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We investigate the Casimir pressure between two dissimilar plates separated by a layer of magnetic fluid. Numerical computations of the Casimir pressure are performed for Au and SiO₂ plates with an intervening layer of water and water-based ferrofluid obtained on addition to water a 5% volume fraction of magnetite nanoparticles with 10 nm diameter. It is shown that an addition of nanoparticles leads to a widening of separation region, where the Casimir interaction is repulsive. This result does not depend on whether the plasma or the Drude model is used for extrapolation of the optical data of Au to low frequencies. The effect of enhanced repulsion due to the presence of ferrofluid may be useful for preventing stiction of closely spaced elements in various microdevices.

The progress in the synthesis of new magnetic nanoparticles and agglomerates stimulates the development of novel ferrofluids with enhanced rheological properties. In the current work ferrofluids based on Co-nanoplatelets and clustered iron oxide nanoparticles have been considered. Steady-shear experiments and yield stress measurements of these ferrofluids have been performed using rotational rheometry.

In this research study, computational investigations are prepared to demonstrate the influence of the nonhomogeny magnetic source on the thermal efficiency of the convergent tube with the nanofluid flow. The major attention of our examination is to analyze the flow stream and temperature spreading on the average Nusselt number of the base fluid with Fe2O3 nanoparticles. The effects of inlet velocity and magnetic intensity on the thermal characteristics of nanofluid stream through the convergent tube are fully investigated. SIMPLEC algorithm is used for the imitation of the incompressible nanofluid flow inside the convergent tube. Our results indicate that growing Reynolds number from 50 to 100 surges heat rates up to 18% in the convergent tube because of the existence of the magnetic field in the vicinity of the tube.

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 $F4$ is more stable and least stable for the model $F1$ in the presence of $N_s$. For different forms of $F(z)$, the results show that the critical Rayleigh number increases with increasing $Da$, 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 $Da$. Besides, isothermal boundaries are found to be more stabilizing when compared to insulated boundaries.

In this work, three passive techniques (ferrofluid, porous zone, curved surfaces) have been merged with one active technique (electric force) to enhance the convective rate. The permeable enclosure contains two curved walls and two straight walls which are not stationary. The concentration of ferrofluid within the domain is constant and associated formulations for properties of ferrofluid have been applied in modeling in which no slip velocity exists among particles. With define, measure, analyze, improve and control (DMAIC), the vorticity equation for partial equations which contain the source terms of electrohydrodynamic (EHD) and permeability, final equations have been achieved and for finding the solution combination of two basic techniques were utilized. Low deviation with a prior article in the validation procedure indicated a good agreement. Nu can augment around 0.49% if platelet particles were applied rather than sphere. Utilizing radiation in simulation makes Nu augment around 82.27%. Elevating Da in the absence and appearance of EHD leads to an augment of Nu around 145.01% and 393.38%. Appearance of EHD enhances the Nu about 96.04%.

The Forchheimer-extended Brinkman’s Darcy-flow model was used to investigate the initiation of ferroconvection in a flat porous layer while accounting for effective viscosity. The rigid ferromagnetic, rigid paramagnetic and stress-free isothermal boundary conditions are the three categories. The eigenvalue issue can be properly addressed for stress-free boundaries; the Galerkin approach is utilized to find the critical stability constraints quantitatively for other barriers. It was discovered that the boundary types had a strong influence on the system’s stabilization. Ferromagnetic boundaries are less preferred than paramagnetic boundaries in control of convection. The dependence of many physical limitations on the linear stability of the system is intentionally given, and it is demonstrated that increasing the value of the viscosity ratio delays the beginning of convection.

Using novel numerical techniques, this paper estimates the effect of EHD force on ferrofluid treatment. Iron oxide additives of various nanoscale forms and dimensions are added to the operating fluid. Because the percentage of nanoparticles exceeds 0.06 and the slip velocity is disregarded, the features of the carrier fluid were modified using an empirical model. The left and bottom surfaces of the moving walls had the highest temperatures and voltages. A non-Darcy presumption was that the region was permeable. A combined FVM and FEM method was utilized to solve this issue. Due to the application of an electric force, the nanofluid is able to move more quickly, and two primary vortices combine to form a single, stronger vortex. As voltage increases, Nu increases by approximately 125.52%. Utilizing greater permeable medium results in a stronger wall collision and a 113.29% increase in Nu. Nu increases by approximately 3.69% when a nanoparticle with a greater shape factor than the sphere is utilized.

