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We present computer simulations of magnetization dynamics of a ferrofluid whose particles can form internal heterogeneous structures. Our results demonstrate that evolution of these structures significantly affects rate of the fluid remagnetization. Due to this evolution, characteristic time of the magnetization relaxation of the simulated ferrofluids after stepwise change of the applied field was in the range of 2.9·10–3 to 1.7 s; that is several orders of magnitude more than the relaxation time in the systems of the single, non-interacting particles. The obtained results can be useful both for basic research of ferrofluids, as well as for bio-medical applications of these systems, like magnetic hyperthermia, magnetic biosensing, etc. [ABSTRACT FROM AUTHOR]

The paper presents the results of computer simulating the structure formation in nanodisperse magnetic fluids and the influence of this process on the kinetics of their magnetization reversal. The work considers a system of identical spherical single-domain ferromagnetic particles suspended in a Newtonian fluid with magnetic moments "frozen" into the particle bodies. The particles are involved in intense Brownian motion. The magnetic interactions of all particles with each other, as well as of the particles with an external magnetic field, are considered. The results have shown that the evolution of the internal structures upon a change in the external field can greatly, by several orders of magnitude, change the characteristic time of ferrofluid magnetization reversal. The results obtained may be useful for the development of both the general theory of these systems and many methods for their high-tech applications. [ABSTRACT FROM AUTHOR]

In this article, the convective flow in a vertical layer subjected to a consistent magnetic field with nonzero gravity condition is investigated. One side of the vertical walls is heated, while the opposite side is cooling. The magnetic field strength, field orientation angle, magnetization effect, and thermal deviation of vertical walls play important roles as controlling parameters on the flow stabilization. The goal of this investigation is to analyze the flow characteristics and find out the significant distinction between water and kerosene based magnetic fluids under the variation of thermal and magnetic effects. The numerical results are obtained by the pseudo-spectral Chebyshev expansion method. The properties of all instability modes caused by three major mechanisms, namely; thermomagnetic, thermogravitational and magneto-gravitational are analyzed. In the normal magnetic field, the wave speed responds faster, and it is recognized by a relatively small wave number in water based fluid than that in kerosene based fluid. In oblique magnetic field, the waves propagate faster in both kerosene and water based fluids with the field inclination angle increases, but they still propagate slower in kerosene based fluid comparatively in water based fluid. According to the linear or non-linear magnetization law, both upward and downward propagating waves in kerosene based fluid propagate slower, and they are recognized by greater wave numbers than that in water based fluid. It is found that the basic flow in water based fluid is much more stable than in kerosene based fluid. [ABSTRACT FROM AUTHOR]

Microfluidic systems have enormous potential for enabling point-of-care diagnostics due to a number of advantages, such as low sample volumes, small footprint, low energy requirements, uncomplicated setup, high surface-to-volume ratios, cost-effectiveness, etc. However, fluid mixing operations are constrained by molecular diffusion since the flow is usually in the laminar regime. The slow nature of molecular diffusion is a technological barrier to implementing fluid transformations in a reasonable time. In this context, magnetically actuated micro-mixers of different sizes, shapes, materials, and actuation techniques provide a way to enhance fluid mixing in microfluidic devices. In this paper, we review the currently existing micro-mixing technologies. From a fundamental perspective, the different magnetization models for permanent and induced dipoles are discussed. The single-particle dynamics in steady and oscillating magnetic fields is studied in order to determine the flow generated and the torque exerted on the fluid due to the magnetic particles. The effect of particle interactions, both magnetic and hydrodynamic, is examined. [ABSTRACT FROM AUTHOR]

The rheological characteristics of manganese zinc (Mn-Zn) ferrite magnetic nanofluid synthesized using co-precipitation technique were examined in the absence and presence of magnetic fields. The research formulates required conditions needed for the formation of a gelly-like structure. The impact of magnetic field and temperature on the rheological properties of Mn-Zn ferrite ferrofluid is investigated. When a magnetic field was applied, higher magnetoviscoelasticity and magnetoviscosity were formed. Other rheological parameters such as damping factor which is crucial for regulating and restricting vibrations in a system was also analyzed. A stiff, gel-like structure is produced when a magnetic field is applied, and the gel-like quality grows as the magnetic field increases; when the magnetic field is removed, the gel-like and rigidity of the structure is lost. At low temperatures, the liquid phase is dominated by solid-like particles, whereas at high temperatures, the liquid-like structure is dominant. This study reveals the conditions required for the creation of high viscous effect and the viscoelastic behavior induced by the field offers important insights for optimizing the Mn-Zn ferrite ferrofluid for a range of applications. Other criterial for gel-like structure formation such as low torque and deflection angle of the ferrofluid were also established. [ABSTRACT FROM AUTHOR]

Ferrofluids are a type of nanometer-scale functional material with fluidity and superparamagnetism. They are composed of ferromagnetic particles, surfactants, and base liquids. The main characteristics of ferrofluids include magnetization, the magnetoviscous effect, and levitation characteristics. There are many mature commercial ferrofluid damping applications based on these characteristics that are widely used in numerous fields. Furthermore, some ferrofluid damping studies such as those related to vibration energy harvesters and biomedical devices are still in the laboratory stage. This review paper summarizes typical ferrofluid dampers and energy harvesting systems from the 1960s to the present, including ferrofluid viscous dampers, ferrofluid inertia dampers, tuned magnetic fluid dampers (TMFDs), and vibration energy harvesters. In particular, it focuses on TMFDs and vibration energy harvesters because they have been the hottest research topics in the ferrofluid damping field in recent years. This review also proposes a novel magnetic fluid damper that achieves energy conversion and improves the efficiency of vibration attenuation. Finally, we discuss the potential challenges and development of ferrofluid damping in future research. [ABSTRACT FROM AUTHOR]

