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Magnetic fluids, also known as ferrofluids, are versatile functional materials with a wide range of applications. These applications span from industrial uses such as vacuum seals, actuators, and acoustic devices to medical uses, including serving as contrast agents for magnetic resonance imaging (MRI), delivering medications to specific locations within the body, and magnetic hyperthermia for cancer treatment. The use of a non-wettable immiscible liquid substrate to support a layer of magnetic fluid opens up new possibilities for studying various fluid flows and related instabilities in multi-phase systems with both a free surface and an interface. The presence of two deformable boundaries within a ferrofluid layer significantly reduces the critical magnetic field strength required to transform the layer into an organized system of drops or polygonal figures evolving according to the intensity, frequency and direction of the considered magnetic field. This paper experimentally investigates this problem by assuming a uniform magnetic field perpendicular to the surface. This specific subject has not been previously explored experimentally. The critical magnetic field intensity required to destabilize the ferrofluid layer is determined based on the layer's thickness and the fluid's initial magnetic susceptibility. It is demonstrated that the critical magnetic field strength needed to disrupt the initially continuous ferrofluid layer increases with the layer's thickness. Conversely, an increase in the ferrofluid's magnetic susceptibility results in a decrease in the critical magnetic field strength. The emerging droplet structures are analyzed in terms of the number of drops, their size, and the periodicity of their arrangement. The number of droplets formed depends on the initial thickness of the layer, the presence or absence of a stable rupture in the upper layer, and the rate at which the magnetic field strength is increased to the critical value. A characteristic viscous time is proposed to evaluate the decomposition of the ferrofluid layer, which depends on the duration of the magnetic field's application. The experimental data on the instability of a ferrofluid layer on a liquid substrate are compared with the theoretical results from the study of "magnetic fluid sandwich structures" conducted by Rannacher and Engel. This comparison highlights the similarities and differences between experimental observations and theoretical predictions, providing a deeper understanding of the behavior of ferrofluid layers under the influence of magnetic fields. [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]

This paper introduces the prototype design, magnetic field analysis and experimental test of a double-rotating ferrofluid vane micropump with an embedded fixed magnet. The micropump is based on the working principle of a positive-displacement pump, as well as the magnetic characteristics and flow properties of magnetic fluid. Through the numerical analysis of the pump cavity magnetic field and the experimental test, the structural parameters of the micropump are optimized reasonably. The pumping flow and pumping height of the micropump were characterized at different driving speeds. The maximum pumping flow rate is approximately 410 μL/min, and the maximum pumping height is approximately 111.4 mm water column. The micropump retains the advantages of simple structure, easy manufacture, flexible control, self-sealing, self-lubrication, low heat production, etc., and can block the pumped liquid backflow. The resulting double-rotating ferrofluid blades can improve pumping efficiency and pumping capacity, and can improve pumping reliability and stability to a certain extent. [ABSTRACT FROM AUTHOR]

Ferrofluids are colloidal suspensions made of nanoscale ferromagnetic particles suspended in a base fluid. It has various medical applications like cell separation, drug targeting, magnetic resonance imaging, etc. Due to the wide range of uses for ferrofluids, this investigation looked into the way that how the fluid flows and transfers heat in the presence of thermal radiation, a porous medium, transverse magnetic field, and heat sink or source impacts. The current study examines a unique corner-heated enclosure problem with a quadratic Boussinesq approximation fluid flow and heat transfer under natural convection circumstances. The square cavity of length (L) is considered with hot and cold slits at the opposite–opposite corners, and it is filled with cobalt ferrite CoFe2O4–water H2O ferrofluid. Middle portion of the all the walls is adiabatic. The implications of magnetohydrodynamic are examined employing CoFe2O4 nanoparticles. The Marker-And-Cell method with finite difference technique is employed for the solution of the dimensionless constitutive equations. In this regard, physics of the problem is well explored for the various parameters such as Rayleigh number (Ra), Darcy number (Da), Hartmann numbers (Ha), solid volume fractions (φ) , thermal radiation parameter (Rd), nonlinear Boussinesq approximation parameter (δ) , and inclination angle of the cavity (ω) and are graphically represented. Suspending the CoFe2O4 ferroparticles in the base liquid water increases heat transmission by 14.5 % . A heat source or sink enhances the fluid velocity and boosts heat transmission by 91.93 % . [ABSTRACT FROM AUTHOR]

Ferrofluids containing magnetic nanoparticles represent a special class of magnetic materials due to the added freedom of particle tumbling in the fluids. We studied this process, known as Brownian relaxation, and its effect on the magnetic properties of ferrofluids with controlled magnetite nanoparticle sizes. For small nanoparticles (below 10 nm diameter), the Néel process is expected to dominate the magnetic response, whereas for larger particles, Brownian relaxation becomes important. Temperature- and magnetic-field-dependent magnetization studies, differential scanning calorimetry, and AC susceptibility measurements were carried out for 6 and 13.5 nm diameter magnetite nanoparticles suspended in water. We identify clear fingerprints of Brownian relaxation for the sample of large-diameter nanoparticles as both magnetic and thermal hysteresis develop at the water freezing temperature, whereas the samples of small-diameter nanoparticles remain hysteresis-free down to the magnetic blocking temperature. This is supported by the temperature-dependent AC susceptibility measurements: above 273 K, the data show a low-frequency Debye peak, which is characteristic of Brownian relaxation. This peak vanishes below 273 K. [ABSTRACT FROM AUTHOR]

The thermal behavior of an electrically non-conducting magnetic liquid flowing over a stretching cylinder under the influence of a magnetic dipole is considered. The governing nonlinear differential equations are solved numerically using a finite element approach, which is properly validated through comparison with earlier results available in the literature. The results for the velocity and temperature fields are provided for different values of the Reynolds number, ferromagnetic response number, Prandtl number, and viscous dissipation parameter. The influence of some physical parameters on skin friction and heat transfer on the walls of the cylinder is also investigated. The applicability of this research to heat control in electronic devices is discussed to a certain extent. [ABSTRACT FROM AUTHOR]