A ferrofluid is a mixture that exhibits both magnetism and fluidity. This merit enables the ferrofluid to be used in a wide variety of areas. Here we show that a floating ferrofluid bridge can be induced between two separated boards under a balanced external magnetic field generated by two magnets, while a flying ferrofluid bridge can be induced under an unbalanced external magnetic field generated by only one magnet. The mechanisms of the ferrofluid bridges were discussed and the corresponding mathematical equations were also established to describe the interacting magnetic force between the ferro particles inside the ferrofluid. This work answered a basic question that, except for the well-known floating water bridges that are related to electricity, one can also build up a liquid bridge that is related to magnetism.

An electromagnetic generator with magnetic spring and ferrofluid is proposed and designed to harvest low-frequency vibration energy from human body motion. The magnetic spring is formed by gravity and magnetic repulsive force between the fixed and the moving permanent magnets. The ferrofluid is aggregated at both ends of the moving permanent magnet, and the ferrofluid layer between the plastic tube wall and permanent magnet can remove the solid-friction as a fluid lubricant. The electromagnetic generator is used to harvest human motion energy. The measured average load powers of electromagnetic generator with ferrofluid 1.5 g from human body motion are 1.3 mW and 7.5 mW during walking and low running, respectively, which is 30 times more than the measured average power of generator without ferrofluid.

The main objective of presented work is a rectangular plate subjected to dynamic in-plane load generated by magnetic field. The plate is made of polyethylene (PE). There are two pockets on each of the two opposite edges of the plate. These pockets with porous structure are filled in with ferrofluid. The coil system consists of two magnetic field coil subsystems. These systems are built of Helmholtz (MC) and Golay coils (GC) and generate nonhomogeneous magnetic field. If the magnetic field is more homogeneous, the compression load is induced. In other cases, local tensile load occurs (compression load dominates). For presented coil systems, the intensity of load was examined for two variants. For the first of them, the intensity of load was dependent on the radius of Golay coil arcs. The change of the radius of saddle coils also influences on the strength of the gradient of the magnetic field. The second one describes the intensity of load which depends on the change of GC radius without changing the strength of magnetic field (the strength of magnetic field is compensated with changing the current flowing through the coils wires or with changing the number of wires). In this paper, the analytical model of the plate is presented. The model of the plate was formulated with the use of classical Kirchhoff–Love hypothesis. Elastic strain energy, kinetic energy as well as work of load were formulated. The equation of motion was derived based on the Hamilton’s principle. The numerical studies were related to the analysis of the intensity of load distribution.

High-quality hydrophobic or hydrophilic ferrofluid based on magnetite (Fe₃O₄) nanoparticles can be synthesized by one-pot direct synthesis which involves thermolysis of Iron(III) acetylacetonate, Fe(acac)₃ in hydrophobic or hydrophilic stabilizing agent, respectively. The structure of the nanoparticles dispersed in the ferrofluid was studied using XRD, FTIR, XPS, and TGA analysis, while morphology and size of the nanoparticles were determined by TEM. The magnetic properties of the samples were measured using VSM and SQUID measurement. The results show that oleylamine (OM) and tri(ethylene glycol) (TREG) coated Fe₃O₄ nanoparticles which are well stabilized in hydrophobic and hydrophilic ferrofluid, respectively, are relatively monodisperse, single crystalline and superparamagnetic in nature with the blocking temperature at around 100 K.

Nitrated lignosulfonates were used to synthesize a water-based magnetic fluid. Lignosulfonates were nitrated by nitric acid under mild conditions and without further purification were used to synthesize a magnetic fluid. Part of the iron(II) were oxidized with an excess of nitric acid, so that the magnetoactive phase under the condensation by action of sodium hydroxide was formed. Optimum conditions for nitration of lignosulfonates and synthesis of a magnetic fluid were experimentally established. The optimum consumption of iron(II) was 1.3–1.5g per 1g of sodium lignosulfonate. Unlike the initial lignosulfonates, nitrated lignosulfonates have peptizing properties, due to which the precipitate formed during condensation was dispersed to nanoparticles (15–30nm). The resulting magnetic fluid had a high magnetic activity and was stable for a long time.

