Structure Of Ferrofluids Research Articles

Characteristics of temperature-dependent shear flow in an ultrasonicated ferrofluid

The rheological effect of manganese zinc (Mn-Zn) ferrite ferrofluid was studied and the impact of temperature on the magnetoviscosity and viscoelastic system of manganese-zinc ferrite ferrofluid generated by co-precipitation process is investigated. At 25 ◦C, a ferrofluid structure that is both hard and elastic is produced. As the temperature rises, the fluid structure loses its elasticity and becomes semi-rigid. When a low relaxation modulus is applied, the fluid behavior, which is temperature-dependent, exhibits the development of linear stress relaxation and steady state flow. When a greater relaxation modulus is used, non-steady state flow results. At a high temperature of 50 ◦C, steady state flow is quickly obtained, whereas at a low temperature, equilibrium or steady state is attained more slowly. At low temperatures, the fluid exhibits a solid-like structure, whereas at high temperatures, a liquid-like structure forms as the fluid’s viscosity decreases. With the creation of yield stress in the region with high shear rates, shear stress increases with temperature, and yield stress increases with temperature. The viscoelastic system is underdamped, and the amount of fluid deflected at 25 ◦C is small, which prevents the disruption of the fluid’s rheology. This develops as a result of the presence of a small deflection angle, which facilitates the development of high magnetoviscosity at low temperature. High viscous effect forms at low temperatures due to the creation of low shear stress and low deflection angle at temperature 25 ◦C. The sample has a single phase and FCC structure, which X-ray diffraction research has verified.

A simplified phase-field lattice Boltzmann method with a self-corrected magnetic field for the evolution of spike structures in ferrofluids

This research presents a numerical analysis of the normal field instability for an initially flat layer of ferrofluid under the influence of magnetic field. A coupling between the simplified lattice Boltzmann method and the self-correcting procedure is developed to capture the velocity field and magnetic field. The proposed method has the ability to simulate complex hedgehog and comb-like spike structures without using an additional magnetization equation. A single dipole permanent magnet is defined instead of multiple point dipoles which makes this method much simpler and more efficient compared to the numerical approaches available in the literature. A comparison between the simulation results and experimental findings is provided to verify the validity of our method. A criterion for the prediction of spikes is presented for uniform magnetic fields. This study also investigates the effects of different types of magnetic fields, their strengths, and the effect of surrounding non-magnetic fluid on the spike structures. Moreover, the description of magnetic field lines, distribution of magnetic flux density, and energy estimation are also provided in this work which gives a useful insight into the hydrodynamic as well as the magnetostatic behavior of ferrofluids.

The role of small separation interactions in ferrofluid structure

Interparticle interactions in colloids are traditionally modeled by means of the DLVO theory, which includes van der Waals and electrical double layer (EDL) interactions However, the validity range limitations become critical in biocompatible magnetic colloids, requiring a more detailed description of the interactions, especially at small intersurface separations. As magnetic colloids, ferrofluids require an extended DLVO (XDLVO) model that includes magnetic interactions. Moreover, the nanoparticles of biocompatible ferrofluids are usually ionic-surfacted, such that their charged surfactants interact both electrically and sterically. In some of such particles, the charge is usually not located at the surface, but at the outer extremities of the surfactant molecules, and this feature restricts the EDL model validity to larger separation distances. We addressed this problem by means of a model proposed by Schnitzer and Morozov, which employs a generalized Derjaguin approximation that makes the EDL repulsion expression valid for all separations. The van der Waals expression of the DLVO theory is also problematic because it shows an unphysical divergence as the intersurface separation tends to zero, a problem that was circumvented by replacing the expression at small separations with another expression based on cohesion energy and the Born-Mayer repulsion. The modifications proposed here are of interest for research on colloids in general and our Monte Carlo simulations show that they acquire even greater importance when it comes to ferrofluids. The influence of magnetic interparticle interactions on the colloid structure is better gauged using these modifications, which prevent magnetic interactions from being obfuscated by artificially large van der Waals and EDL interactions. This conclusion makes the small separation treatment particularly important for the study of magnetic colloids.

Fully nonlinear simulations of ferrofluid patterns in a radial magnetic field

Simulations reveal the emergence of complex starlike ferrofluid structures having sharp finger tips. These patterns result from an interplay of surface tension, magnetic, and viscous effects. This work supplements previous studies, which examined the system via linear and weakly nonlinear analyses.