Purpose: Ferrofluid seals are known for their low friction torque and high tightness. However, they have some limitation due to the allowable rotational speed. The work presented here analyzes the performance of newly designed seals which are a combination of a ferrofluid and a centrifugal seal. The new seals can operate at high speeds. The purpose of this study is to theoretically predict the performance of combined seals. Design/methodology/approach: Three seals were designed and selected for analysis. A version of the seals with a nonmagnetic insert is also considered, the purpose of which is to facilitate the installation and return of ferrofluid during low rotational speeds. The analyses were based on combining the results of numerical simulation of magnetic field distribution with mathematical models. Findings: A combination of ferrofluid sealing and centrifugal sealing is possible. Analyses showed that the combined seal could hold a minimum pressure of 190 kPa in the velocity range of 0–100 m/s. The problem with this type of seal is the temperature. Originality/value: New seal designs are presented. Key parameters that affect the seal operation are discussed. A methodology that can be used in the design of such seals is presented. Peer review: The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2023-0221/. [ABSTRACT FROM AUTHOR]

In this article, the equations governing the constant ferromagnetic current are investigated. The Lorentz force restrains this ferrofluid flow in a semi-porous valve. Analyzes were performed on three sub-particle fluids: kerosene and blood, water and magnetite. Modeling in the Cartesian coordinate system using the relevant equations was investigated. A slight thinning should be considered in the lower part of this channel. This research has used two Akbari-Ganji methods (AGM) and finite element method (FEM) to solve the equations. Nonlinear differential equations are solved using the above two methods. In the finite element model, the effect of changing the Hartmann number and the Reynolds number on the flow velocity and the derivatives of the velocity and shear stress of the fluid were investigated. As the Hartmann number increases, the velocity decreases in both directions. The Reynolds number changes in different slip parameters, which shows the opposite behavior for the two directions. Also, the insignificant effect of volume fraction of nanoparticles on velocity and its derivatives and shear stress was investigated. The results of solving the equations with the above two methods were compared with HAM. The results obtained using AGM and FEM and their comparison with previous researches have led to complete agreement, which shows the efficiency of the techniques used in this research. [ABSTRACT FROM AUTHOR]

Mechanical vibrations can be effectively converted into electrical energy using a liquid type of energy harvesting device comprised of a ferrofluid and a permanent magnet-inductor coil assembly. Compared to solid vibration energy harvesting devices, the liquid nature of the ferrofluid overcomes space conformity limitations which allow for the utilization of a wider range of previously inaccessible mechanical vibration energy sources for electricity generation and sensing. This report describes the design and the governing equations for the proposed liquid vibration energy harvesting device and demonstrates vibration energy harvesting at frequencies of up to 33 Hz while generating up to 1.1 mV. The proposed design can continuously convert mechanical into electrical energy for direct discharge or accumulation and storage of electrical energy. [ABSTRACT FROM AUTHOR]

Mixed convection of ferrofluid in square configurations having various iso‐perimetric shaped obstacles has been investigated for fluid flow and heat transfer characteristics. The iso‐perimetric shapes are allowed to vary in size and tested at three different vertical positions subject to adiabatic and isothermal conditions. Simulations have been carried out for various choices of controlling parameters using the higher order mixed finite element method. In particular, the higher order and stable finite element pair of cubic and quadratic (ℙ3/ℙ2$$ {\mathrm{\mathbb{P}}}_3/{\mathrm{\mathbb{P}}}_2 $$) approximations on the hybrid grid is utilized to achieve the discretization of the governing non‐dimensional equations of mass, momentum and energy. The adaptive Newtons method is chosen to solve the discrete system of nonlinear equations. The present solver has been tested against a well‐known open source finite element analysis tool (FEAT2) and published results in the literature. The impact of governing parameters is investigated in the ranges as nanoparticles volume fraction ϕ(0%≤ϕ≤15%$$ \phi\ \Big(0\%\le \phi \le 15\% $$), the Richardson number Ri(0.01≤Ri≤5)$$ Ri\ \left(0.01\le Ri\le 5\right) $$, the perimeter of the obstacle Γ(0.2π≤Γ≤0.6π)$$ \Gamma\ \left(0.2\pi \le \Gamma \le 0.6\pi \right) $$, and positions of the obstacles C(0.25≤C≤0.75)$$ C\ \left(0.25\le C\le 0.75\right) $$. It is concluded that an increase in the perimeter of the inner obstacles causes a weakness in the fluid flow while a good thermal enclosure is obtained. Moreover, the isothermal obstacles enhance the average Nusselt number more than the adiabatic obstacles in all cases. [ABSTRACT FROM AUTHOR]

The yield stress of ferrofluids has been a hot topic in their rheological studies, and it is still controversial which method could obtain the more accurate yield stress value. In this work, we used varieties of methods to obtain the yield stress of the ferrofluid, including steady shear and oscillatory shear. We obtained the static yield stress and dynamic yield stress of the ferrofluid by Herschel–Bulkley fitting of the flow curves measured by the controlled shear rate mode and the controlled shear stress mode, and found that at any magnetic field strength, the static yield stress was always significantly greater than the dynamic yield stress. We then performed oscillatory shear of the ferrofluid and determined the yield stress using three methods, where the yield stress value corresponding to the intersection of the storage modulus G ′ and the loss modulus G ″ is similar to the static yield stress value fitted by H-B at low magnetic field strength. The power law function and shear stress as a function of shear strain consistently gave the maximum value of the yield stress. Ultimately, we believe that which method is ultimately chosen depends on the practical application conditions of interest. [ABSTRACT FROM AUTHOR]

The paper proposes the bandpass filter model consisting of the geometric model of a cuvette filled with a magnetic fluid. The proposed model is developed using Navier–Stokes equations, level set method, and Maxwell's equations. The numerical simulation is used to study the properties of periodic filamentary structures in the ferrofluid locating in the external magnetic field, depending on the magnetic strength and ferrofluid parameters. Transmission spectra are calculated for the broadband terahertz radiation passing through the created periodic structures. The dependences are suggested and reference points are determined for the formation of periodic structures in the ferrofluid occupying the external magnetic field. A qualitative assessment is conducted for transmission spectra of terahertz bandpass filters. [ABSTRACT FROM AUTHOR]

