Ultrasound attenuation in ferrofluids

The absorption of acoustic energy by internal degrees of freedom of short chains is proposed as a new viable mechanism of ultrasound attenuation in ferrofluids. It is demonstrated that even though the volume fraction of the chains may be quite small, such an effect may reach the order of magnitude of viscous damping. In addition, by investigating the statistical properties of dimers in ferrofluids, it is shown that an applied magnetic field modifies the sound attenuation in a highly anisotropic manner. The proposed mechanism provides new insight into the fundamental issue of colloidal response, and, in particular, may lead to its utilization in novel experimental concepts.     

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.     

垂直圆管氮气-磁性液体两相流电磁感应现象

基于磁性液体与非磁性气泡磁导率的差异,理论分析与实验研究了氮气-磁性液体两相流的电磁感应现象。以颗粒体积浓度为3.83%的水基Fe₃O₄磁性液体为工质,同时采用物性与其相近的去离子水进行流型的可视化对比研究。研究结果表明,当管内为泡状流、弹状流、搅拌流时均能产生毫伏级的感应电压,且感应电压的峰峰值随着气体流量的增加而增大,另外根据感应电压随时间变化的波形可以辨识出管内的流型。     

Optical properties of one-dimensional soft photonic crystals with ferrofluids

We review the recent theoretical study on the optical properties of one-dimensional soft photonic crystals (1D SPCs) with ferrofluids. The proposed structure is composed of alternating ferrofluid layers and dielectric layers. For the ferrofluid, single domain ferromagnetic nanoparticles can align to a chain under the stimuli of an external magnetic field, thus changing the microstructure of the system. Meanwhile, nonlinear optical responses in ferrofluids are also briefly reviewed.     

Experimental investigation on the magneto-optic effects of ferrofluids via dynamic measurements

An experimental investigation on the origin of the magneto-optic effects of ferrofluids is presented. Dynamic measurements of their transverse and longitudinal magneto-optic effects and their magnetic properties have been performed. As ferrofluids are colloidal suspensions of magnetic particles in a liquid, the influence of two parameters has been studied: the magnetic anisotropy energy of particles and the carrier liquid viscosity for four ferrofluid samples. The interpretation and the comparison of results give some clues for a better understanding of magneto-optical effects of ferrofluids.     

The modulation of coupling in the relaxation behavior of light transmitted through binary ferrofluids

With an external magnetic field, a relaxation process is observed when the light transmit through binary ferrofluids composed of ferrimagnetic CoFe₂O₄ and paramagnetic p-NiFe₂O₄ nanoparticles similar to ferrofluids consisting only of CoFe₂O₄. Since only the ferrimagnetic nanoparticles are able to form field-induced chainlike structures for such binary ferrofluids by magnetic interaction between the particles, so the relaxation behavior of the transmitted light is caused mainly by the ferrimagnetic system. In the binary ferrofluids, the paramagnetic nanoparticles, regarded as magnetically polarized gas molecules, are restrained to occupy the space between the ferrimagnetic chains and distribute following the CoFe₂O₄ particle chains covering and diverging, producing a modulation effect on the relaxation behavior of the transmitted light. The modulation effect can be characterized by range and time parameters that describe the relationship of the relaxation behavior of the transmitted light to the properties of the binary ferrofluids and single CoFe₂O₄ ferrofluids.     

应用变换光学理论调控电磁场光束束宽

 根据变换光学理论,提出了利用超常材料调控光束束宽及将有限束宽光束变成线光束的理论方案,得到了所需超常材料的介电常数和磁导率的解析结果。应用这些结果,可以实现对光束束宽的调控或者将有限束宽光束压缩成线光束。利用有限元分析方法进行全波仿真,仿真结果与理论相符合,可应用于压缩或扩展电磁波束的能量密度。线光束中心处的场能量密度极大,可应用于增强非线性光学效应等方面。     

Relaxation behavior measuring of transmitted light through ferrofluids film

In this paper, relaxation behavior of transmitted light through thin ferrofluid film under an applied magnetic field is measured. The results show that the intensity of transmitted light through a ferrofluid film increases quickly as soon as an external magnetic field is applied then weakens with time. If uniformity of the field is poor, the transmission of light continuously decreases in a measured duration. Otherwise, the transmission of light will tend increasingly towards a stable value after it decreases to a minimum value while the gradient of the field is low. The relaxation time would increase to an order of some hundreds seconds magnitude and is dependent on the strength of magnetic field and viscosity of the ferrofluids. The field-induced relaxation behaviors of transmitted light through ferrofluids correspond to anisotropic microstructure of the ferrofluids under applied magnetic field. PACS 75.50.Mm; 78.20.Ls     

Water dynamics in ionic magnetic colloids studied by H nuclear magnetic resonance

In a previous nuclear magnetic resonance (NMR) study we observed that the NMR spectra of water in both surfacted and ionic ferrofluids are asymmetric and several orders of magnitude wider than the one of pure water. It has been proposed that this effect is produced by extremely strong magnetic field gradients in the intergrain volume and/or by surface interactions between the carrier liquid molecules and the grains surface. In the case of aqueous ionic ferrofluids the latter possibility should be interpreted as electric interactions between water (polar) molecules and the charges in the grain surface.