To find the relationship between the volume of ferrofluids injected into the sealing gap and the sealing pressure of magnetic fluid seal, a magnetic fluid sealing experimental equipment is well designed. The ferrofluids used in magnetic fluid seal experiments are expertly synthesized silicone oil based ferrofluids, which have a saturated magnetization as high as 31.4 kA/m. The finding of experiments and simulation results indicates that the sealing pressure has a positive relationship with the volume of ferrofluids when the volume is smaller than 0.5 mL. Based on those results, a theoretical model is established. The key factor affecting the sealing pressure is the thickness of liquid sealing barrier L , which is related to the volume of ferrofluids. [ABSTRACT FROM AUTHOR]

In this work, we study the heating process of colloidal ferrofluids by a magnetic field. The heating of the fluid occurs by the magnetic relaxation of the nanoparticles which provide thermal energy for the host liquid. In the limit of small volumes, the relaxation process occurs through the Néel mechanism since the magnetic nanoparticles present superparamagnetic behavior. Within this limit, we have used a microscopic model for the coupling to phonons and external magnetic field in order to model the relaxation mechanism and to obtain an expression for the heating rate of the fluid as a function of microscopic parameters. The analysis allows determining appropriate conditions for an optimal heating rate for ferrofluids based on superparamagnetic nanoparticles. [ABSTRACT FROM AUTHOR]

A relatively stable, non-magnetic, torus-shaped fluid droplet within a linearly magnetizable surrounding ferrofluid medium, and subject to the annular magnetic field induced by an electric current in a wire passing perpendicularly through its centre, has been found through the use of coupled finite element/boundary element computer simulations. [ABSTRACT FROM AUTHOR]

A one-step preparation method for cobalt- and iron-containing nanomaterials based on poly-N-phenylanthranilic acid (P-N-PAA) and magnetic nanoparticles (MNP) was developed for the first time. To synthesize the MNP/P-N-PAA nanocomposites, the precursor is obtained by dissolving a Co (II) salt in a magnetic fluid based on Fe3O4/P-N-PAA with a core-shell structure. During IR heating of the precursor in an inert atmosphere at T = 700–800 °C, cobalt interacts with Fe3O4 reduction products, which results in the formation of a mixture of spherical Co-Fe, γ-Fe, β-Co and Fe3C nanoparticles of various sizes in the ranges of 20 < d < 50 nm and 120 < d < 400 nm. The phase composition of the MNP/P-N-PAA nanocomposites depends significantly on the cobalt concentration. The reduction of metals occurs due to the hydrogen released during the dehydrogenation of phenylenamine units of the polymer chain. The introduction of 10–30 wt% cobalt in the composition of nanocomposites leads to a significant increase in the saturation magnetization of MNP/P-N-PAA (MS = 81.58–149.67 emu/g) compared to neat Fe3O4/P-N-PAA (MS = 18.41–27.58 emu/g). The squareness constant of the hysteresis loop is κS = MR/MS = 0.040–0.209. The electrical conductivity of the MNP/P-N-PAA nanomaterials does not depend much on frequency and reaches 1.2 × 10−1 S/cm. In the argon flow at 1000 °C, the residue is 77–88%. [ABSTRACT FROM AUTHOR]

The flow of Burgers' fluid in the magnetic field new mathematical modeling is introduced in this article which is heated convectively and maintained. The thermal energy transport aspects are examined by employing the space- and temperature-related heat source. In the present investigation, the homogeneous-heterogeneous reactions will present the features of scrutiny of the fluid concentration. For the purpose of dimensionless similarity transformations, ordinary differential equations (ODEs) are utilized practically. Developed ODEs are solved by introducing the concepts of Runge–Kutta–Fehlberg's fourth-fifth method. The graphs show the pertinent outcome. The relaxation time parameter is exhibited by diminishing the thermal distribution of Burgers' fluid property, and this will depend on the relaxation time factor. Biot number and retardation time factor behaviors are analyzed by opposing the behavior of the material factor of Burgers' fluid. The response of homogeneous strength is deteriorated by the concentration rate of the fluid, and this will augment the data using the heterogeneous response with greater magnitude. By using already published studies, it is investigated that the present investigation is validated. [ABSTRACT FROM AUTHOR]

Magnetic Particle Imaging (MPI) is an emerging imaging technique that uses magnetic nanoparticles as tracers. In order to analyze the quality of nanoparticles developed for MPI, a Magnetic Particle Spectrometer (MPS) is often employed. In this paper, we describe results for predictions of the nanoparticle harmonic spectra obtained in a MPS using three models: the first uses the Langevin function, which does not take into account finite magnetic relaxation; the second model uses the magnetization equation by Shliomis (Sh), which takes into account finite magnetic relaxation using a constant characteristic time scale; and the third model uses the magnetization equation derived by Martsenyuk, Raikher, and Shliomis (MRSh), which takes into account the effect of magnetic field magnitude on the magnetic relaxation time. We make comparisons between these models and with experiments in order to illustrate the effects of field-dependent relaxation in the MPS. The models results suggest that finite relaxation results in a significant drop in signal intensity (magnitude of individual harmonics) and in faster spectral decay. Interestingly, when field dependence of the magnetic relaxation time was taken into account, through the MRSh model, the simulations predict a significant improvement in the performance of the nanoparticles, as compared to the performance predicted by the Sh equation. The comparison between the predictions from models and experimental measurements showed excellent qualitative as well as quantitative agreement up to the 19th harmonic using the Sh and MRSh equations, highlighting the potential of ferrohydrodynamic modeling in MPI. [ABSTRACT FROM AUTHOR]