The effect of long time exposure to light of a water-based ferrofluid on its low frequency complex magnetic permeability

The paper reports on the effect of prolonged exposure to light of a water-based ferrofluid, with magnetite particles, double stabilized with oleic acid, on its low frequency complex magnetic permeability.The investigated ferrofluid was divided into three samples and placed in identical transparent glass vials. The reference sample, denoted by SR was kept in dark (in the laboratory), other sample, denoted by SL, was exposed to the light of a table lamp, with LED bulb (in the laboratory) and the third sample, denoted by SG, was exposed to natural sunlight, in a garden (subjected to the succession of days and nights as well as to the weather conditions). The duration of exposure to light was 80 days, during the summer.Complex magnetic permeability measurements over the frequency range 1 kHz–2 MHz show that after exposure to light both SL and SG samples exhibit Brownian maxima, unlike the reference sample, which has no maximum.Dynamic light scattering measurements show that in all three ferrofluid samples, the magnetic nanoparticles are mainly organized in condensed phase droplets and after exposure to light, the diameter of agglomerations increases.The change in the magnetic permeability spectrum of the investigated samples is explained based on the change in the local structure of ferrofluids, due to the electron-hole pairs resulted by photoelectric effect in magnetite, which affect the electrochemical balance of the colloidal system.

Design of Magnetic Circuit and Finite Element Analysis of Stepped Ferrofluid Seal with Small Sealing Gap

For the sake of study the pressure capability of ferrofluid seals with small sealing gap, a diverging stepped ferrofluid structure with small sealing gap was designed according to the theory of stepped ferrofluid and magnetic path design, and the length and cross-sectional area of the permanent magnet (NeFeB) are obtained. On the basis of these parameters, the ferrofluid distribution of seal gap is calculated by using the magnetic ferrofluid element method, and the pressure capability is obtained, then the reliability of the magnetic path design is obtained according to the capability of withstand voltage. There are some findings as following after analysis and comparison. It is the magnetic flux leakage at the junction of the pole boot and NeFeB that cause pressure capability calculated by the magnetic path method is lower than that calculated by the finite element method.

SAXS Analysis of Magnetic Field Influence on Magnetic Nanoparticle Clusters

In this work, we investigated the local colloidal structure of ferrofluid, in the presence of the external magnetic field. The nanoparticles studied here are of the core-shell type, with the core formed by manganese ferrite and maghemite shell, and were synthesized by the coprecipitation method in alkaline medium. Measures of Small Angle X-ray Scattering (SAXS) performed in the Brazilian Synchrotron Light Laboratory (LNLS) were used for the study of the local colloidal structure of ferrofluid, so it was possible to study two levels of structure, cluster and isolated particles, in the regimes with and without applied magnetic field. In the methodology used here there is a combination of the information obtained in the system with and without magnetic field application. In this way, it is possible to undertake a better investigation of the colloidal dispersion. The theoretical formalism used: (i) the unification equation proposed by Beaucage G.; (ii) the analysis of the radial distribution function p ( r ) and (iii) theoretical calculation of the radius of gyration as a function of the moment of inertia of the spherical of n-nanoparticles.

Observing the Jumping Laser Dogs

Jumping sun dogs are rapid light flashes changing over clouds, with some of them located close to the places of halo formation in thunder storms clouds. This paper presents an outline of some aspects that are required for understanding the jumping sun dogs, using some experiments with light scattering in complex fluids. In our analogy, we have observed the jumping laser dogs, in which the ice crystals are replaced by needlelike structures of ferrofluid, the electric field in the atmosphere is represented by an external magnetic field, and the laser beam scattered by the ferrofluid structure has the same role of the sun as the source of light scattered by the ice crystals subjected to changing electric fields in thunderstorm clouds.

The dynamics analysis of a ferrofluid shock absorber

The paper presents a shock absorber using three magnets as the inertial mass. Movement of the inertial mass inside a cylindrical body filled with ferrofluid will lead to a viscous dissipation of the oscillating system energy. The influence of a dumbbell-like ferrofluid structure on the energy dissipation is considered and the magnetic restoring force is investigated by experiment and theoretical calculation. A theoretical model of the hydrodynamics and energy dissipation processes is developed, which includes the geometrical characteristics of the body, the fluid viscosity, and the external magnetic field. The theory predicts the experimental results well under some condition. The shock absorber can be used in spacecraft technology.

Direct observation of electric field induced pattern formation and particle aggregation in ferrofluids

Ferrofluids typically respond to magnetic fields and can be manipulated by external magnetic fields. Here, we report on formation of visually observable patterns in a diluted low-polarity ferrofluid exposed to external electric fields. This presents a specific type of ferrofluid structure driven by a combined effect of electrohydrodynamics and electrical body forces. The free charge and permittivity variation are considered to play a key role in the observed phenomenon. The corresponding changes in the ferrofluid structure have been found at nanoscale as well. By small-angle neutron scattering (SANS), we show that the magnetic nanoparticles aggregate in direct current (dc) electric field with a strong dependence on the field intensity. The anisotropic aggregates preferably orient in the direction of the applied electric field. Conducting SANS experiments with alternating current (ac) electric fields of various frequencies, we found a critical frequency triggering the aggregation process. Our experimental study could open future applications of ferrofluids based on insulating liquids.