In this work we study a series of ionic and surfacted ferrofluids prepared at different magnetic grain concentrations and sizes, and with different surface charge densities. Our experiments clearly show that the sign and the density of the electric charge on the magnetic grains have no influence on NMR spectra. On the other hand, spectral widths increase with the magnetic grain concentration.     

Proper bases and internal energy in electromagnetic field theory

Orthonormal proper bases are determined for an electromagnetic field. An expression is proposed for the internal electromagnetic field energy that is a four-dimensional scalar. The Maxwell equations are written in the proper bases.Translated form Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 8–12, October, 1989.     

Information (no-energy) waves described by global potentials

The case where the magnetic flux inside an electronic loop varies with time is considered for an experiment in which the Aharonov-Bohm effect is observed. The electromagnetic field and potentials of a solenoid with an alternating current are studied. It is shown that the vector potential outside the solenoid contains a term corresponding to a zero electromagnetic field at the same point of space. This part of the potential (global potential) describes standing waves whose properties differ from those of ordinary electromagnetic waves and which can be conventionally called information potential waves. A method of detecting information waves is proposed involving an effect similar to the Aharonov-Bohm effect (quasi-Aharonov-Bohm effect). Unlike ordinary electromagnetic waves, these information waves are not involved in any energy interactions but they can carry information.     

磁性流体薄片高电压电流光学测量的研究

研究一种用磁性流体薄片对高压电流进行光学测量的电流传感器。设计原理基于磁性流体的光透射率随垂直磁场强度的变化而变化的关系。磁性流体受到垂直外磁场作用,原先无序的磁性颗粒将凝聚在一起形成条条磁链,从而引起磁性流体的光透射率的变化。采用磁性流体作为传感媒介设计的传感结构避免了基于法拉第效应的光磁式电流互感器对环境干扰敏感造成信噪比不高和传统的电磁式电流互感器的高压绝缘的缺陷。通过改变磁性流体的浓度和基液,或者改变传感头的结构,来达到实际需要的传感灵敏度和响应时间。     

Alternative mechanism for omega(0)/2 emission in laser-produced plasmas

Several models have been proposed to explain the broad spectral features characteristic of omega(0)/2 emission observed in laser-produced plasmas. In this article, the electromagnetic decay instability is examined as an alternative explanation for this emission. It is shown that the electromagnetic decay instability is able to explain some of the spectral features observed from laser-produced plasmas. In addition, the electromagnetic decay instability is consistent with two other features observed in experiments: the efficient generation of electromagnetic energy and the discrepancy in the levels of emission between the omega(0)/2 emission and the 3omega(0)/2 emission.     

Magnetic Behaviour and Heating Effect of Fe3O4 Ferrofluids Composed of Monodisperse Nanoparticles

Fe3O4 ferrofluids containing monodisperse Fe3O4 nanoparticles with different diameters of 8, 12, 16 and 18nm are prepared by using high-temperature solution phase reaction. The particles have single crystal structures with narrow size distributions. At room temperature, the 8-nm ferrofluid shows superparamagnetic behaviour, whereas the others display hysteresis properties and the coercivity increases with the increasing particle size. The spin glass-like behaviour and cusps near 190K are observed on all ferrofluids according to the temperature variation of field-cooled （FC） and zero-field-cooled （ZFC） magnetization measurements. The cusps are found to be associated with the freezing point of the solvent. As a comparison, the ferrofluids are dried and the FC and ZFC magnetization curves of powdery samples are also investigated. It is found that the blocking temperatures for the powdery samples are higher than those for their corresponding ferrofluids. Moreover, the size dependent heating effect of the ferrofluids is also investigated in ac magnetic field with a frequency of 55 kHz and amplitude of 200 Oe.     