Herein we present the results of specific loss power (SLP) analysis of polydisperse water based ferrofluids, Fe3O4/PEG200 and Fe3O4/PEG6000, with average Fe3O4 particle size of 9 nm and 11 nm, respectively. Specific loss power was measured in alternating magnetic field of various amplitudes and at fixed frequency of 580.5 kHz. Maximum SLP values acquired were 195 W/g for Fe3O4/PEG200 and 60 W/g for Fe3O4/PEG6000 samples. The samples were labeled as superparamagnetic by magnetization measurements, but SLP field dependence showed deviation from the behavior predicted by the commonly employed linear response theory. The scope of this theory for both samples with wide particle size distribution is discussed. Deviation from the expected behavior is explained by referring to polydisperse nature of the samples and field dependent relaxation rates. [ABSTRACT FROM AUTHOR]

• The sealing performance of micro-nano composite magnetic powder on silicone oil is higher than that of water. • Temperature-resistant magnetic powder was prepared by doping microcrystalline graphite. • The microstructure model of temperature-resistant magnetic powder was proposed. • Temperature-resistant magnetic powder effectively reduces sealing temperature rise and friction torque. This paper mainly focuses on the research on micro-nano composite magnetic powder (MNC-MP) sealing liquid medium. Micro-nano composite magnetic powder and temperature-resistant magnetic powder (TR-MP) were prepared, and their magnetization curves were measured. The influence of temperature on the saturation magnetization of the TR-MP was analyzed. The results show that after heat treatment at 100 °C for 10 min, the specific saturation magnetization remains unchanged at 102.55A·m2/kg. The pressure resistance performance of the MNC-MP sealing liquids of different viscosities and the pressure change of the TR-MP sealing silicone oil were studied in the static sealing test. The results show that the higher the viscosity of the MNC-MP sealing liquid, the better the sealing performance, and pressure remains stable at 120 kPa when sealing dimethyl silicone oil. The dynamic sealing test analyzed the relationship between the pressure and rotation speed of the MNC-MP sealing silicone oil. We concluded that when MNC-MP seals dimethyl silicone oil, the effective time of the seal decreases with the increase of rotational speed, and the leakage rate after failure increases as the rotational speed increases. The changes in temperature and torque of MNC-MP and TR-MP sealing silicone oil with time at different rotational speeds were compared. Compared with MNC-MP, the temperature of the TR-MP seal can be reduced by 23.74 %, and the friction torque can be reduced by 17.58 %. [ABSTRACT FROM AUTHOR]

In this study, sintered NdFeB magnets with high residual flux densities and magnetic coercivities were manufactured by filling a die cavity with granulated NdFeB magnetic powder during pressing. In the binder-bound granules prepared by a spray drying method, the bonding force between the primary particles was relatively strong. However, the magnets obtained from these granules could not achieve high residual magnetic flux densities because it was difficult to align each particle in these granules along the magnetization easy axis through magnetic field pressing. Therefore, a weak liquid bridge force was utilized for particle binding. After preparing the liquid-bound granules containing 0.25 mass% terpineol, which does not evaporate easily at room temperature (-293 K) but undergoes debinding on baking, the angle of repose of the magnetic powder (representing its flowability index) decreased from 63° to 44.2°, while the deterioration of its magnetic properties was considerably suppressed. As a result, the residual magnetic flux density of the sintered magnet was equal to 99.3% of the value obtained for the non-granulated powder. In addition, the liquid-bound granules exhibited high filling rates and low weight variations, demonstrating their applicability for mass production. [ABSTRACT FROM AUTHOR]

Mn-Zn ferrite-based magnetic fluid is temperature-sensitive. Because of the low Curie temperature of Mn-Zn ferrite particles, magnetization is dramatically attenuated as the temperature rises. This temperature-dependent magnetization can be used to transport heat by magnetically driving a fluid via the application of a non-uniform magnetic field and heat. A potential application of this scheme is long-distance heat transport without the use of mechanical pumps. In the present work, a miniaturized magnetically-driven heat transport device with an inner diameter of 1.54 mm and a flow path length of 1,500 to 6,000 mm was fabricated. The influence of flow path length on the flow rate induced by the magnetic body force was investigated experimentally and theoretically. Although the flow rate decreased with increasing flow path length due to friction loss, the driving pressure increased with rising heater temperature because of the temperature-dependent magnetization. The influence of flow path length on the driving pressure is relatively insignificant. An increase in the flow path length leads to effects that both attenuate and enhance the driving pressure. Substantial long-distance (6,000 mm) heat transport was realized by applying a non-uniform magnetic field and heat. [ABSTRACT FROM AUTHOR]

We study a nonlinear coupling system of partial differential equations describing the dynamic of a magnetic fluid with internal rotations. The present mathematical model generalizes those discussed previously in the literature since actually the fluid is electrically conducting inducing additional nonlinearities in the problem and the dynamics of the magnetic field is described by the quasi-static Maxwell equations instead of the usual magnetostatic ones. We prove existence of weak solutions with finite energy first for the unsteady problem then for the steady one. [ABSTRACT FROM AUTHOR]

