Ground state structures in ferrofluid monolayers

Computer and theoretical investigation of particle arrangements in a thin film of a magnetic fluid at low temperatures is presented. The approach developed by us combines the simplicity of simulations and accuracy of the analytical model and allows studying particle aggregates and their properties. The systems under investigation were: a monodisperse and a bidisperse model in the absence of an external magnetic field, and a monodisperse model under the influence of an external field. Careful analysis of the most probable microstructures in a ferrofluid thin layer has been carried out at 0 K. The analysis of the stability of structures under thermal fluctuations allows making a conclusion about the microstructure of investigated system at low temperatures.     

Ground state microstructure of a ferrofluid thin layer

Using a fine weave of theoretical analysis and computer simulations, we found various aggregates of magnetic single-domain nanoparticles, which can form in a quasi-two-dimensional (q2D) ferrofluid layer at low temperatures. Our theoretical investigation allowed us to obtain exact expressions and their asymptotes for the energies of each configuration. Thus, for ferrofluid q2D layers it proved possible to identify the ground states as a function of the particle number, size, and other system parameters. Our suggested approach can be used for the investigation of ground state structures in systems with more complex interparticle interactions.     

Structure factor of ferrofluids with chain aggregates: Theory and computer simulations

In this paper we present theoretical and simulation results on the structure factor of mono- and bidisperse ferrofluids with chain aggregates, both with and without an applied external magnetic field. Chain distribution is obtained by the density functional theory (DFT). The radial distribution function (RDF) is calculated directly on the basis of the chain distribution and Fourier transformed to calculate the structure factor. An extensive comparison of the theoretical predictions to the results of the molecular dynamics computer simulations is provided. The proposed combined approach allows to elucidate the connection between experimentally observed small angle neutron scattering (SANS) images and the ferrofluid microstructure.     

Effects of oscillatory shear flow on chain-like cluster dynamics in ferrofluids without magnetic fields

Brownian dynamics simulations of interacting magnetic particles in a quasi-two-dimensional ferrofluid system are performed at zero temperature, under the influence of oscillatory shear flow in the absence of external magnetic fields. Starting from chain-like clusters of the particles, we study the time-dependent behavior of both magnetization and microstructures of the ferrofluid by changing values of two parameters, the shear rate strength and frequency of oscillatory shear flow. Simulation results show that there are three different dynamical regimes for the chain clusters dynamics, depending on these two parameters. Scaling behavior of the asymptotic magnetization is also observed for a certain range of parameters.     

Pair correlations in a bidisperse ferrofluid in an external magnetic field: theory and computer simulations

The pair distribution function g(r) for a ferrofluid modeled by a bidisperse system of dipolar hard spheres is calculated. The influence of an external uniform magnetic field and polydispersity on g(r) and the related structure factor is studied. The calculation is performed by diagrammatic expansion methods within the thermodynamic perturbation theory in terms of the particle number density and the interparticle dipole-dipole interaction strength. Analytical expressions are provided for the pair distribution function to within the first order in number density and the second order in dipole-dipole interaction strength. The constructed theory is compared with the results of computer (Monte Carlo) simulations to determine the range of its validity. The scattering structure factor is determined using the Fourier transform of the pair correlation function g(r) ? 1. The influence of the granulometric composition and magnetic field strength on the height and position of the first peak of the structure factor that is most amenable to an experimental study is analyzed. The data obtained can serve as a basis for interpreting the experimental small-angle neutron scattering results and determining the regularities in the behavior of the structure factor, its dependence on the fractional composition of a ferrofluid, interparticle correlations, and external magnetic field.     

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.     

Magnetic colloidal properties of ionic ferrofluids

Magnetic properties of colloidal suspensions of γ-Fe₂O₃ particles, obtained through a chemical synthesis, are investigated. Using an optical technique it is verified that these ionic aqueous ferrofluids are stable in high fields. The magnetization saturation of the particles is found independent of their size. Electron microscopy, magnetization and birefringence measurements allow us to separate the two superparamagnetic processes existing in such ferrofluid solutions: Bulk and Néel rotations. The Néel process is investigated through remanent magnetization of frozen solution.     

The quasi-magnetic-hysteresis behavior of polydisperse ferrofluids with small coupling constant

The magnetization behaviors of ferrofluids based on γ-Fe₂O₃/Ni₂O₃ composite nanoparticles of size about 11 nm have been investigated. The dipole coupling constant λ of these particles is so small (0.43) that they cannot form aggregates through magnetic interaction alone. Experimental results have shown that for a polydisperse ferrofluid with a particle volume fraction of ?_V=2.4%, the magnetization curve exhibits quasi-magnetic-hysteresis behavior, i.e., the demagnetization curve lies above the magnetization curve in a high field. However, for a more dilute γ-Fe₂O₃/Ni₂O₃ ferrofluid with ?_V=0.94%, the magnetization curve does not show such behavior. According to the bidisperse model for polydisperse ferrofluids, these magnetization behaviors may be attributed to field-induced effects of self-assembled pre-existing chain-like aggregates. For such pre-existing chain-like aggregates, the orientation of the moments inside the particles is not co-linear, so that during the magnetization and demagnetization processes, their apparent magnetizations at the high-field limit are different. As a consequence, the magnetization curve of the ferrofluid with ?_V=2.4% displays quasi-magnetic-hysteresis.     