The timely detection and diagnosis of diseases and accurate monitoring of specific genetic conditions require rapid and accurate separation, sorting, and direction of target cell types toward a sensor device surface. In that regard, cellular manipulation, separation, and sorting are progressively finding application potential within various bioassay applications such as medical disease diagnosis, pathogen detection, and medical testing. The aim of this paper is to present the design and development of a simple traveling wave ferro-microfluidic device and system rig purposed for the potential manipulation and magnetophoretic separation of cells in water-based ferrofluids. This paper details in full: (1) a method for tailoring cobalt ferrite nanoparticles for specific diameter size ranges (10–20 nm), (2) the development of a ferro-microfluidic device for potentially separating cells and magnetic nanoparticles, (3) the development of a water-based ferrofluid with magnetic nanoparticles and non-magnetic microparticles, and (4) the design and development of a system rig for producing the electric field within the ferro-microfluidic channel device for magnetizing and manipulating nonmagnetic particles in the ferro-microfluidic channel. The results reported in this work demonstrate a proof of concept for magnetophoretic manipulation and separation of magnetic and non-magnetic particles in a simple ferro-microfluidic device. This work is a design and proof-of-concept study. The design reported in this model is an improvement over existing magnetic excitation microfluidic system designs in that heat is efficiently removed from the circuit board to allow a range of input currents and frequencies to manipulate non-magnetic particles. Although this work did not analyze the separation of cells from magnetic particles, the results demonstrate that non-magnetic (surrogates for cellular materials) and magnetic entities can be separated and, in some cases, continuously pushed through the channel based on amperage, size, frequency, and electrode spacing. The results reported in this work establish that the developed ferro-microfluidic device may potentially be used as an effective platform for microparticle and cellular manipulation and sorting. [ABSTRACT FROM AUTHOR]

Magnetic pore fabrics (MPFs) serve as efficient proxy for pore alignment and preferential flow directions. Anisotropy of magnetic susceptibility of impregnated samples reflects the shape and arrangement of ferrofluid‐filled pores. Reliable interpretation requires that the entire connected pore space is reached by ferrofluid. This study investigates impregnation efficiencies of various methods (standard vacuum, pressure injection, magnetic flowthrough, and resin impregnation) and ferrofluids (water‐based versus oil‐based) on sandstone and calcarenite with different wettabilities. Mass‐based and susceptibility‐based impregnation efficiencies further evaluate the influence of impregnation time and flow rate. Because directional impregnation may introduce artificial fabrics, mutually perpendicular cores were measured, and subsamples served to capture spatial variability. Impregnation efficiencies vary between <1% and >100%, with resin impregnation and pressure injection using oil‐based ferrofluids being least efficient. Oil‐based ferrofluid generally leads to lower susceptibility‐based compared to mass‐based impregnation efficiencies, suggesting particle filtering. Water‐based ferrofluids show larger ranges of susceptibility‐based compared to mass‐based impregnation efficiencies. In particular, impregnation efficiencies >100% exclusively occur for susceptibility‐based impregnation efficiencies of water‐based ferrofluid, using either standard vacuum or pressure injection methods. MPFs after magnetic flowthrough impregnation reflect the impregnation direction rather than pore fabric. A major challenge is to apply enough force to reach sufficiently high impregnation efficiencies without creating MPFs that do not represent pore fabrics. While we cannot define a best practice for ferrofluid impregnation yet, this study improves our understanding of the impregnation process, and provides experimental procedures to identify artifacts. Plain Language Summary: The details of a rock's pore space define how fluids move from pore to pore, and is thus important for groundwater or oil flow. One simple and efficient way to describe pore space properties and predict flow directions is using magnetic methods. For this, pores are filled with a strongly magnetic fluid, a so‐called ferrofluid, and then the magnetic properties of the impregnated rock are measured as a function of measurement direction. Two prerequisites to obtain reliable results are that a large part of the pores is invaded by the ferrofluid, and that the pore space remains intact during the experiment. When more force is applied, the chances that pores are filled become higher, but at the same time also the risk of creating artificial fabrics increases. Here, various methods and ferrofluid types are tested on rocks with different properties. There is no single best method; however, the results presented here help increase our understanding of the impregnation process. Key Points: Impregnation efficiency of standard vacuum, pressure injection, magnetic flowthrough, and resin impregnation was determinedMagnetic flowthrough impregnation introduces artificial fabrics where the maximum susceptibility reflects impregnation directionNeed to check for impregnation artifacts by testing impregnation efficiencies and MPFs of perpendicular cores [ABSTRACT FROM AUTHOR]

This study aims at finding the linear theory for the onset of ferromagnetic convective flow in a heterogeneous Brinkman porous layer with uniformly distributed internal heat source in the presence of vertical magnetic field. The resulting critical values are obtained numerically using the Galerkin technique for isothermal/insulated rigid-ferromagnetic boundaries for different forms of vertical heterogeneity permeability function F (z). The results converge for six terms in the Galerkin expansion. The effect of types of F (z) and N s is found to either delay or speedup the flow of the ferrofluids. The stability of the system for the model F 4 is more stable and least stable for the model F 1 in the presence of N s . For different forms of F (z) , the results show that the critical Rayleigh number increases with increasing D a , while decreases with increasing R m and M 3 . The values of a c increase with R m , but they decrease with increasing M 3 and D a. Besides, isothermal boundaries are found to be more stabilizing when compared to insulated boundaries. [ABSTRACT FROM AUTHOR]

The present work investigates the ferrohydrodynamic flow and heat transfer due to a uniform source and irrotational vortex under the influence of a stationary magnetic field. A uniform source generates only a two-dimensional flow. However, in the presence of the vorticity with a uniform source, the flow becomes three-dimensional. The governing equations are expressed as a system of nonlinear coupled differential equations. The transformed differential equations are solved using the finite element approach for both the two-dimensional and three-dimensional flow models. With variations in the strength of the source parameter, Reynolds number, and ferromagnetic interaction numbers, the behavior of two-dimensional and three-dimensional flow is investigated. In three-dimensional flow, the influence of swirling effects on the velocity and temperature profiles are weak as compared to two-dimensional flow. The main role of the three-dimensional vortex flow of ferrofluid is to generate rotational viscosity, and it is not possible in the case of two-dimensional flow case. [ABSTRACT FROM AUTHOR]