The dynamics of a magnetic fluid in a rotating magnetic field in the presence of thermal noise were studied by performing numerical simulations based on the Fokker-Planck equation. We first clarified the dynamic properties by numerical simulation such as the frequency dependence of the fluid magnetization, the field-dependent relaxation time, and the M-H curve in a rotating magnetic field. Using the simulation results, we modified an existing analytical model and obtained an empirical expression to quantitatively describe the particle dynamics. The simulation results were compared with experimental results in a rotating magnetic field. The frequency dependence of the magnetization of the magnetic fluid was measured over the linear and nonlinear regions. In order to make a quantitative comparison, the hydrodynamic and effective core size distributions were independently estimated from measurements of the ac susceptibility and the M-H curve. The phase lag and amplitude in a rotating magnetic field obtained from our simulation agreed well with the experimental results. [ABSTRACT FROM AUTHOR]

A model based on Monte Carlo technique is applied to investigating the superparamagnetic magnetite (Fe3O4) colloidal nanocrystal clusters (CNCs) proposed by Ge et al. [Nano Lett. 7, 3203 (2007)]. In other words, the model investigates the following three aspects of CNCS: the morphology of magnetic particles, the formation of field-induced chainlike patterns, and the induced evolution of the magnetization processes. It is shown that the parameters such as diameter, surfactant molecules per unit, and volume concentration of the magnetic fluid are significant factors that enable one to efficiently manipulate the morphology and magnetization process, which eventually leads to the efficient control of the fabrication and multiple applications. The experiment results also evidenced the presence of this self-assembled chain structures. [ABSTRACT FROM AUTHOR]

We developed an inexpensive and simple system for three-dimensional self-assembly of micron-sized nonmagnetic particles into millimeter-scale structures using the differential pressure exerted by ferrofluids in the presence of magnetic field gradients. We demonstrate it by assembling separate individual 5, 10, and 21 μm diam polystyrene beads into millimeter-sized spherical and ellipsoidal structures. The system can also self-organize its smaller components by volume and provide compressive forces of hundreds of piconewtons on millimeter-scale structures. Extensions of this method have assembled multicellular systems. [ABSTRACT FROM AUTHOR]

We present Brownian dynamics simulations of real charge-stabilized ferrofluids, which are stable colloidal dispersions of magnetic nanoparticles, with and without the presence of an external magnetic field. The colloidal suspensions are treated as collections of monodisperse spherical particles, bearing point dipoles at their centers and undergoing translational and rotational Brownian motions. The overall repulsive isotropic interactions between particles, governed by electrostatic repulsions, are taken into account by a one-component effective pair interaction potential. The potential parameters are fitted in order that computed structure factors are close to the experimental ones. Two samples of ferrofluid differing by the particle diameter and consequently by the intensity of the magnetic interaction are considered here. The magnetization and birefringence curves are computed: a deviation from the ideal Langevin behaviors is observed if the dipolar moment of particles is sufficiently large. Structure factors are also computed from simulations with and without an applied magnetic field H: the microstructure of the repulsive ferrofluid becomes anisotropic under H. Even our simple modeling of the suspension allows us to account for the main experimental features: an increase of the peak intensity is observed in the direction perpendicular to the field whereas the peak intensity decreases in the direction parallel to the field. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We derive the complete set of macroscopic dynamic equations for ferrofluids in the nematic liquid crystal phase under an external magnetic field. We include the magnetization as an independent dynamic degree of freedom. Special emphasis is laid on possible static and dynamic cross couplings between magnetization, director reorientations and flow. As examples we discuss the field dependence of the sound spectrum and the rheology of shear flow. [ABSTRACT FROM AUTHOR]

In this work, we show that the static magnetization curve of high-concentrated ferrofluids can be accurately approximated by the Mittag–Leffler function of the inverse external magnetic field. The dependence of the Mittag–Leffler function’s fractional index on physical characteristics of samples is analysed and its growth with the growing degree of system’s dilution is revealed. These results provide a certain background for revealing mechanisms of hindered fluctuations in concentrated solutions of strongly interacting of the magnetic nanoparticles as well as a simple tool for an explicit specification of macroscopic force fields in ferrofluid-based technical systems.

Magnetization relaxation equation (MRE) plays a primary role in numerous phenomena and applications involving ferrofluid dynamics. However, as yet there exist no MREs derived from first principles and applicable to concentrated and strongly interacting ferrofluids. In this paper, we derive a novel MRE based on the projection operator technique. It sufficiently accounts for interparticle correlations beyond the scope of previous models. The MREs by Martsenyuk, Raikher, and Shliomis and by Zubarev and Yushkov (ZY), respectively, for ideal and weakly nonideal (WNI) ferrofluids, are recovered as low-order approximations. We also investigate the magnetoviscous effects. For the first time, we unveil qualitatively the different roles played by short- and long-range interparticle correlations. The long-range correlation effect dominates in a WNI ferrofluid, and both our MRE and the ZY model are in quantitative agreement with simulations on field-dependent rotational viscosity. However, for strongly nonideal ferrofluids, short-range correlations can become substantial and compete with long-range correlations to reduce rotational viscosities. Our MRE is the first dynamic model faithfully capturing both short- and long-range correlations, thereby applicable to ferrofluids characterized by a broad range of concentrations and interacting strengths. It is expected to be a cornerstone for quantitative modeling of the dynamic response of ferrofluids to external fields or flow deformations. Because most commercial ferrofluids are designed to be strongly nonideal to enhance magnetic response, our theory may provide fresh insights for applications of realistic ferrofluids in industry and biomedicine. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the structural, magnetic, and antibacterial activities of CrxFe3−xO4 ferrofluids. CrxFe3−xO4 nanoparticles were synthesized from iron sand by a co-precipitation method using Cr3+ ion variants (x content) of 0, 0.05, 0.1, 0.15, and 0.2. The prepared nanoparticles were then characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and Fourier transform infrared (FTIR). The results of XRD analysis showed that the addition of Cr+ ions caused a shift in the diffraction peak, indicating the successful penetration of Cr+ ions into the Fe3O4 octahedral atomic site. The average particle size of Fe3O4 nanoparticles determined by TEM morphology analysis was 13.7 nm. In addition, the magnetic properties of CrxFe3−xO4 ferrofluids analyzed via VSM showed that the saturation magnetization ranged between 6.95 and 42.71 emu/g, while the magnetic moments were in the range of 7.47 × 10−19–19.76 × 10−19 J/T, which varied with the addition of Cr3+ ions. Furthermore, an analysis of the antibacterial activities of CrxFe3−xO4 ferrofluids revealed that bacterial viability decreased with increase in the number of Cr3+ ions. Thus, the CrxFe3−xO4 ferrofluids prepared from iron sand are potentially excellent antibacterial agents. [ABSTRACT FROM AUTHOR]

