnetic functionally graded materials (FGMs). In the present study, the effects of magnetic field gradient, magnetic interaction between MPs and concentration of NPs on the distributions of particles in the suspension are investigated using a two-dimensional Monte Carlo simulation. The results show that a gradient distribution of MPs is formed under gradient magnetic fields and increases with increasing the field gradient. However, as the interaction between MPs increases, the distribution gradient decreases, accompanied by the formation of chain-like MP clusters. Moreover, NPs are found to hinder the translation of MPs along the field direction. As the NP concentration increases, the translation of MPs becomes difficult.     

Structurization of ferrofluids in the absence of an external magnetic field

Structural transformations in a model ferrofluid in the absence of an external magnetic field have been theoretically studied. The results agree with well-known laboratory experiments and computer simulations in showing that, if the concentration of particles and their magnetic interaction energy are below certain critical values, most particles form separate linear chains. If these parameters exceed the critical values, most particles concentrate so as to form branched network structures. The passage from chains to network has a continuous character rather than represents a discontinuous first-order phase transition.     

Magnetophoresis and diffusion of colloidal particles in a thin layer of magnetic fluids

Experimental and theoretical studies were carried out to investigate the spatial distribution of colloidal particles in magnetic fluids formed under the influence of magnetophoresis and gradient diffusion in a strong magnetic field. Several theoretical models, describing the equilibrium concentration profiles for rigid chain-like and quasispherical aggregates, are discussed. The experiment was made for four samples of magnetic fluids, differing in the average diameter of magnetic particles and the width of the particle size distribution. The analysis of the experimental data shows that the aggregates essentially change the concentration profile, making it nonlinear even in small (2 mm) magnetic fluid samples. Good agreement between the experimental and theoretical curves is observed in the case when the aggregates contain on the average 40-50 particles. The average diameter of single particles, calculated from the concentration profile curves, coincides with the average diameter, found from the magnetogranulometric analysis.     

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.     

Quasi-2D Monte Carlo simulations of phase transitions and internal aggregate structures in a highly dense suspension composed of magnetic plate-like particles

The phase transitions and the internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis have been investigated by means of the Monte Carlo method. The present study considered a quasi-2D system in order to clarify the influences of the volumetric fraction of particles and the magnetic field strength on particle aggregations and phase transitions. The internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation functions, orientational pair correlation functions, nematic and polar order parameters. The main results obtained here are summarized as follows. When the influence of the magnetic interaction between particles is of the same order of that of the perpendicular magnetic field strength, the particles form column-like clusters, and the internal structure of the suspension shows solid-like structures. For the case of a strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under the situation of a strong applied magnetic field, and the internal structure exhibits solid-like ones. The brick wall-like structures also appear for a relatively weak magnetic field applied along the in-plane direction despite a slightly smaller volumetric fraction compared with the case of the perpendicular applied magnetic field.     

On the theory of phase transitions in magnetic fluids

Particles of magnetic fluids (ferrofluids), as is known from experiments, can condense to bulk dense phases at low temperatures (that are close to room temperature) in response to an external magnetic field. It is also known that a uniform external magnetic field increases the threshold temperature of the observed condensation, thus stimulating the condensation process. Within the framework of early theories, this phenomenon is interpreted as a classical gas-liquid phase transition in a system of individual particles involved in a dipole-dipole interaction. However, subsequent investigations have revealed that, before the onset of a bulk phase transition, particles can combine to form a chain cluster or, possibly, a topologically more complex heterogeneous cluster. In an infinitely strong magnetic field, the formation of chains apparently suppresses the onset of a gas-liquid phase transition and the condensation of magnetic particles most likely proceeds according to the scenario of a gas-solid phase transition with a wide gap between spinodal branches. This paper reports on the results of investigations into the specific features of the condensation of particles in the absence of an external magnetic field. An analysis demonstrates that, despite the formation of chains, the condensation of particles in this case can proceed according to the scenario of a gas-liquid phase transition with a critical point in the continuous binodal. Consequently, a uniform magnetic field not only can stimulate the condensation phase transition in a system of magnetic particles but also can be responsible for a qualitative change in the scenario of the phase transition. This inference raises the problem regarding a threshold magnetic field in which there occurs a change in the scenario of the phase transition.     

分段表面放电沉积效率研究

利用一种结构紧凑的分段表面放电辐射源模块，详细研究了在不同电压、电容、气压实验条件下回路等效电阻、等效电感及放电能量沉积效率的变化规律，利用四分幅相机拍摄获得了不同实验条件下的放电等离子体通道图像，分析讨论了放电等离子体运动对放电能量沉积效率的影响，提出了提高能量放电沉积效率的有效途径。     

Rheological properties of polydisperse magnetic fluids. Effect of chain aggregates

A theoretical model of medium-density polydisperse magnetic fluids is proposed. The model takes into account that the major fraction of particles in typical ferrofluids is characterized by a magnetic core diameter of about 10 nm. In addition, there is a certain proportion of large particles with a core diameter of about 16 nm. As a result of the magnetic dipole interaction, the large particles form chain aggregates. Small particles, for which the magnetic dipole interaction energy (both with each other and with large particles) is smaller than the thermal energy, remain in the individual nonaggregated state. The distribution of chains with respect to the number of (large) particles and some rheological characteristics of the ferrofluids are determined. The proposed model is capable of explaining, in principle, the giant magnetoviscosity effect and a strong dependence of the rheological properties of ferrofluids on the shear rate observed in some recent experiments.     

