Preparation and characterization of silicon oil based ferrofluid

Stable silicon oil based ferrofluid was prepared in the present investigation. Silicon oil surfactant ethoxy terminated polydimethylsiloxane was used to modify the Fe₃O₄ nanoparticles. The Fe₃O₄ nanoparticles were firstly coated with a SiO₂ layer by the hydrolysis of tetraethoxysilane. Then using the active hydroxyl groups on the surface of the SiO₂, silicon oil surfactant was covalently grafted onto the Fe₃O₄ nanoparticles surface. The ethoxy terminated polydimethylsiloxane has similar molecular chain structure and good compatibility with that of the carrier liquid, thus ensuring stable dispersion of modified Fe₃O₄ in the carrier silicon oil. The interaction between Fe₃O₄ and the modifier was characterized by IR and XPS. The crystal structure and the magnetic properties of the Fe₃O₄ nanoparticles were determined by XRD and VSM, respectively. The size and morphology of the particles were observed using TEM. The properties of the silicon oil based ferrofluid were characterized by Gouy magnetic balance. The results indicated that the ferrofluid had high magnetism and good stability. The rheological properties and thermostability of the ferrofluid were also investigated.     

Magnetite ferrofluid with high specific absorption rate for application in hyperthermia

A magnetite ferrofluid coated by dextran with a high specific absorption rate (SAR) of 75 W/g in an AC field of 55 kHz and 200 Oe was prepared by the gel crystallization method with ultrasonic treatment. For comparison, uncoated magnetite particles with a mean diameter of 50 nm were also fabricated. Several possible mechanisms such as Brownian, Neel and diffusion relaxation processes on heating effects and their influence on SAR are discussed. Several factors which can increase the value of SAR were discussed, including dextran coating, ultrasonic treatment, proper particle size and the presence of defects and disorder in the particles.     

Using genetic algorithms to characterize ferrofluid topographies in externally applied magnetic fields

Both ferrofluidics and genetic algorithms are relatively new fields. Due to complex physical interactions, ferrofluidic topographies and assemblies have only been solved using finite time step, Lattice Boltzmann, and finite-element methods in very simple magnetic field configurations. In this paper, we show that it is possible (and highly advantageous) to employ genetic algorithms to solve for the fluid topographies, which can be extended to include more complex magnetic fields.     

Stationary thermal convection in a viscoelastic ferrofluid

We report theoretical and numerical results on convection for a magnetic fluid in a viscoelastic carrier liquid. We focus in the stationary convection for idealized boundary conditions. We obtain explicit expressions of convective thresholds in terms of the control parameters of the system. Close to bifurcation, the coefficients of the corresponding amplitude equation are determined analytically. Finally, the secondary instabilities are performed.     

Correlation for Nusselt number in pure magnetic convection ferrofluid flow in a square cavity by a numerical investigation

Magnetic convection heat transfer in a two-dimensional square cavity induced by magnetic field gradient is investigated numerically using a semi-implicit finite volume method. The side walls of the cavity are heated with different temperatures, the top and bottom walls are isolated, and a permanent magnet is located near the bottom wall. Thermal buoyancy-induced flow is neglected due to the nongravity condition on the plane of the cavity. Conditions for the different values of non-dimensional variables in a variety of ferrofluid properties and magnetic field parameters are studied. Based on this numerical analysis, a general correlation for the overall Nusselt number on the side walls is introduced for a wide range of effective parameters. Results showed that maximum error produced by use of this correlation is about 6 percent.     

Experimental investigation for enhanced ferrofluid heat transfer under magnetic field effect

This paper reports an experimental work on the convective heat transfer of ferrofluid flowing through a heated copper tube in the laminar regime in the presence of magnetic field. Significant enhancement on the heat transfer of ferrofluid by applying various orders of magnetic field is observed in this experiment. Also in this experiment, the effect of magnetic nanoparticles concentrations and magnet position have been investigated. The main reason for the enhancement of heat transfer coefficient could be caused due to remarkable changes in thermophysical properties of ferrofluid under the influence of applied magnetic field.     

Low-field DC-magnetization study of Ho-doped Mn–Zn ferrite ferrofluid

The physical and magnetic properties of magnetic nanoparticles are crucial for their effectiveness and reliability in biomedical applications. In this article, we report the synthesis of a stable Ho-substituted Mn–Zn ferrite ferrofluid and its physical and magnetic properties. Substitution by rare earth metal plays an important role in determining the magneto-crystalline anisotropy in 4f-3d inter-metallic compounds. Ho³⁺ substitution not only enhanced the magnetic anisotropy but also produced strong spin frustration at low temperature. The field dependence of blocking temperature shows H^2/3 dependency in the entire range of field, i.e., 10–700 Oe, indicating the emergence of Ising spins characteristics in the present system.     