SEM investigation of pillared microstructures formed by electrochemical anodization

Two methods of multistep electrochemical processing of aluminum films during porous anodic oxide films formation were developed to improve the morphology of pillared microstructures. Analysis of experimental data shows that the developed methods allow us to obtain the ordered pillared microstructures with high aspect ratio (≥4), radius 15 to 35 nm, and pillar density (3.0to8.5)×10⁹ cm^-2. High reproducibility of physical and morphological parameters of the structured layers for large-area samples is achieved. Such pillared matrices can be used for manufacturing of the microelements based on the mesoscopic and quantum effects: solar cells, controlled and uncontrolled emitter matrices for flat panel displays and emission cathodes, cathode luminescent displays, functional screens and polarizors for optoelectronics, etc. Received: 22 June 1998/Accepted: 24 June 1998     

Characterization of energy barrier and particle size distribution of lyophilized ferrofluids by magnetic relaxation measurements

The magnetic properties of a ferrofluid are strongly influenced by its particle size distribution. We analyzed a ferrofluid with an unknown particle size distribution as well as fractionated samples of the original material. The ferrofluid in our investigations consists of a mixture of maghemite and magnetite. We investigated these different samples using temperature-dependent magnetorelaxometry method. The evaluation of the Néel relaxation signal allows us a direct determination of the energy barrier distribution, which is one of the most important parameters of such systems of magnetic nanoparticles. The calculated particle volumes were compared with particle sizes determined by transmission electron microscopy.     

Enhanced nonlinear optical responses of materials: Composite effects

We review recent theoretical progress in understanding physical processes of composite effects on enhanced third-order nonlinear optical responses of various kinds of the recently-proposed nonlinear optical materials, namely, colloidal nanocrystals with inhomogeneous metallodielectric particles or a graded-index host, metallic films with inhomogeneous microstructures adjusted by ion doping or temperature gradient, composites with compositional gradation or graded particles, and magneto-controlled ferrofluid-based nonlinear optical materials.     

Experimental investigations on a branched tube model in magnetic drug targeting

Magnetic drug targeting (MDT) has been established as a promising technique for tumour treatment. Due to its high targeting efficiency unwanted side effects are considerably reduced, since drug-loaded nanoparticles are concentrated within a target region due to the influence of a magnetic field. This work presents experimental results that are based on systematic quantitative measurements on a branched tube model as a model system for a blood vessel supplying a tumour. The systematic measurements are summarized in novel drug targeting maps, combining e.g. the net amount of targeted nanoparticles, the magnetic volume force and also the position of the magnet. The model, the injection procedure and the ferrofluid are chosen close to the parameters of a medical application. This will allow transfer of the results to future medical investigations. This work will present a targeting map, where the concentration of the injected ferrofluid is in the range of experiments with an ex vivo bovine artery model.     

Reciprocity of Faraday effect in ferrofluid: Comparison with magneto-optical glass

The transmission light intensity method is carried out on a classical platform to study the reciprocity of Faraday effect in water-based Fe₃O₄ ferrofluid and its diluents. Setting the polarization direction of the analyzer at an angle of 45° to that of the polarizer, the switchable DC magnetic field and the alternating magnetic field are imposed to ferrofluid. The ferrofluid film is replaced by magneto-optical glass for contrastive experiments. The results indicate that ferrofluid is different with magneto-optical glass. Even though the direction of magnetic field is reversed, the rotation direction of the polarized light does not change for ferrofluid. The theoretical model of magneto-optical rotation was used to describe the origin of the reciprocity of Faraday effect in ferrofluid and the non-reciprocity in magneto-optical glass. These findings suggest that the magnetic moments of nanoparticles in ferrofluid tend to the same orientation with the magnetic field because of the rotation of particles.     

Frequency-domain birefringence measurement of biological binding to magnetic nanoparticles

Optical detection of the frequency-dependent magnetic relaxation signal is used to monitor the binding of biological molecules to magnetic nanoparticles in a ferrofluid. Biological binding reactions cause changes in the magnetic relaxation signal due to an increase in the average hydrodynamic diameter of the nanoparticles. To allow the relaxation signal to be detected in dilute ferrofluids, measurements are made using a balanced photodetector, resulting in a 25 μV/√Hz noise floor, within 50% of the theoretical limit imposed by photon shot noise. Measurements of a ferrofluid composed of magnetite nanoparticles coated with anti-IgG antibodies show that the average hydrodynamic diameter increases from 115.2 to 125.4 nm after reaction with IgG.     