The present study is carried out to examine the effects of magnetic field-dependent viscosity on steady axi-symmetric ferrofluid flow due to rotating disk in porous medium. The momentum equations give rise to highly nonlinear partial differential equations, which are converted to a system of nonlinear coupled ordinary differential equations on using Karman's similarity transformation. Then a numerical technique, which is the combination of finite difference and shooting methods, is employed in MATLAB environment to get the numerical solution of the problem. Beside the velocity and pressure profiles, the effect of MFD viscosity parameter and porosity parameter are also examined on radial, tangential skin frictions and on boundary layer displacement thickness. The results thus obtained numerically over the entire range of physical parameters are presented graphically.

The purpose of present study is to investigate the effects of field dependent viscosity on swirling flow of an incompressible electrically non-conducting ferrofluid over a porous rotating disk with suction and heat transfer at the wall. Karman's similarity transformations are used to convert the governing boundary layer equations involved in the problem to a system of nonlinear coupled differential equations. The solution of this system is obtained by using a second-order numerical scheme which combines the features of Finite Difference method and Newton's zero finding algorithms. The flow characteristics including velocity and temperature profiles and boundary layer displacement thickness are studied for various values of MFD (magnetic field dependent) viscosity and suction parameter. Beside these, skin friction coefficients and the rate of heat transfer are also calculated on the surface of the disk. Magnetic field dependent viscosity and suction at the surface of porous rotating disk affect significantly the velocity and temperatures fields, rate of heat transfer and other flow characteristics in the generated ferrofluid boundary layer.

Nonlinear vibration and instability of a boron nitride micro-tube (BNMT) conveying ferrofluid under the combined magnetic and electric fields are investigated. Based on Euler–Bernoulli beam (EBB), piezoelasticity strain gradient theory and Hamilton's principle, high order equations of motion are derived for three boundary conditions namely as clamped–clamped (C–C), simply–simply (S–S) and clamped–simply (C–S). The differential quadrature method (DQM) is applied to discretize the motion equations in order to obtain the nonlinear frequency and critical fluid velocity using a direct iterative method. A detailed parametric study is conducted to elucidate the influences of the various boundary conditions, size diameter and magnetic field on vibrational characteristic of BNMT. Numerical results indicate that the effect of magnetic field appears in higher speed of ferrofluid and increases the critical velocity or enlarges the stability region. The results are in good agreement with the previous researches. The results of this study can be used to manufacture smart micro/nano electromechanical systems in advanced biomechanics applications with magnetic and electric fields as parametric controllers.

Using combination of ferrofluid (FF) and Fe-based amorphous alloy in the advanced treatment of high concentration, organic wastewater was investigated. The addition of Fe_73.5Nb₃Cu₁Si_13.5B₉ amorphous alloy powders into a FF give rise to a dramatic enhancement in decreasing chemical oxygen demand (COD) and decolorization. The removal rate of COD by using FF that combined Fe_73.5Nb₃Cu₁Si_13.5B₉ metallic glass (MG) particles reached 92% in the presence of H₂O₂, nearly more than 50% higher than that by using only FF. Furthermore, compared with the FF, the decolorizing effect of the combination was 20% higher. It has been found that MG powders with the amorphous structures have high efficiency of waste water treatment and lead to high catalytic ability.

The effect of feedback control on the onset of Bénard-Marangoni ferroconvection in a horizontal ferrofluid layer heated from below is investigated theoretically. The lower boundary is rigid and the upper free boundary is assumed to be flat and undeformable. A linear stability analysis is used and the Galerkin method is employed to find the critical stability parameters numerically. It is found that the onset of instability can be delayed through the use of feedback control.

Biomanipulation based on micro/nano structures is an attractive approach for biotechnology. To manipulate biological systems by magnetic forces, the magnetic labeling technology utilized magnetic nanoparticles (MNPs) as a common rule. Ferrofluid, well-dispersed MNPs, can be used for magnetic modification of the surface or as molds to form organized microstructures. For magnetic-based micro/nano structures, different methods to modulate magnetic field at the microscale have been developed. Specifically, this review focused on a new strategy which uses the concept of micromagnetism of patterned magnetic thin film with specific domain walls configurations to generate stable magnetic poles for cell patterning.

The progress in the synthesis of new magnetic nanoparticles and agglomerates stimulates the development of novel ferrofluids with enhanced rheological properties. In the current work ferrofluids based on Co-nanoplatelets and clustered iron oxide nanoparticles have been considered. Steady-shear experiments and yield stress measurements of these ferrofluids have been performed using rotational rheometry.