Numerical simulations of (water-Fe3O4) ferrohydrodynamics (FHD) mixed convection inside a vertical channel are performed. The magnetic field is produced by three sources positioned outside the channel’s right wall. The latter is provided with localized heat sources surmounted by variously shaped porous blocks: rectangular, trapezoidal, and triangular. The general model of Darcy-Brinkman-Forchheimer is employed to describe the fluid flow in the porous regions, and the resulting equations are numerically solved by the finite volume approach. The influence of significant parameters, including the magnetic number (Mn), the Richardson number (Ri), and the shape of blocks, is examined. The results essentially reveal that the enhanced heat transfer brought by the magnetic field and its intensity increase is suppressed by the augmentation of Ri until a critical value, rising with Mn, beyond which the global Nusselt number increases again. The mean friction coefficient increases with increased Mn and reduced Ri. Compared to the case with no magnetic field, the maximum enhancement in heat transfer rate is around 132% for the rectangular blocks, 146% for the trapezoidal blocks, and 160% for the triangular blocks, while the maximum increase in pressure drop is approximately 45% for all the shapes. The triangular shape seems the most efficient because it leads to high heat transfer rates and low mean friction coefficients; its performance factor is 2.32 for a dominant magnetic field and 2.62 for a dominant buoyancy force. The current research's conclusions will help optimize the operation of various thermal engineering systems, including electronic devices, where the improved heat removal rate will keep the electronic components at a safe operating temperature. [ABSTRACT FROM AUTHOR]

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively. [ABSTRACT FROM AUTHOR]

A stable colloid called ferrofluid is made up of tiny magnetic particles, often magnetite (Fe3O4), that have been bonded with an amphiphilic dispersion layer and are then suspended in a suitable liquid solvent carrier. Current industrial uses for ferrofluid include dynamic sealing, inertial and viscous damping, magnetic drug targeting, liquid microrobots, etc. In this article, we studied the heat transfer and MHD micropolar ferrofluid flow caused by non-linearly stretching surface. The results are presented for hybrid alumina-copper/ethylene glycol (Al2O3 -Cu/EG) nanofluid. The governing non-linear equations describing flow are transformed into a system of ordinary differential equations using similarity transformations. Using the BVp4c method, the microstructure and inertial properties of a magnetite ferrofluid across a non-linear stretched sheet are studied. The influence of relevant parameters on stream function, velocity, micro-rotation velocity, and temperature are obtained and represented graphically. The computed results are original, and it has been observed that if we increase the magnetic parameter, the stream function and the velocity decrease, while the temperature and micro-rotation velocity increase. As the Prandtl number increases, the temperature profile decreases. It has been observed that the Nusselt number or heat transfer rate of hybrid nanofluid is better as compared to nanofluid flow. [ABSTRACT FROM AUTHOR]

The stability of a ferrofluid with a fairly arbitrary nonuniform magnetic susceptibility between two parallel walls, subject to a non-uniform magnetic field acting normal to the walls, is investigated. The susceptibility may depend on position and the field strength, and the stationary state is such that the gradient of the susceptibility with respect to the modulus of the field is negative. Previous work suggests that the configuration may be linearly unstable, as regions of higher susceptibility do not coincide with regions of strongest field, and this is proved here. Adding a constant field in the plane of the layer suppresses parallel instabilities, but has no effect on those orthogonal to it. However, it is demonstrated that stability can be achieved by applying a rapidly rotating field. [ABSTRACT FROM AUTHOR]

One primary concern in colloid science is understanding the relationship of its macroscopic rheology and diffusion behavior with the observed microscopic arrangements of the nanoparticles in the fluid. This manuscript addresses the study of these dynamical properties through a first-principle stochastic method. Both properties directly relate to the observed fluid structure factor, which depends on a few known material parameters. However, in the literature, this static quantity is reported up to the first prominent peak of its small momentum transfer of the scattered radiation, leading to inaccurate determination of the transport properties. Here, it is proposed to use the rescaled mean spherical approximation under the requirement of fitting the experimental data of the structure beyond the dependence of more significant wave numbers. The predicted viscosity agrees with the observed ones at a low volume fraction of particles for ferrofluids dispersed in polymer solvents. This rheological quantity is inversely related to the self-diffusion coefficient of a tracer particle. [ABSTRACT FROM AUTHOR]

In this study, a numerical investigation of the effect of different magnetic fields on ferrofluid-fluid mixing processes in a two-dimensional microchannel is performed An improved version of smoothed particle hydrodynamics, SPH, by shifting particle algorithm and dummy particle boundary condition, is implemented to solve numerical continuity, ferrohydrodynamics-based momentum and mass transfer equations. SPH is formulated through the irregular arrangement of the nodes where the fields are approximated using the fifth-order Wendland kernel function. After validating the computational approach, the influence of the number (from one to three) of parallel electrical wires positioned perpendicular to the microchannel on the mixing efficiency is studied for the first time. It has originally been found that the mixing efficiency highly non-linearly depends on the Reynolds number and the number of electrical wires. For Re ≤ 20 the mixing efficiency is almost the same for two and three electrical wires and about two times higher than one electrical wire. For Re ≥ 80, the mixing efficiency of three wires is much higher than one and two electrical wires. Optimum performance of the micromixer is achieved with three electrical wires, since the mixer performs well on a broader range of Re than the other two studied cases. The outcomes of this study, obtained by a meshless method, are important for the industrial design of micromixers. [ABSTRACT FROM AUTHOR]

The anisotropy of magnetic susceptibility of ferrofluid‐impregnated samples is an efficient and powerful proxy for pore space anisotropy and preferred flow directions. One of the main assumptions in pore fabric studies is that all pores >10–20 nm are homogeneously filled with ferrofluid, and that the ferrofluid has constant properties throughout the pore space and over time. If only part of the pore space is filled, this can lead to artifacts. Additionally, because magnetic anisotropy of a given pore space depends on fluid susceptibility, quantitative interpretations may be affected by the interval between impregnation and measurement time, or by the age of the ferrofluid during impregnation, unless fluid properties remain constant. A careful investigation of the time variation of ferrofluid properties and magnetic pore fabrics in synthetic and natural samples shows time‐dependence of susceptibility and hysteresis properties, related to dissolution of particle surfactants, and deterioration of colloidal stability. The latter leads to particle aggregation and sedimentation, which changes the anisotropy properties, and also affects impregnation behavior. Natural samples impregnated by oil‐based ferrofluid also experience a 2.5–3‐fold increase in mean susceptibility, with important consequences for the susceptibility‐based determination of impregnation efficiency. Based on our results, we recommend that ferrofluid properties are determined at the time of impregnation, and that samples are measured shortly after impregnation. Plain Language Summary: One way to describe a rock's pore space, that is, properties relevant for the flow of groundwater or hydrocarbons, is to fill all pores with a strongly magnetic fluid and then measuring the magnetic properties and their directional dependence. This method is reliable if all pores are filled with the magnetic fluid, and the magnetic properties of the fluid are known. Challenges occur when the magnetic particles in the fluid cluster together and become too big to enter smaller pores, only part of the pores are filled with magnetic fluid, the magnetic fluid changes its properties over time, or the magnetic properties of the rock change upon contact with the fluid. These processes are investigated here, using both synthetic samples and rocks. We show that particles cluster and magnetic properties change over time, showing the need to re‐evaluate some previous statements. Key Points: Changing ferrofluid properties, for example, particle aggregation, affect impregnation process and magnetic pore fabric degree, shape or orientationSusceptibility of rock impregnated with oil‐based fluid increases over time, affecting impregnation efficiency calculationsStatements on impregnation efficiency of oil‐versus water‐based ferrofluids need to be re‐evaluated [ABSTRACT FROM AUTHOR]