Ferrofluidic Open Loop Pulsating Heat Pipes: Efficient Candidates for Thermal Management Of Electronics

Thermal management of electronic devices is presently a serious concern. This article investigates the thermal performance of a five-turn open-loop pulsating heat pipe in both start-up and steady thermal conditions. The effects of working fluid, namely water and ferrofluid, heat input, charging ratio, ferrofluid concentration, orientation, as well as application of magnetic field, are explored. Experimental results show that using ferrofluid enhances the thermal performance in comparison with the case of distilled water under certain conditions. In addition, applying a magnetic field on the open-loop pulsating heat pipe charged with ferrofluid improves its thermal performance. Charging ratios that lead to lower thermal are mentioned. Optimum concentration of ferrofluid in steady-state performance is 2.5 g/L. This study helps to design electronic cooling devices more efficiently.     

The measurements of anisotropy of ultrasound propagation and magnetic susceptibility in viscous ferrofluid

Results of experimental study of anisotropy of ultrasound propagation and anisotropy of magnetic susceptibility in a ferrofluid of considerable viscosity are reported. The dependence of the propagation velocity and absorption coefficient of ultrasonic wave on the angle between the direction of measurement and that of the magnetic field provides important information on the ferrofluid structure in a magnetic field. The results show that the ultrasonic absorption coefficient of a wave propagating in a viscous ferrofluid in a DC magnetic field depends mainly on the rotational degree of freedom. Measurements of the anisotropy of the magnetic susceptibility of a ferrofluid bring the information on the magnetic moment of the magnetic particles and their mean radius.     

磁性液体复合体在外磁场中的赝双折射效应和二向色性

研究了由非磁性聚苯乙烯颗粒弥散于煤油基Fe_3O_4磁性液体中制备而成的磁性液体复合体.该复合体双折射效应和线二向色性随外磁场变化.在相同的磁场条件下,复合体的双折射效应较纯磁性液体有减弱而二向色性较后者有所增强.文中采用一简化模型对结果给出了解释.     

Kink-induced transport and segregation in oscillated granular layers

We use experiments and molecular dynamics simulations of vertically oscillated granular layers to study horizontal particle segregation induced by a kink (a boundary between domains oscillating out of phase). Counterrotating convection rolls carry the larger particles in a bidisperse layer along the granular surface to a kink, where they become trapped. The convection originates from avalanches that occur inside the layer, along the interface between solidified and fluidized grains. The position of a kink can be controlled by modulation of the container frequency, making possible systematic harvesting of the larger particles.     

Multiple-step martensitic transformations in Ni-rich NiTi alloys--an in-situ transmission electron microscopy investigation

Multiple-step martensitic transformations in Ni-rich NiTi shape memory alloys have so far been rationalized on the basis of dislocation stress fields, coherency stress fields around Ni 4 Ti 3 precipitates and evolving Ni concentrations between precipitates during ageing. The primary objective of the present paper is to show that such transformations can also occur owing to heterogeneous microstructures that form during ageing of solution annealed defect-free materials. These microstructures are characterized by Ni 4 Ti 3 grain-boundary precipitation and by precipitate-free grain interiors. Two microstructures which give rise to two and three distinct differential scanning calorimetry (DSC) peaks on cooling from the B2 regime are subjected to in-situ cooling and heating cycles in the transmission electron microscope. Martensitic transformations are directly studied and the observations provide a new explanation for multiple-step martensitic transformations in Ni-rich NiTi alloys. Most importantly the results of the present study allow us to understand why DSC chart features on cooling from the B2 regime change during ageing, where they lose their one-step character (after solution annealing at 1123 K for 900 s) and evolve from two-step (after ageing at 773 K for 3.6 ks) to three-step (after ageing at 773 for 36 ks K) transformations.     

梯度磁场作用下光纤中磁流体光透射特性研究的建模与分析

对光纤中磁流体在梯度磁场作用下的光透射特性进行了研究,提出光纤中磁流体的光透射率变化主要来源于梯度磁场引起的磁流体密度分布变化。根据郎之万函数和流体理论,推导了光纤中磁流体在梯度磁场作用下的密度分布,并根据Beer-Lambert定律,得到磁流体光功率透射衰减和纳米粒子局部密度的关系,从而建立光纤中磁流体在梯度磁场作用下光透射特性的理论模型。进而对光纤中磁流体在不同梯度磁场作用下的光透射功率进行数值分析,得到不同磁场强度和磁场梯度下光纤中磁流体透射功率的变化规律。最后将数值分析的结果和实验数据进行对比,验证了模型的合理性, 同时也验证了梯度磁场作用下磁流体光透射功率的变化主要来源于磁流体密度分布变化的推论。