脉冲高功率离子二极管的动力学模拟

基于等离子体的爆炸发射模型,利用自洽的2.5维的胞中粒子（PIC）模拟程序MAGIC模拟了平板型磁绝缘离子二极管中电子和质子的动力学特性。给出了电压为300kV,外加磁场为2倍临界磁场情况下的二极管特性,阴阳极间隙中带电粒子的空间和相空间分布,以及净电荷密度分布和电场分布,结果表明,引出束流密度比单离子Child-Langmuir公式计算的结果大5倍;外加磁场导致在阴极附近形成虚阴极。空间电荷使得阴阳极间隙中电场扰动和增强。     

Bi-functional Au/FeS (Au/Co3O4) composite for in situ SERS monitoring and degradation of organic pollutants

Journal of Nanoparticle Research - Magnetic particles (MPs) are known to respond to a magnetic field and can be moved by magnetic force, which make them good carriers in bioengineering and...     

Mechanism of chain formation in nanofluid based MR fluids

Mechanism of structure formation in bidispersed colloids is important for its physical and optical properties. It is microscopically observed that the mechanism of chain formation in magnetic nanofluid based magnetorheological (MR) fluid is quite different from that in the conventional MR fluid. Under the application of magnetic field the magnetic nanoparticles are filled inside the structural microcavities formed due to the association of large magnetic particles, and some of the magnetic nanoparticles are attached at the end of the chains formed by the large particles. The dipolar energy of the large particles in a magnetic nanofluid matrix becomes effective magnetic permeability (μ_eff) times smaller than that of the neutral medium. Inclusion of magnetic nanoparticles (∼10 nm) with large magnetic particles (∼3-5 μm) restricts the aggregation of large particles, which causes the field induced phase separation in MR fluids. Hence, nanofluid based MR fluids are more stable than conventional MR fluids, which subsequently increase their application potentiality.     

Parametric resonance in magnetic fluids

A new resonance effect in the nonlinear behaviour of magnetically anisotropic objects in an alternating external magnetic field is proposed. Ferromagnetic particles with a “frozen” magnetic moment (due to a strong magnetic anisotropy), when located in an external alternating magnetic field, are able to rotate (or vibrate) and to transfer energy from the external field to the medium. The numerical solution of the appropriate parametrically driven nonlinear equation shows all types of nonlinear dynamic behaviour, including transition to chaos. The sensitivity of the proposed phenemenon could be used for an experimental analysis of the size distribution of the ferromagnetic particles in a ferrofluid or of the size of “magnetic holes”.     

On the time evolution of transmittivity in magnetic fluids

When a magnetic fluid is subjected to a magnetic field, a part of the magnetic particles in the fluid agglomerates to form chains. Thus, the ferrofluid becomes optically anisotropic. In this work we describe optically observed patterns in some magnetic fluid films in applied parallel magnetic fields and optical effects of these, especially the optical transmittance. The most interesting experimental observation is that concerning the time dependence of relative transmittivity . For kerosene base ferrofluids relax rapidly at coupling and decoupling magnetic field, but for a transformer-oil magnetic fluid the relaxation times can attain (5–10) minutes, depending on the intensity of applied magnetic field.     

磁性液体中非磁性小球与磁性纳米颗粒的相互作用及磁组装

利用磁性液体与聚苯乙烯小球溶液混合得到的复合磁性液体, 研究了聚苯乙烯小球和磁性纳米颗粒在外加磁场作用下的动力学过程. 实验结果表明, 当外加磁场的方向平行于样品平面时, 聚苯乙烯小球在沿着磁场的方向上表现出相互吸引而形成链状结构, 其动力学过程可分为聚苯乙烯小球被反磁化产生相互吸引而形成短链的快过程以及短链间相互吸引形成长链的慢过程; 当外加磁场的方向垂直于样品平面时, 相邻聚苯乙烯小球表现出排斥的相互作用而形成短程有序的二维结构, 当磁场强度增加到一定的阈值时, 聚苯乙烯小球和磁性纳米颗粒形成的团簇会产生相互吸引而组装成复合式的花瓣结构. 关键词： 磁性液体 磁组装 非磁性颗粒     

Magnetization relaxation of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics driven by DC/AC magnetic field

The response of uniaxial anisotropic ferromagnetic particles with linear reaction dynamics subjected to alternating current(AC) or direct current(DC) bias magnetic field is evaluated by the reaction–diffusion equation for the probability distribution function of the molecular concentration in the spherical coordinate system. The magnetization function and the probability distribution function of the magnetic particles in the reaction system are derived by using the Legendre polynomials and Laplace transform. We discuss the characteristics of magnetization and probability distribution of the magnetic particles with different anisotropic parameters driven by a DC and AC magnetic fields, respectively. It is shown that both the magnetization and the probability distribution decrease with time increasing due to the reaction process. The uniformity of the probability distribution and the amplitude of the magnetization are both affected by the anisotropic parameters.Meanwhile, the difference between the case with linear reaction dynamics and the non-reaction case is discussed.     