We studied the polarization of light in a thin film of ferrofluid subjected to a magnetic field using the Mueller matrix formalism. By observing the results of some experiments, we relate the observed light patterns with Stokes vectors that can be operated by Mueller matrices, which represent the magnetic field applied to the sample. We observed that the changes in the dichroism of this system can be monitored along the sample, allowing for the visualization of magneto-optical effects mainly for linear polarized light, and the effects of circular polarized light are related to birefringence. [ABSTRACT FROM AUTHOR]

In this study, experiments, simulations, and optimization were performed to evaluate heat transfer performance of ferrofluids. Ferrofluids are colloidal suspensions containing magnetic-nano particles with a diameter of 5 to 15 nm in a base fluid such as oil or water. Recently, as many devices are miniaturized, the design of heat dissipation systems are being diversified to consider cost and safety, and it is becoming important to separate an ancillary device for cooling from main unit. In ferrofluids, the behavior and vortex of magnetic-nano particles are actively generated by an external magnetic field, and the cooling system can be designed in a simplified manner by using this characteristic. The main design parameter is the arrangement of permanent magnets, and the output variable is the temperature inside the magnetic nanofluid. The permanent magnet can be moved up and down, and the temperature inside the magnetic nanofluid was measured at various locations. A predictive model was created using a design of experiments (DOE) and response surface method (RSM) using selected design and temperature variables. Based on the generated regression model, an optimization was applied to find a permanent magnet arrangement that maximizes heat transfer performance. Through the optimization technique used in this study, economic efficiency in terms of time and cost was obtained by reducing the number of experiments. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe-Zr-B magnetic fluids were successfully prepared. • Magnetic sizing agents can be made from Fe-Zr-B magnetic fluids. • Magnetic fibers can be easily obtained by using magnetic sizing agents. • The saturation magnetization strength of BF-FeZrB reaches 63.5 emu/g. In this paper, FeZrB amorphous nanoparticles were prepared and used to obtain a magnetic fluid which can be used as a magnetic sizing agent for basalt fibers. Unmodified FeZrB nanoparticles (Unmodified FeZrB), citric acid-modified FeZrB nanoparticles (FeZrB@CA) and sodium oleate-modified FeZrB nanoparticles (FeZrB@NaOL) were synthesised by chemical reduction. Three kinds of FeZrB magnetic sizing agents were obtained by dispersing nanoparticles into the mixture consisting of PVA and KH-550. The magnetic basalt fibers were prepared by spraying the magnetic sizing agents in the process of drawing basalt fibers. The structure, morphology, surface state and magnetic properties of unmodified and modified FeZrB amorphous nanoparticles were characterized by XRD, SEM, TEM, FT-IR and VSM. The properties of magnetic basalt fibers were investigated by SEM, VSM and EDS. The results showed that the XRD spectra showed sharp diffraction peaks in both Unmodified FeZrB and FeZrB@NaOL nanoparticles, and the two groups of samples had different degrees of crystallinity. Only a single broad peak can be observed at around 45° in FeZrB@CA, which confirms the amorphous structure of FeZrB@CA. SEM and TEM images show that the FeZrB amorphous nanoparticles exhibit spherical with sizes ranging from 7 nm to 25 nm. Electron diffraction patterns showed that FeZrB@CA had a diffraction halo typical of an amorphous structure, and the remaining two samples displayed diffraction rings, indicating some degree of crystallinity. The FT-IR spectra confirm the formation of CO bonds in both FeZrB@CA and FeZrB@NaOL nanoparticles, suggesting the successful modification of FeZrB nanoparticles. The saturation magnetization of Unmodified FeZrB, FeZrB@CA, and FeZrB@NaOL nanoparticles were determined as 83.4 emu/g, 98.9 emu/g, and 60.3 emu/g, respectively. The SEM images showed that the surface of fibers was completely covered by the magnetic sizing agents. Moreover, the saturation magnetization of BF-Unmodified FeZrB, BF-FeZrB@CA, BF-FeZrB@NaOL (magnetic basalt fibers) could reach 47.9 emu/g, 63.5 emu/g, and 28.9 emu/g, respectively. The formation mechanism of magnetic basalt fibers was elucidated, which is attributed to the Si O bonds between basalt fibers and KH-550, as well as CONH bonds between KH-550 and FeZrB@CA. Consequently, the FeZrB magnetic sizing agents could adhere to the fiber surface. This study provides a simple method for developing magnetic sizing agents and offers a new perspective for imparting magnetism to basalt fibers and preparing magnetic fibers. [ABSTRACT FROM AUTHOR]

• Dual-rotating magnetic source positive-displacement ferromagnetic fluid micropump. • Coupling magnetic force of the ferromagnetic fluid plug enhanced. • Influence of magnetic pole direction on magnetic field in microchannel examined. • Various factors such as maximum pumping flow rate and pumping height improved. • Pumping failure due to gradual collapse of ferromagnetic fluid clogging. Despite extensive research on ferromagnetic fluid micropumps as novel microactuators, their low pumping flows and driving forces limit their development. There is an urgent need to improve the performance of ferrofluid pumps from the perspective of structural improvement. Based on the driving and sealing principles of ferrofluids, this study proposed a dual-rotating magnetic source positive-displacement ferromagnetic fluid micropump. The results show that the coupling magnetic force of the ferromagnetic fluid plug was enhanced by the dual rotating magnetic sources. Furthermore, the influence of the magnetic pole direction on the magnetic field in the microchannel and the optimal center declination angle of the double-rotating magnetic source were determined. The maximum pumping flow rate, pumping height, maximum tolerated speed, and pumping stability of the double-rotating magnetic source ferromagnetic fluid micropump were significantly improved. Moreover, the key to the pumping failure of the micropump was the process of foam formation at the tail of the ferromagnetic fluid plug in the microchannel until the entire plug gradually collapsed. Furthermore, the speed of the ferromagnetic fluid plug and strength of the magnetic field were the most critical factors affecting the process. [ABSTRACT FROM AUTHOR]