This article establishes the global existence of weak solutions to a model proposed by Rosensweig [Ferrohydrodynamics, Cambridge Monographs on Mechanics, Cambridge University Press, Cambridge, 1985] for the dynamics of ferrofluids. The system is expressed by the conservation of linear momentum, the incompressibility condition, the conserv ation of angular momentum, and the evolution of the magnetization. The existence proof is inspired by the DiPerna--Lions theory of renormalized solutions. In addition, the rigorous relaxation limit of the equations of ferrohydrodynamics towards the quasi-equilibrium is investigated. The proof relies on the relative entropy method, which involves constructing a suitable functional, analyzing its time evolution, and obtaining convergence results for the sequence of approximating solutions. [ABSTRACT FROM AUTHOR]

A magnetic fluid ring, which can be compared to a liquid O-ring, held at a given place by a magnetic field is used for various applications, such as seals, compressors, dampers, or sensors. The work of this type of system is the result of a number of factors, such as the distribution and magnitude of the magnetic field in the gap, rheological and magnetic properties, physical properties of the surrounding environment (interfacial phenomena) and operating conditions (differential pressure and relative velocity). The research area of this publication is related to the recognition and assessment of how the magnetic fluid properties influence the mechanism of the local loss of continuity for a single liquid ring. This applies to the case when the differential pressure on two sides of the magnetic fluid-free surface exceeds a critical value and a leak channel is formed. Previous studies have shown that the process of equalizing the pressure difference in this type of system is related to pressure jumps. This phenomenon has not been linked to such properties as saturation magnetization, dynamic viscosity or surface tension. This paper takes these factors into account. In addition, the shape of the stage (trapezoid and rectangle) on which the fluid is held and the magnetic field value are considered. The loss of continuity is an important issue and the obtained results allow this problem to be better understood and the magnetic fluid to be chosen for appropriate working conditions, e.g., in the case of multistage seals. The studied phenomenon may also contribute to the development of new types of pressure regulators or sensors. [ABSTRACT FROM AUTHOR]

Assuming the magnetic fluid a homogeneous dipole gas implies its magnetization description by the Langevin law derived for an ensemble of molecules of paramagnetic gas. In the paper, an approach used in the literature and aimed at finding the regularities of the magnetization process of paramagnets and of the possibilities of its routine application to characterize similar processes in magnetic colloids is critically analyzed. Basing on the results of experimental studies of the temperature and concentration dependences of magnetic susceptibility, effective fields in such media have been calculated and the conditions for applying models typical of molecular paramagnets have been found. [ABSTRACT FROM AUTHOR]

Magnetic fluid seal is the most important and mature application of magnetic fluid. Usually, gravity is ignored when analysing the performance of magnetic fluid seal and designing magnetic fluid sealing devices. With the expansion of magnetic fluid seal applications, gravity could seriously affect the performance of magnetic fluid seal, especially when the "O" ring diameter of magnetic fluid and sealing gap are both large. In this paper, the influence of gravity on the anti-pressure ability of magnetic fluid seal is theoretically analysed and proven by the experiments. The results show that magnetic fluid seal has a limit size that is directly proportional to the magnetic field gradient in the sealing gap and is inversely proportional to the ratio of magnetic fluid density to the magnetization. If the diameter of the magnetic fluid "O" rings, whose centre axis is perpendicular to the direction of gravity, is far smaller than the limit size, the influence of gravity on the anti-pressure ability of magnetic fluid seal can be neglected. However, if the size is close to or greater than the limit size, gravity would lead to a drop in the anti-pressure ability of magnetic fluid seal. [ABSTRACT FROM AUTHOR]