Magnetic fluid in ionizing electric field

This article describes influence of strong (ionizing) electric field on sprayability of magnetic fluid containing colloid particles with size in the range from 10 to 20 nm of magnetite Fe₃O₄. Magnetic fluids can be based for example on both transformer oil and physiological solution for application in medical using (in human medical science research), that supports a fluid colloidal system. Further component of magnetic fluid is surfactant. It is acting as surface-active substance that prevents from nanometric dimension particle settlement. Magnetic fluid gets off nozzle with diameter in range 0.3–1.0 mm from container in surroundings of ionizing (i.e. strong) electric field (E > 10⁷ V m^?1). As a consequence of action of electric field it gives out suppression surface tension in fluid what leads onwards to decomposition of magnetic fluid ligament at the end of nozzle. The diameter of nozzle oneself respects basic theoretical calculations in regards of fluid concentration and thereinbefore its selected size. Magnetic fluid in dependency on its used liquid base has weak-polar till polar orientation polarization character. It gives out sprayability in non-homogeneous electric field E in combination with magnetic field of intensity H. Orientation of vectors Ê and ?, resp. induction of magnetic field B is defined by parallel or vertical direction. Results are confronted with measurements realized explicitly only at action of electric field (variable B = 0). In the case of magnetic field applications with permanent magnet together with electric non-homogeneous field it gives out unconventional dynamics of electrical charging particles of macroscopic dimension. Orientation particle track is influenced by orientation of field vector combinations. This phenomenon develops magneto-dielectric anisotropy, which oneself manifests behaviour of electrophysical quantities characterizing examination system. In consideration of technology utilization of this method it is very important to respect applied magnetic fluid concentration. Electrical characteristics were examined for volume concentration of magnetite particles in the range from 0.125% to 18%. Nevertheless efficiency optimization of given media suggests to boundary concentration of magnetic fluid of 4.0%, when it is in the regions of weak polar till polar material. Electrophysical research refers to exploitation of applied magnetic layer technology on dielectric insulating substances with inorganic origin as well as thin layer technology coating plastic foils created from macromolecular organic substance.     

Chain structures and clusters of particles with the mixed dipole–quadrupole interaction in smectic freely suspended nanofilms

The formation of unusual chain structures and clusters of particles with the mixed dipole–quadrupole interaction has been found in smectic nanofilms. Unlike topological dipoles and quadrupoles, the interaction between which leads to the formation of structures with finite interparticle distances, the particles with the mixed interaction touch each other and form stable chains and two-dimensional clusters. The orientation of particles in chains is intermediate between dipole and quadrupole chains. The variation of the interparticle distance and orientation of chains is explained qualitatively on the basis of the calculation of the с-director (field lines) near particles and the mutual arrangement of particles providing the minimum distortion of field lines.     

Structure formation in electromagnetically driven granular media

We report structure formation in submonolayers of magnetic microparticles subjected to periodic electrostatic and magnetic excitations. Depending on the excitation parameters, we observe the formation of a rich variety of structures: clusters, rings, chains, and networks. The dynamics and shapes of the structures are strongly dependent on the amplitude and frequency of the external magnetic field. We find that for pure ac magnetic driving the low-frequency magnetic excitation favors compact clusters, whereas high frequency driving favors chains and netlike structures. An abrupt phase transition from chains to a network phase was observed for a high density of particles.     

一种新型的基于磁性液体的光纤Sagnac磁场传感器

提出一种使用磁性液体的新型光纤Sagnac磁场传感器.磁性液体具有磁致可变双折射效应和二向色性,在外加磁场作用下,液体中的磁性纳米粒子沿磁场方向结链规则排列,形成各向异性.将其制成液体薄膜,放入具有一段保偏光纤的Sagnac环中,使光纤Saganc干涉仪的正弦形状干涉光谱可随外磁场变化.光纤中传输光垂直经过磁性液体薄膜...     

Two-dimensional Monte Carlo simulations of structures of a suspension comprised of magnetic and nonmagnetic particles in uniform magnetic fields

The structures of suspensions comprised of magnetic and nonmagnetic particles in magnetic fields are studied using two-dimensional Monte Carlo simulations. The magnetic interaction among magnetic particles, magnetic field strength, and concentrations of both magnetic and nonmagnetic particles are considered as key influencing factors in the present work. The results show that chain-like clusters of magnetic particles are formed along the field direction. The size of the clusters increases with increasing magnetic interaction between magnetic particles, while it keeps nearly unchanged as the field strength increases. As the concentration of magnetic particles increases, both the number and size of the clusters increase. Moreover, nonmagnetic particles are found to hinder the migration of magnetic ones. As the concentration of nonmagnetic particles increases, the hindrance on migration of magnetic particles is enhanced.