Swarm robotics finds inspiration in nature to model behaviors, such as the use of pheromone principles. Pheromones provide an indirect and decentralized communication scheme that have shown positive experimental results. Real implementations of pheromones have suffered from slow sensors and have been limited to controlled environments. This paper presents a novel technology to implement real pheromones for swarm robotics in outdoor environments by using magnetized ferrofluids. A ferrofluid solution, with its deposition and magnetization system, is detailed. The proposed substance does not possess harmful materials for the environment and can be safely handled by humans. Validation demonstrates that the substance represents successfully pheromone characteristics of locality, diffusion and evaporation on several surfaces in outdoor conditions. Additionally, the experiments show an improvement over the chemical representation of pheromones by using magnetic substances and existing magnetometer sensor technologies, which provide better response rates and recovery periods than MOX chemical sensors. The present work represents a step toward swarm robotics experimentation in uncontrolled outdoor environments. In addition, the presented pheromone technology may be use by the broad area of swarm robotics for robot exploration and navigation. [ABSTRACT FROM AUTHOR]

From a technological point of view one of the most important characteristics of magnetorheological fluids and ferrofluids is their fast (on the 10 − 3 s time scale) response to external magnetic fields. We report response time measurements based on the time domain magnetic susceptibility response of commercially available fluids. The dynamic susceptibility is measured over time in the presence of external magnetic field by a method using an inductive resonant circuit. The response time of the fluids is extracted as the characteristic time constant from the response to ramp excitation. We investigated the influence of the slope of the ramp excitation on the response time. The macroscopic response times were correlated with the characteristic time scales of the microscopic processes behind the susceptibility response. In case of the magnetorheological fluids the overall response time have been associated with the characteristic times of structure formation. The relatively slow response of the ferrofluid corresponded to a relaxation dominated by Brownian mechanism, which indicated the presence of multi-core clusters. [ABSTRACT FROM AUTHOR]

Core losses of transformers motivate many engineers and scientists to design and implement different transformers for their specific aims. Since there exists a growing interest on high frequency applications in today's world, design and optimization studies of a magnetic fluid core transformer (MFCT), having an easy and cheap production approach in high frequency applications, are considered in the present paper. The desired design should operate in a more efficient way within a wide frequency band. The MFCT considered here can be a solution to the eddy currents and core losses encountered in the conventional transformers with its low conductivity, oil-based magnetic fluid and super paramagnetic characteristic. The magnetic fluid in the proposed work consists of a combination of ferromagnetic particles made by iron in an averaged diameter of 70 μm with an adjustable magnetism compared to the traditional magnetic fluids and an engine oil, thereby the magnetic permeability of the overall fluid core can be fully adjustable by the variation of mass proportion between the oil and iron powder as an easy process without any chemical process. A COMSOL Multi-Physics design is performed via a finite element package in three dimensions. It is proven that the iron particles exhibit a complicated pattern inside the engine oil and produce a well-defined high frequency output at the secondary windings in a wide range of frequency. [ABSTRACT FROM AUTHOR]

Background: Flexible aerospace structures present dynamic characteristics of low natural frequency. For a long-term free-floating spacecraft, theses flexible aerospace structures are prone to vibrate due to various excitation. However, it is extremely difficult to eliminate these low-frequency vibrations. Purpose: The main aim of this paper is to verify the damping performance of a novel ferrofluid inertial damper with the optimal stiffness in reducing the low-frequency free vibration of structures. Methods: The ferrofluid inertial damper consists of an inertial mass block, ferrofluids, and two magnetic field sources. The inertial mass block is levitated in two layers of ferrofluid absorbed on magnetic field sources. When the main system vibrates, the ferrofluid can generate a very small restoring force and damping force between the inertial mass block and the main system. A series of simulations and experiments are used to optimize the restoring force. Furthermore, the influence of the ferrofluid mass on the restoring force is studied. The damping performance is verified by the free oscillation of a flexible copper plate. Results: Two sets of geometric parameters whose restoring forces meet the requirement of the optimal stiffness are obtained. Compared to the copper plate damped by itself, the oscillation time of the copper plate with the ferrofluid inertial damper can be reduced by 97.73%. Conclusion: The inertial mass block has a fast response to external vibrations. The ferrofluid inertial damper has very excellent performance for damping the free oscillations of a copper plate. [ABSTRACT FROM AUTHOR]

The effect of magnetic particle volume fraction on the lubrication performance of ferrofluid solid-liquid interface was studied by molecular dynamics simulation. The ferrofluid model of Fe304 with volume fraction of 5%, 9.6% and 14.7% was established, and the ferrofluid solid-liquid interface lubrication model was constructed. The structure of the lubrication model was optimized and annealed, and the optimized model was simulated. The simulation results show that with the increase of the magnetic particle volume fraction, the friction coefficient is reduced, so the lubrication performance of the ferrofluid is improved. The friction coefficient of the ferrofluid with volume fraction of 14.7% magnetic particles is about 67% lower than that of the ferrofluid with the volume fraction of 5% magnetic particles. Based on the data of mean square displacement, kinetic energy, van der Waals potential energy and temperature change, the effect of magnetic particle content on the lubrication performance of ferrofluid was explored from a microscopic point of view, and the mechanism of the effect of magnetic particle content on the lubrication of ferrofluid solid-liquid interface was revealed. The results show that the increase of magnetic particle content will enhance the diffusion ability of molecules in the magnetic fluid, make some molecules in the magnetic fluid adhere to the surface of the solid layer, and form a penetration layer or diffusion layer with strong friction reducing and anti-wear, so as to improve the lubrication performance of the magnetic fluid. In addition, the increase of magnetic particle content will reduce the van der Waals force between molecules in the magnetic fluid, change the viscosity of the magnetic fluid, and enhance the thermal conductivity and heat transfer performance, so as to improve the lubrication performance of solid-liquid interface. [ABSTRACT FROM AUTHOR]