Ferrite-based magnetic nanofluids used in hyperthermia applications

Magnetic ferrofluids (magnetic nanofluids) have received special attention due to their various biomedical applications such as drug delivery and hyperthermia treatment for cancer. The biological applications impose some special requirements. For example, the well-known iron oxide ferrofluids become undesirable because their iron atoms are poorly distinguishable from those of hemoglobin. A conceivable solution is to use mixed-ferrites (MFe₂O₄ where M=Co, Mn, Ni, Zn) to have a range of magnetic properties. These ferrites have attracted special attention because they save time, and because of their low inherent toxicity, ease of synthesis, physical and chemical stabilities and suitable magnetic properties. Based on the importance of ferrite particles in ferrofluids for hyperthermia treatment, this paper gives a summary on the physical concepts of ferrofluids, hyperthermia principal, magnetic properties and synthesis methods of nanosized ferrites.     

Saturation of the Substorm Mode by Magnetic Trapping

The purely growing electromagnetic instability, which was proposed by the author in an earlier publication as the onset mechanism for an isolated substorm, is shown to get saturated by the trapping of particles in the magnetic potential of the wave. The quasilinear diffusion plays a secondary role and is responsible for the generation of a high energy tail.     

基于变换光学的椭圆形透明聚集器的设计研究

基于麦克斯韦方程组在不同坐标系下具有形式不变性以及变换光学理论, 通过设计材料的本构参数(介电常数和磁导率)来引导电磁波的传播, 提出了具有电磁透明和电磁聚集两种功能的新型电磁器件-椭圆形透明聚集器的设计方法. 电磁波透明体不会阻挡电磁波的传播并且能够与斗篷内部进行交互; 电磁波聚集器是当电磁波入射到该装置上时, 电磁波能够被设计的装置按照要求集中到一个区域或者一个点, 实现电磁波能量的集中. 本文利用压缩变换和扩展变换推导出了这种电磁器件中各层的相对介电常数和相对磁导率的张量表达式, 并利用基于有限元算法的电磁仿真软件对该电磁器件进行了全波仿真验证, 得到了入射波从各个不同方向入射时磁场z 分量的分布图, 仿真结果证实了该设计方法和电磁参数的正确性. 最后还讨论了电磁器件存在损耗时的情况, 当损耗逐渐增大时, 器件的功能在一定程度上受到了削弱. 本文的设计方法为其他新型电磁器件的设计提供了一种新的思路.     

Equivalent damping and frequency change for linear and nonlinear hybrid vibrational energy harvesting systems

A unified approximation method is derived to illustrate the effect of electro-mechanical coupling on vibration-based energy harvesting systems caused by variations in damping ratio and excitation frequency of the mechanical subsystem. Vibrational energy harvesters are electro-mechanical systems that generate power from the ambient oscillations. Typically vibration-based energy harvesters employ a mechanical subsystem tuned to resonate with ambient oscillations. The piezoelectric or electromagnetic coupling mechanisms utilized in energy harvesters, transfers some energy from the mechanical subsystem and converts it to an electric energy. Recently the focus of energy harvesting community has shifted toward nonlinear energy harvesters that are less sensitive to the frequency of ambient vibrations. We consider the general class of hybrid energy harvesters that use both piezoelectric and electromagnetic energy harvesting mechanisms. Through using perturbation methods for low amplitude oscillations and numerical integration for large amplitude vibrations we establish a unified approximation method for linear, softly nonlinear, and bi-stable nonlinear energy harvesters. The method quantifies equivalent changes in damping and excitation frequency of the mechanical subsystem that resembles the backward coupling from energy harvesting. We investigate a novel nonlinear hybrid energy harvester as a case study of the proposed method. The approximation method is accurate, provides an intuitive explanation for backward coupling effects and in some cases reduces the computational efforts by an order of magnitude.     

Size effects in monodomain magnetite based ferrofluids

Ferrofluids based on two types of hybrid particles Fe₃O₄/β-cyclodextrin were prepared: Using monodomain (below 60 nm) magnetite nanoparticles with (A) non-superparamagnetic (non-SPM) behaviour and (B) with superparamagnetic (SPM) behaviour. We found a strong dependence of the hybrid particles’ magnetic properties on their size and homogeneity. In both types of ferrofluids we observed hyperthermia upon applying an ac electromagnetic field with frequency 40 kHz and amplitude 30 kA/m. The maximal ΔТ upon irradiation with duration of about 12 min for the non-SPM particles was 12 °C, while for the SPM ones it was 3.5 °C.     

A new spectrometric method,using a magneto-optical effect,to study magnetic liquids

We present an original polarimetric method to study the magnetic relaxation of ferrofluids. The Brownian relaxation of three ferrofluids with Fe₂CoO₄ particles dispersed in water, dibutyl phthalate and diethylene glycol is presented and compared with the theoretical model of Debye adapted to the magnetism. The results show that this system has a good sensitivity even at low volume concentration of particles and under low ac magnetic field. The analysis of the relaxation parameters leads to a good estimation of particles diameter and put in evidence a distribution of relaxation time.