The effect of the dipolar particle interactions on the behavior of ferrofluids under a shear flow is not yet well understood. The equilibrium magnetization in the absence of flow is studied in Paper I [A. P. Rosa, G. C. Abade, and F. R. Cunha, "Computer simulation of equilibrium magnetization and microstructure in magnetic fluids," Phys. Fluids 29(9), 092006 (2017)]. In this paper, we present the results of magnetization and rheology in terms of a rotational viscosity obtained by applying Brownian dynamics simulations for a periodic magnetic suspension, where the many body long-range dipole-dipole interactions are calculated by the Ewald summation technique. The dependence of these macroscopic properties on the dipolar interactions is explored in ferrofluids undergoing both weak and strong shear flows in the presence of a uniform magnetic field. Through the simulations, the suspension microstructure is also analyzed in order to characterize the interplay between the structure and the investigated macroscopic properties. We show that for weak shear flows the dipole-dipole interactions produces a magnetization increasing. In contrast, a decrease in the ferrofluid magnetization with the shear rate is substantially intensified as the dipolar interactions are accounted for. Therefore, for strong shear flows, the dipolar interactions always have an effect of decreasing magnetization. In addition, while the dipolar particle interactions produce an increase in the rotational viscosity for weak flows, variations in the same property are not perceptible under the condition of strong flows. The numerical simulations show chain-structure formation oriented in the direction of the magnetic field (i.e., perpendicular to the direction of the shear) for weak flows, which explains the remarkable increasing of the suspension rotational viscosity as a function of the applied magnetic field and of the dipolar interactions parameters. A detailed comparison shows that our simulation results of magnetization and the rotational viscosity are in excellent agreement with approximate theoretical predictions reported in the literature for the case of noninteracting particles. [ABSTRACT FROM AUTHOR]

Highlights • Ferrofluids based on Mg x Fe 3−x O 4 NPs were synthesized by a chemical co-precipitation. • NPs size decreases while their aggregation in ferrofluids increases with Mg content. • All samples show superparamagnetic behaviour at room temperature. • Saturation magnetization increase of ≈40% compared to undoped Fe 3 O 4 was obtained. • SAR values of ferrofluides in AC fields displayed decrease with Mg increase. Abstract Ferrofluids based on magnesium substituted magnetite nanoparticles Mg x Fe 3−x O 4 (x = 0.1; 0.2; 0.4) were synthesised by a chemical co-precipitation method. Their physical properties have been compared with those of the magnetite based ferrofluid obtained by the same synthesis route. Both XRD and TEM studies showed particle size decrease with the increased Mg content while DLS experiments pointed to the more prominent aggregation of Mg-containing nanoparticles. Magnetic properties investigation conducted on the powder (i.e. dried) specimens showed decrease of magnetization values with increased Mg content except for the lowest concentration of x = 0.1 where substantial saturation magnetization rise of about 40% was recorded at room temperature. Heating abilities of the studied ferrofluids under the applied AC fields (SAR values) also showed decreasing trend with the increased Mg content even for x = 0.1 sample despite its elevated magnetization value. This trend has been understood as a consequence of the changed intrinsic nanoparticle properties such as size and magnetic anisotropy, as well as contribution of a collective behaviour due to an increased nanoparticle aggregation in Mg-doped systems. [ABSTRACT FROM AUTHOR]

With the development of science technology, ferrofluid seal has been intensively applied in mechanical industry, but more and more larger sealing stages are needed. At present, the literature mostly concentrated on numerical analysis with few pole pieces ferrofluid seals and focused on the critical pressure value in one-stage magnetic fluid seal, lacking of numerical simulation and experimental validation on the static pressure of multi-stage ferrofluid seals. On this basis, the analysis of larger number of sealing stages is presented. And numerical simulation along with experiment investigation was used to study the seal capacity of ferrofluids in different seal gaps and different seal stages in this paper. The results show that the theoretical value is well coincident with experimental value when the sealing gap is less than 0.2 mm. Increase the number of the pole tooth can significantly improve the seal capacity of ferrofluid seal, but the sealing pressure of ferrofluids does not increase linearly with the addition of the number of pole teeth. For the new structure, the pressure of one-stage pole tooth has more than doubled as the sealing clearance is 0.2 mm and 3.5 times increase when the sealing gap is 0.3 mm. [ABSTRACT FROM AUTHOR]

Using an altering chemically induced transition route, magnetic nanoparticles as well as nanoparticles modified with oleic acid have been prepared. The modified nanoparticles have been used to synthesize a high-quality kerosene-based ferrofluid, in which the mass fraction percentage of particles consisting of a γ-Fe2O3 core and an oleic acid coating, ϕm, amounted to 55%. Ferrofluids having lower concentrations of particles were obtained by diluting the mother ferrofluid. Magnetization measurements showed the as-synthesized ferrofluids to have excellent dispersity of the particles and field-induced inter-particle interactions. Optical transparency measurements confirmed that the ferrofluids showed a sensitive field-induced effect of chain-like aggregation, with redispersion of the particles after removing the magnetic field. According to results concerning both particle structure and ferrofluid density, the volume fraction percentage of particles, including both the γ-Fe2O3 core and the oleic acid coating, ϕ′v, as well as that of the γ-Fe2O3 alone, ϕv, can be deduced. [ABSTRACT FROM AUTHOR]

A one-step polyol method was utilized to prepare a stable aqueous iron/iron oxide ferrofluid. The dried powders were characterized by x-ray diffraction, electron microscopy, x-ray absorption spectroscopy, and vibrating sample magnetometry for the determination of phase, morphology, and magnetic properties. To show its potential for imaging applications, the ferrofluid was also investigated as a magnetic resonance imaging contrast agent. [ABSTRACT FROM AUTHOR]

We report the systematic dc and ac susceptibility studies on the particle blocking and carrier fluid freezing effects on the magnetization and relaxation processes in two different ferrofluids composed of Fe3O4 nanoparticles (mean size of ∼14 nm) suspended in hexane and dodecane, which respectively have freezing temperatures below (178 K) and above (264 K) the blocking temperature of magnetic nanoparticles (∼200 K). Experimental results reveal that these effects play a key role in the formation of glasslike peaks and magnetic anomalies in ferrofluids. Quantitative fits of the frequency dependent ac susceptibility to the Vogel–Fulcher model τ=τo exp[Ea/k(T-To)] clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics. [ABSTRACT FROM AUTHOR]