The classical linear stability analysis is used to examine the effect of thermal radiation on the onset of Darcy-Brinkman ferroconvection. The boundaries of the fluid layer are treated as black bodies and the optical properties of the transparent ferromagnetic fluid are independent of the wave length of radiation. The fluid and solid matrix are assumed to be in local thermal equilibrium. Considering realistic boundary conditions, the principle of exchange of stabilities is shown to be valid and the critical values pertaining to the stationary instability are obtained by means of the higher order Galerkin method. It is observed that the basic temperature profile turns out to be exponential and symmetric as the radiative parameters increase and that the effect of thermal radiation is to delay the onset of Darcy-Brinkman ferroconvection. The destabilizing influence of magnetic forces is affected by the radiative parameters. The effect of magnetic, radiative and porous parameters on the convection cell size is also discussed. [ABSTRACT FROM AUTHOR]

Convective instability of a horizontal ferromagnetic fluid saturated porous layer with magnetic field dependent (MFD) viscosity subjected to gravity field varying with distance in the layer is investigated. The fluid motion is described by the Brinkman model. The method of small perturbation is applied and the resulting eigenvalue problem is solved using the higher order Galerkin technique. The stationary instability is shown to be the preferred mode of instability and the resulting eigenvalue problem is solved by taking into account the realistic rigid-rigid-isothermal boundary conditions. The study reveals that the effect of MFD viscosity is to delay the onset of ferroconvection and the stabilizing effect of MFD viscosity is reduced when the magnetic Rayleigh number is sufficiently large. In the presence of variable gravity, the effect of magnetic and non-magnetic parameters on ferroconvective instability is also discussed. [ABSTRACT FROM AUTHOR]

The present numerical study is based on the forced magnetohydrodynamic (MHD) convection of a ferrofluid through a backward facing step (BFS). A cylinder with two fixed fins and fixed dimensions is implanted inside fluid. The dimensionless governing equations have been solved using the multigrid finite element method. Several parameters were considered, such as the Hartmann number 0≤ Ha ≤100, the magnetic field inclination angle 0°≤ ɣ ≤90°, the Reynolds number 10≤ Re ≤200, the nanoparticle volume fraction 0%≤ φ ≤10%, and the fins inclination angle 0°≤ a ≤180°. The results have shown that the presence of the fins improves the heat transfer, especially at the position a = 90° where the Nuave number increases with a ratio of 113% for Re = 200. [ABSTRACT FROM AUTHOR]

The main advantages of the energy approach to solving the problem of determining magnetic forces acting on solid bodies immersed into magnetized ferrofluids (FFs) are shown. Characteristic disadvantages of the standard approach to the calculation of magnetic forces using the Bernoulli equation for FFs and an equation for the magnetic pressure jump at the interface are considered. A review of works devoted to the study of forces acting on solid bodies immersed in magnetized FFs is presented. This literature review convincingly demonstrates the need for and potential advantage of using the energy approach to these problems, since the analytical expressions significantly depend on the body shape and obtaining the final numerical results is complicated by the error of magnetic field calculation at the "solid body–FF" interface where the normal component of induction and the tangential component of the magnetic field exhibit a discontinuity. In contrast, the energy approach allows using the standard functions of program packages for determining thermodynamic potentials. The choice of a thermodynamic potential correctly describing experimental data is discussed. The method of magnetic energy determination is justified by the problem setting and verified by comparison of the results of several numerical solutions obtained using the open software package FEMM for FFs obeying a nonlinear magnetization law. This analysis was previously performed neither experimentally nor theoretically in view of the commonly accepted use of simplifying assumptions (approximations of weak and strong magnetic fields or a noninductive approximation). Here, the energy approach to determining forces acting on solid bodies in FFs has been justified by pairwise comparison of the results obtained in the framework of this approach to the data of laboratory experiment and the results of standard calculations. [ABSTRACT FROM AUTHOR]

The advantages of magnetic fluid seals, such as low friction moment, high tightness, and high durability, have been known for many years. However, they are used particularly in air or vacuum environments and the application of this type of seal in water still remains a problem. This is mainly due to the friction occurring between the two fluids and interfacial instability. This publication focuses on investigating the influence of magnetic fluid properties on the mechanism of pressure transfer between sealing stages. The main aim was to better understand the leak mechanism in this type of seal and to be able to select ferrofluids (FFs) resistant to water for long-term operation after short-term testing. The tested seals rotated over 4.3 million times without failure. The results of the research carried out are recommendations for the selection of magnetic fluid, sealing stage width, and FF volume. [ABSTRACT FROM AUTHOR]

We studied the effects of image formation in a device known as Ferrocell, which consists of a thin film of a ferrofluid solution between two glass plates subjected to an external magnetic field in the presence of a light source. Following suggestions found in the literature, we compared the Ferrocell light scattering for some magnetic field configurations with the conical scattering of light by thin structures found in foams known as Plateau borders, and we discuss this type of scattering with the concept of diffracted rays from the Geometrical Theory of Diffraction. For certain magnetic field configurations, a Ferrocell with a point light source creates images of circles, parabolas, and hyperboles. We interpret the Ferrocell images as analogous to a Möbius transformation by inversion of the magnetic field. The formation of circles through this transformation is known as horocycles, which can be observed directly in the Ferrocell plane. [ABSTRACT FROM AUTHOR]