空气域中铁磁流体动态控制研究

根据铁磁流体磁化后会受到磁力,退磁后不存在任何磁滞的超顺磁性质,提出了铁磁流体的动态控制方法,对喷射在空气域中的铁磁流体液柱直接添加电磁场实现其动态偏转驱动。在修正后的纳维-斯托克斯(N-S) 控制方程中加入表面张力、重力及磁力,并结合磁感应方程,建立了铁磁流体动力学(FHD)模型。利用二次开发的Fluent流体仿真软件建立了铁磁流体在空气域中喷射的流体体积函数(VOF)多相流模型,仿真了在不同磁场强度下铁磁流体的液相分布及分散状况,分析了磁性对其动力学行为的影响。结果表明,随着磁场强度和喷射距离的增大,铁磁流体沿磁场方向速度及偏移量增大,其发散情况逐渐明显。     

Frequency-domain birefringence measurement of biological binding to magnetic nanoparticles

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

Ground state structures in ferrofluid monolayers

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

Preparation of a biocompatible magnetic film from an aqueous ferrofluid

Very promising nanoparticles for biomedical applications or in medical drug targeting are superparamagnetic nanoparticles based on a core consisting of iron oxides (SPION) that can be targeted through external magnets. Polyvinyl alcohol (PVA) is a unique synthetic biocompatible polymer that can be chemically cross-linked to form a gel. Biotechnology applications of magnetic gels include biosensors, targeted drug delivery, artificial muscles and magnetic buckles. These gels are produced by incorporating magnetic materials in the polymer composites. In this paper we report the synthesis of an aqueous ferrofluid and the preparation of a biocompatible magnetic gel with polyvinyl alcohol and glutharaldehyde (GTA). HClO₄ was used to induce the peptization since this kind of ferrofluid does not have surfactant. The magnetic gel was dried to generate a biocompatible film.     

Numerical investigation of optically induced microconvection in thin ferrofluid layers

We consider a periodic concentration grating induced in a thin layer of ferrofluid by an optical grating of interfering laser beams under the action of an externally applied uniform magnetic field. The direction of the applied field is parallel to the concentration gradient. Under certain conditions microconvective instability may develop during relaxation of the grating, driven by the internal demagnetizing field due to the nonhomogenous distribution of concentration. A series of numerical simulations has been performed to determine the role of microconvection in the dissipation of the concentration grating.     

The stability of ferrofluid flow in a vertical layer subject to lateral heating and horizontal magnetic field

We use the Langevin law of magnetization to study the linear stability of a convective flow in a flat vertical layer of ferrofluid subject to a transverse temperature gradient and a uniform magnetic field. The stability of the flow against planar, spiral and three-dimensional perturbations is examined, and the stability boundaries and characteristics of critical disturbances are determined. The competition between the monotonic mode and two types of wave modes is analyzed taking into account the properties of the fluid (magnetic susceptibility and Prandtl number) and the magnetic field strength. The domain of parameters where the oscillatory thermomagnetic wave instability exists is found.     

Synthesis and rheological properties of an iron oxide ferrofluid

A ferrofluid (FF) was synthesized in air using a co-precipitation method. Some rheological properties and magnetoviscous effects of this sample were studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used for characterization of the solid particles, and the rheological properties were investigated with a special rheometer with variable magnetic field. Magnetic particles with mean particle size of 10.6 nm were obtained. Rheological results show that the shear thinning behavior in the absence and presence of magnetic field is different from that based fluid behavior. Moreover, contrary to expectation, the magnetoviscous effect showed an initial increase at low shear rates (near 15 s⁻¹) and decrease at higher shear rates. The rheological properties of FF depend on the rearrangement of nanoparticles. In addition, time is an effective factor in the formation and destruction of magnetically induced structures.     

Parametric investigation of heating due to magnetic fluid hyperthermia in a tumor with blood perfusion