A magnetic fluid micromixer with high mixing efficiency using nonuniform magnetic field produced by tapered magnets is proposed. From the numerical results, the mixing efficiency increases with the saturation magnetization of the magnets. The larger the saturation magnetization is, the stronger the magnetic force acted on the magnetic fluid will be. On the other hand, asymmetric magnetic stray field that induces circulation between two fluids leads to higher mixing efficiency than a symmetric one. [ABSTRACT FROM AUTHOR]

In this paper, the effect of different parameters on Fe3O4/water nanofluid preparation characteristics is investigated experimentally. Two criteria, stability and magnetism, are employed to characterize prepared ferrofluids. Dynamic light scattering methods (DLS) distribution and transmission electron microscopy (TEM) images are applied in nanoparticle size investigation. Two-step preparation method is used to prepare the ferrofluid samples. Zeta potential and vibrating sample magnetometer (VSM) methods are used to study the stability and magnetism characteristics of prepared ferrofluid samples, respectively. The effect of six parameters (surfactant material, surfactant mass, heater stirring speed, heater stirring time, pH, initial sonication time and final sonication time) with three levels and one parameter (surfactant material) with six levels on the stability and magnetization is considered. The Taguchi method is applied in design of experiments, and 18 samples are prepared. The results show that the effective parameters on the stability of the prepared ferrofluid as their importance are: surfactant material, pH number, initial sonication time, surfactant mass, final sonication time, heater stirring speed and heater stirring time, respectively. According to magnetization viewpoint, the order of importance for effective parameters is: surfactant material, surfactant mass, pH number, final sonication time, heater stirring time, initial sonication time and heater stirring speed, respectively. [ABSTRACT FROM AUTHOR]

Abstract An experimental 1H NMR relaxometry investigation on iron oxide nanoparticles with different magnetic core size and coated with PolyAcrylic Acid (PAA), is presented. A full structural, morphodimensional and magnetic characterization of the nanoparticles has been performed by means of X-ray diffraction, Dynamic Light Scattering, Transmission Electron Microscopy, Atomic Force Microscopy and SQUID DC magnetometry. The application of a heuristic model for the field dependence of the NMR relaxivity curves allowed us to evaluate the distance of minimum approach of the solvent molecules from the magnetic centers, and to conclude that the local correlation times, namely the Neél time τ N and the diffusion time τ D related to the magnetization reversal and to the diffusion process respectively, depend strongly on the core size. A preliminary evaluation of their r 2 efficiency as Magnetic Resonance Imaging (MRI) contrast agents is also performed by means of a universal scaling law model. The results of our experimental investigation should allow to tailor the physical properties of the nanoparticles for obtaining systems with a resultant contrast efficiency optimized for the in-vivo application of MRI at pre-clinical and clinical level. Highlights • Synthesis of stable colloidal suspensions of iron-oxide-based magnetic nanoparticles. • Ferrofluids coated by PolyAcrylic Acid possess variable magnetic core diameters. • The nuclear spin dynamics mechanisms of ferrofluids are studied by means of Nuclear Magnetic Resonance. [ABSTRACT FROM AUTHOR]

Magnetic fluid is a new type of functional materials. The seal of reciprocating shaft is a typical application of the magnetic fluid. However, there is little research on the magnetic fluid reciprocating seals, thus improving the sealing life and seal capacity is the key problem of studying the reciprocating seals. This paper presents a structure of the seal to test the sealing life and other characteristics. The magnetic field of the seal is estimated using the finite-element method. The thickness of the magnetic fluid on the surface of reciprocating shaft and the flow rate of magnetic fluid are obtained theoretically. The experimental result shows that the static seal capacity is related to the fluid volume. The static seal capacity obtained by experiment is approximately equal to the value through the simulation with sufficient magnetic fluid. The relationship between sealing life and the motion of the reciprocating shaft is obtained experimentally. The sealing life decreases rapidly with the increase of the stroke and velocity. [ABSTRACT FROM AUTHOR]

Single crystals of Fe-substituted Cu 2 Mn 1-x Fe x BO 5 ludwigites have been synthesized using flux technique (x = 0.2, 0.4, 0.5 – in the initial flux system). Structural properties of the synthesized compounds were studied by the single crystal and powder X-ray diffraction analysis. Obtained results were analyzed in the relationship with parent compound Cu 2 MnBO 5 . It was revealed that the type of monoclinic distortions of Fe-substituted ludwigites is different from the structure of Cu 2 MnBO 5 . The real cation composition and local structure of Cu 2 Mn 1-x Fe x BO 5 ludwigites were studied using XANES and EXAFS techniques, respectively. Analysis of field and thermal dependencies of magnetization showed a high dependence of the magnetic properties of these ludwigites on x with changing the type of magnetic ordering. [ABSTRACT FROM AUTHOR]