• Magnetic Control of Natural Heat Transfer inside a Porous Dual Triangular Cavity. • Langevin equation is applied to computing the magnetization effects. • Tilting enclosure results in a reduction in effect of the magnetic convection. • At high value of Ra the heat transfer is maximum for inclined vertical enclosure. • Increasing ε reduces the overall heat transfer. • For low Ra, the thermomagnetic convection dominates the flow behavior. Various passive and active methods are employed to control heat transfer in various industries. The application of external magnetic fields and porous materials usage are among these methods. Understanding the interplay between magnetic and gravitational convection in porous media is therefore essential for such systems. In the present study, mixed magnetic-natural convection heat transfer through a dual porous enclosure filled by a ferrofluid is numerically investigated. Two permanent magnets are placed at two sides of the enclosure. The equations governing the flow and energy transfer in the cavity are derived in the non-dimensional form. Langevin equation is applied to computing the magnetization of the ferrofluid. The various equations are solved by means of the finite element method. The influence of key factors including buoyancy and magnetic Rayleigh numbers Ra and Ra m , inclination angle of the enclosure, Darcy number and the porosity of the porous medium on the flow and thermal behaviors of the fluid are studied and discussed. It was found that enhancing the natural gravitational convection hinders the thermomagnetic convection in the top part of the enclosure. When the enclosure is tilted, the magnetic convection becomes less affected by the natural convection. An average of 28% increase in the heat transfer rate is found when the cavity is tilted vertically in comparison to a horizontal one. Raising Ra and Ra m enhances the gravitational and magnetic convections, respectively. For high Ra, the heat transfer is maximum when the enclosure is inclined vertically, i.e. when the gravitational convective effects are minimized. The heat transfer rate can rise by up to 180% when strong magnetic effects are present compared to pure gravitational convection. The flow circulation is intensified when Da and ε are raised. Increasing ε reduces the overall heat transfer due to the inhibited impact of the porous medium. These outcomes can provide perspectives for heat transfer optimization and thermal management systems and inform the design of new components. [ABSTRACT FROM AUTHOR]

In this article, a novel methodology to tune parameters of a resonant accelerometer realized by a flexible structure is presented. A model describing the behavior of the tuning mechanism has been proposed and experimentally validated as well as a suitable design flow is presented through case studies. The device under test, whose dimensions are in the mesoscale, adopts a resistive sensing readout strategy implemented through strain gauges, directly inkjet printed on the flexures (springs) supporting the central membrane. The tuning mechanism, which exploits the interaction between an external magnetic field and a magnetic fluid (ferrofluid) embedded in the device’s springs, has been experimentally investigated by using a lab-scale prototype. The procedure for the optimal design of the sensor has also been addressed. The prototype shows a responsivity of 721.0 $\mu $ strain/m/s2 at the resonance frequency of 30.5 Hz and a noise floor of 7.5e $- 5 { \text {m/}}\text {s}^{{2}} {/}\sqrt {\text {Hz}} $ estimated in a bandwidth of 500 Hz. Responsivities of $657.0~\mu $ strain/m/s2 and $580.0~\mu $ strain/m/s2 as well as noise floor of 8.7e $- 5 { \text {m/}}\text {s}^{{2}} {/}\sqrt {\text {Hz}} $ and 10.9e $- 5 { \text {m/}}\text {s}^{{2}} {/}\sqrt {\text {Hz}} $ have been estimated in case of the tuning magnetic fields of $H =6.8\text{e}-3$ T and $H =11.4\text{e}-3$ T, respectively, showing resonance frequencies of 31.0 and 32.5 Hz. The main novelty of the proposed approach arises from the tuning strategy that is important for many real applications requiring a fine-tuning of the sensor parameters while the use of a cheap inkjet printing technology for the device realization represents a substantial advantage in terms of costs and to enable the rapid prototyping of customizable devices on flexible substrates. [ABSTRACT FROM AUTHOR]

We present a novel methodology for magnetic field sensing by exploiting surface plasmon resonance (SPR) sensors based on D-shaped plastic optical fiber (POF) and magnetic fluids (MFs). The proposed methodology adopts a multimode POF patch covered with ferrofluid placed between the light source and the SPR-POF platform. The working principle exploits the changing of the SPR spectra due to variations of the SPR resonance conditions generated by the changing in the input light in the SPR-POF sensor due to the attractive force exerted by the target magnetic field on the POF covered with ferrofluid. A model showing a good fitting between the simulated and measured values of the magnetic field is also presented. A proof of concept for this novel sensing approach has been obtained exploiting a prototype of a sensor that has been characterized in the range of values between 0.15 and 1.2 mT. Particularly, we have estimated, in the linear range of the sensor response, sensitivity equal to about 6800 pm/mT and resolution equal to about 0.029 mT. Moreover, the comparison between the expected and the experimental behavior has shown a very good match with a mean squared error of about 5%. [ABSTRACT FROM AUTHOR]

The benefit of transformer oil seeded with magnetic nanoparticles (forming a transformer oil-based ferrofluid) is numerically studied in an axisymmetric model of 40 kVA transformer. The maximum temperature in the system is decreased by 2.2 °C when using ferrofluid instead of transformer oil. This effect is mainly due to the magnetic body force, which modifies the convective flow in the system and enhances heat transfer. The leakage magnetic field is highly localized and limits the effect of thermomagnetic convection. Varying the geometry of the primary coil assembly allows to increase this effect and to decrease the maximum temperature. [ABSTRACT FROM AUTHOR]

In this article, the experimental investigation and analysis of the effect of rheological properties on the starting torque of the magnetic fluid seal are reported. The change of the starting torque with a standing time for perfluoropolyether-based magnetic fluid seal and diester-based magnetic fluid seal, as well as the viscosity of magnetic fluids under the relevant magnetic field and shear rate, were measured. It was observed that the magnetic fluid with higher viscosity correspondingly exhibited higher starting torque. The starting torque of seals with two different magnetic fluids exhibited the same variation tendency. The starting torque was exponentially increased with the standing time and then tended to be a stable state with small scale fluctuation. The average maximum starting torque of PFPE-based ferrofluid seal after standing five days was ten times as large as the value of the initial standing, whereas that of diester-based ferrofluid seal was 3.5 times as large the value of the initial standing. The rheological behavior was the key factor influencing the starting torque of the magnetic fluid seal. Particle agglomeration and shear thinning were the main microscopic mechanisms leading to the change of starting torque with a standing time. The correlation between macroscopic rheological properties and starting torque was established based on the microstructure variation. [ABSTRACT FROM AUTHOR]

We consider in this paper numerical approximations of a phase field model for twophase ferrofluids, which consists of the Navier--Stokes equations, the Cahn--Hilliard equation, the magnetostatic equations, and the magnetic field equation. By combining the projection method for the Navier--Stokes equations and some subtle implicit-explicit treatments for coupled nonlinear terms, we construct a linear, decoupled, fully discrete finite element scheme to solve the highly nonlinear and coupled multiphysics system efficiently. The scheme is provably unconditionally energy stable and leads to a series of decoupled linear equations to solve at each time step. Through numerous numerical examples in simulating benchmark problems such as the Rosensweig instability and droplet deformation, we demonstrate the stability and accuracy of the numerical scheme. [ABSTRACT FROM AUTHOR]