Magnetic fluid hyperthermia (MFH) is a cancer treatment that can selectively elevate the tumor temperature without significantly damaging the surrounding healthy tissue. Optimal MFH design requires a fundamental parametric investigation of the heating of soft materials by magnetic fluids. We model the problem of a spherical tumor and its surrounding healthy tissue that are heated by exciting a homogeneous dispersion of magnetic nanoparticles infused only into the tumor with an external AC magnetic field. The key dimensionless parameters influencing thermotherapy are the Péclet, Fourier, and Joule numbers. Analytical solutions for transient and steady hyperthermia provide correlations between these parameters and the portions of tumor and healthy tissue that are subjected to a threshold temperature beyond which they are damaged. Increasing the ratio of the Fourier and Joule numbers also increases the tumor temperature, but doing so can damage the healthy tissue. Higher magnetic heating is required for larger Péclet numbers due to the larger convection heat loss that occurs through blood perfusion. A comparison of the model predictions with previous experimental data for MFH applied to rabbit tumors shows good agreement. The optimal MFH conditions are identified based on two indices, the fraction I_T of the tumor volume in which the local temperature is above a threshold temperature and the ratio I_N of the damaged normal tissue volume to the tumor tissue volume that also lies above it. The spatial variation in the nanoparticle concentration is also considered. A Gaussian distribution provides efficacy while minimizing the possibility of generating a tumor hot spot. Varying the thermal properties of tumor and normal tissue alters I_Tand I_N but the nature of the temperature distribution remains unchanged.     

Towards ferrofluids with enhanced magnetization

It is well known that the saturation magnetization of a sterically stabilized magnetic fluid (ferrofluid) is limited by the presence of a surfactant coating on the surface, and in some cases, by an effectively demagnetized surface layer in the solid magnetic particle. These surface layers take up a disproportionate volume in the colloidal dispersion thereby severely limiting the volume fraction of the core magnetic substance. This work proposes and analyzes Janus particles having the objective of increasing the magnetic loading beyond the present day constraints. Using numerical computation of the virial coefficient it is calculated that the magnetic volume fraction of magnetite ferrofluids might be increased by a factor approaching 2 and that of iron-based ferrofluids by a factor of 3.     

On stationary convection and oscillatory motions in ferromagnetic convection in a ferrofluid layer

It is shown analytically that the ‘principle of the exchange of stabilities’ (PES), in general, is not valid in ferromagnetic convection in a ferrofluid layer, for the case of free boundaries and hence a sufficient condition is derived for the validity of the PES. Upper bounds for the complex growth rate are then obtained. It is proved that the complex growth rate σ=σ_r+iσ_i (where σ_r and σ_i are, respectively, the real and imaginary parts of σ) of an arbitrary oscillatory motion of growing amplitude, in ferromagnetic convection in a ferrofluid layer, for the case of free boundaries lies inside a semicircle in the right half of the σ_rσ_i-plane whose center is at the origin and ²(radius)=RM₁/P_r, where R is the Rayleigh number,M₁ is the magnetic number and P_r is the Prandtl number. Further, bounds for the case of rigid ferromagnetic boundaries are also derived separately.     

Effect of magnetic field-dependent viscosity on revolving ferrofluid

The effect of magnetic field-dependent viscosity on the revolving axi-symmetric steady flow of ferrofluid in a disc is investigated by solving the boundary layer equations using Neuringer-Rosenweig model. Besides numerically calculating the velocity components and pressure for different values of magnetic field-dependent viscosity with variation in dimensionless parameter α (Karman’s parameter), we also have calculated the thickness of the boundary layer and the total volume flowing outward the z-axis. Here, the solutions of non-linear differential equations are obtained in the form of asymptotic series.     

Biomagnetic fluid flow in a channel with stenosis

In this study, the fundamental problem of the biomagnetic (blood) fluid flow in a channel with stenosis under the influence of a steady localized magnetic field is studied. The mathematical model used for the formulation of the problem is consistent with the principles of ferrohydrodynamics (FHD) and magnetohydrodynamics (MHD). Blood is considered as a homogeneous Newtonian fluid and is treated as an electrically conducting magnetic fluid which also exhibits magnetization. For the numerical solution of the problem, which is described by a coupled, non-linear system of PDEs, with appropriate boundary conditions, the stream function-vorticity formulation is adopted. The solution is obtained by the development of an efficient pseudotransient numerical methodology using finite differences. This methodology is based on the development of a semi-implicit numerical technique, transformations and stretching of the grid and proper construction of the boundary conditions for the vorticity. Results concerning the velocity and temperature field, skin friction and rate of heat transfer indicate that the presence of the magnetic field influences the flow field considerably.     

Structure of Co-Zn ferrite ferrofluid: A small angle neutron scattering analysis

A hydrothermal synthesis route is used to synthesize nanomagnetic particles of Co_0.3Zn_0.7Fe₂O₄ ferrite ferrofluids with particle diameter ranging from 5.5–9 nm. XRD analysis shows the formation of a single phase spinel structure. EDX results confirm the stoichiometric composition of the cations. Small angle neutron scattering technique is used to determine the size and size distribution of Co_0.3Zn_0.7Fe₂O₄ ferrofluid. The sizes thus obtained are in the range of 5.4 to 8.4 nm. These results are in agreement with magnetic measurements.