Abstract A large radial clearance (0.7 mm) static and dynamic magnetic fluid seal is designed and its performance is tested. The radial clearance of 0.7 mm is quite higher than the most of the commercial seals which have maximum radial clearance of 0.3 mm. Hydrostatic magnetic fluid “plug” is created in 5.94 mm glass tube and it is tested for various plug heights with respect to the pole piece height. Results show that as plug height increases the pressure holding capacity increases and then for optimum fluid height it becomes maximum. The increase in fluid magnetization also increases the pressure holding capacity of fluid plug. Using these results the dynamic seal having a large radial clearance (0.7 mm) is designed and simulated using FEMM. The prototype test set-up is fabricated with multiple magnetic sources for 0.7 mm radial clearance between the rotating shaft (DIA: 25 mm) and the pole piece. The seal assembly is tested for transformer oil based magnetic fluid for variable speed and the number of stages under vacuum pressure of 10−3mbar. Results show three stages are sufficient to achieve 10−2mbar pressure difference at 1500 rpm. Such seal can work as the bearing protection in low vacuum devices. Highlights • Large diameter hydrostatic magnetic fluid “plug” seal is tested for various plug height. • Results of loading capacity matches with the values obtained from the magnetization curve. • The prototype test set-up (shaft dia: 25 mm) for large radial clearance is fabricated for study. • The three stages of magnetic fluid seal holds 10−2 mbar pressure difference at 1500 rpm. [ABSTRACT FROM AUTHOR]

The paper gives an assessment of the viscosity increment (magnetoviscous effect) in a thin near-wall layer of a column of magnetic fluid which oscillates in a tube when applying a strong transverse magnetic field. The value of the magnetoviscous effect was calculated using a formula derived from two different theoretical approaches and applied to previously published experimental results of complex measurements of the oscillation frequency and saturation magnetization of a magnetic fluid under the assumption that there is no viscosity field dependence. [ABSTRACT FROM AUTHOR]

We prove the global existence strong solutions for the system of partial differential equations corresponding to the Shliomis model for magnetic fluids in exterior domains without regularization terms in the magnetization equation under the assumption of small data and also small coupling parameter. [ABSTRACT FROM AUTHOR]

We establish the existence of static doubly periodic patterns (in particular rolls, squares and hexagons) on the free surface of a ferrofluid near onset of the Rosensweig instability, assuming a general (nonlinear) magnetization law. A novel formulation of the ferrohydrostatic equations in terms of Dirichlet-Neumann operators for nonlinear elliptic boundaryvalue problems is presented. We demonstrate the analyticity of these operators in suitable function spaces and solve the ferrohydrostatic problem using an analytic version of Crandall-Rabinowitz local bifurcation theory. Criteria are derived for the bifurcations to be sub-, super- or transcritical with respect to a dimensionless physical parameter. [ABSTRACT FROM AUTHOR]

Magnetically responsive and flexible bacterial cellulose (BC) membranes were successfully fabricated using a simple diffusion of a ferrofluid solution. BC hydrogels were either water-substituted by alcohol (BC-N) or freeze dried (BC-F) prior to their immersion in the ferrofluid. The presence of both crystalline BC and Fe 3 O 4 phases, and the homogeneous distribution of nanoparticles (NPs) in BC nanofibrils were observed. Higher concentrations of Fe 3 O 4 NPs were found in the BC-N samples than for the BC-F samples. Higher magnetization in the BC-N samples was observed compared to the BC-F samples. Mechanical properties tests showed the higher strength and Young’s modulus for the BC-F samples was possibly due to their more compacted nanostructure compared to BC-N. Using this simple process, the magnetic BC membranes show elastic properties upon deformation, returning to their original shape without damage. Also, they were highly sensitive to external magnetic forces giving them potential for many applications. [ABSTRACT FROM AUTHOR]

This investigation purposes to study the magnetic fluid based squeeze film behavior on transversely rough stepped plates with the influence of couple stress. Using the well-known stochastic model of Christensen and Tonder the roughness effect has been evaluated. The magnetic fluid flow model of Neuringer - Roseinweig has been adopted to obtain the influence of magnetization. The governing Reynolds' type equation is derived on the basis of stokes microcontinum theory for couple stress fluid. For the expression of pressure distribution, the stochastically averaged Reynolds' type equation is solved. which results in calculation of load carrying capacity. The graphical outcomes also presented in tabular form suggest that although the bearing suffers on account of roughness, the magnetization and couple stress effect save the situation, as this combination does not allow the load carrying capacity to fall rapidly. However, in the case of negatively skewed roughness the magnetization goes a long way in dropping the adversarial influence of roughness by selecting an appropriate value of couple stress parameter when variance (-ve) is involved. It is found that the couple stress effect, alone may not be sufficient to counter the negative influence of transverse roughness and porosity. However, in almost all situations the ferrofluid lubrication adds significantly to the positive effect of couple stress to overcome the adversarial outcome of porosity and roughness. Further, the position of step plays a vital role for an all-round enhancement of the bearing performance. [ABSTRACT FROM AUTHOR]

One of the main engineering applications of ferrofluids is in magnetic fluid seals, working in rotary motion. An area, which has so far been insufficiently examined, is the mechanism of leaks between seal stages in this type of seal. This is an important issue in order to understand the principle of the seal operation and to properly design this type of seal. This paper presents the results of research into the mechanism of leaks between stages in multi-stage magnetic fluid seals. The tests were conducted when the pressure of the sealing air increased and reached its maximum value. The influence of the magnetic fluid properties, such as saturation magnetization or dynamic viscosity, was taken into account. Studies were performed for different seal temperatures. Experiments allowed models for determining the maximum pressure of the multi-stage seal to be proposed, based on the information about this pressure for a one-stage seal. This paper presents the comparative results of the critical pressure value obtained from experimental studies and calculations based on numerical simulations. [ABSTRACT FROM PUBLISHER]