Dispersion of ferrofluid aggregates in steady flows

Using focused shadowgraphs, we investigate steady flows of a magnetically non-susceptible fluid interacting with ferrofluid aggregates comprised of superparamagnetic nanoparticles. The ferrofluid aggregate is retained at a specific site within the flow channel using two different applied magnetic fields. The bulk flow induces shear stresses on the aggregate, which give rise to the development of interfacial disturbances, leading to Kelvin-Helmholtz (K-H) instabilities and shedding of ferrofluid structures. Herein, the effects of bulk Reynolds number, ranging from 100 to 1000, and maximum applied magnetic fields of 1.2 × 105 and 2.4 × 105 A/m are investigated in the context of their impact on dispersion or removal of material from the core aggregate. The aggregate interaction with steady bulk flow reveals three regimes of aggregate dynamics over the span of Reynolds numbers studied: stable, transitional, and shedding. The first regime is characterized by slight aggregate stretching for low Reynolds numbers, with full aggregate retention. As the Reynolds number increases, the aggregate is in-transition between stable and shedding states. This second regime is characterized by significant initial stretching that gives way to small amplitude Kelvin-Helmholtz waves. Higher Reynolds numbers result in ferrofluid shedding, with Strouhal numbers initially between 0.2 and 0.3, wherein large vortical structures are shed from the main aggregate accompanied by precipitous decay of the accumulated ferrofluid aggregate. These behaviors are apparent for both magnetic field strengths, although the transitional Reynolds numbers are different between the cases, as are the characteristic shedding frequencies relative to the same Reynolds number. In the final step of this study, relevant parameters were extracted from the time series dispersion data to comprehensively quantify aggregate mechanics. The aggregate half-life is found to decrease as a function of the Reynolds number following a power law curve and can be scaled for different magnetic fields using the magnetic induction at the inner wall of the vessel. In addition, the decay rate of the ferrofluid is shown to be proportional to the wall shear rate. Finally, a dimensionless parameter, which scales the inertia-driven flow pressures, relative to the applied magnetic pressures, reveals a power law decay relationship with respect to the incident bulk flow.

Low Field Magnetoresistance at Room Temperature in CrO2 Doped Polyvinyl Alcohol Films

ABSTRACTFerromagnetic CrO2 nanoparticles embedded in a diamagnetic matrix of polyvinyl alcohol (PVA) presents an example of granular giant magnetoresistance (GMR) in a diluted magnetic composite structure. Usefully significant MR occurs at room temperature, viz. -6.8% in 3.0 wt% CrO2-PVA, at field as small as 1.1 kOe. We suggest that the system is a natural analogue to dilute ferro-fluid structures composed of nanoscale magnetic particles in a viscous organic fluid (diamagnetic as well as an electrical insulator), i.e., this material displays a GMR effect without an introduction of chemical interfaces. It has great potential in various applications and can also facilitate studies of magnetotransport properties in GMR materials.

Comparative structure analysis of non-polar organic ferrofluids stabilized by saturated mono-carboxylic acids

The structure of ferrofluids (magnetite in decahydronaphtalene) stabilized with saturated mono-carboxylic acids of different chain lengths (lauric, myristic, palmitic and stearic acids) is studied by means of magnetization analysis and small-angle neutron scattering. It is shown that in case of saturated acid surfactants, magnetite nanoparticles are dispersed in the carrier approximately with the same size distribution whose mean value and width are significantly less as compared to the classical stabilization with non-saturated oleic acid. The found thickness of the surfactant shell around magnetite is analyzed with respect to stabilizing properties of mono-carboxylic acids.

Lamellar-like structures in ferrofluids placed in strong magnetic fields

We consider a ferrofluid system consisting of magnetic particles interacting with a magnetic dipole–dipole interaction. We study the strong magnetic field regime where all magnetic dipoles are completely polarized in the direction of the magnetic field. We introduce a lattice gas model that serves to describe space ordering phenomena in such systems. It is found that, within mean field theory, this model predicts a second order phase transition to a phase with inhomogeneous lamellar-like ordering below a certain critical temperature.

Nonlinear magnetic susceptibility of ferrofluids

In ferrofluids, external magnetic fields can produce a nonlinearity in the dependence of magnetic susceptibilities on the field strength, due to both normal saturation and abnormal saturation. We derive the nonlinear magnetic susceptibility by using a Langevin model. Then, we investigate the nonlinear ac responses of the magnetic susceptibility by means of a perturbation expansion method and an orthogonal numerical method. The two methods are numerically shown in excellent agreement. We find that the responses are sensitive to suspension structures and field frequencies. Thus, by detecting the ac responses, it seems possible to real-time-monitor the structure of ferrofluids.

Direct and inverse domain structures in ferrofluids

The work deals with the theoretical study of domain structures in a ferrofluid, filling a thin flat gap with hard non-magnetic walls, placed into magnetic field, perpendicular to the gap plane. Two experimentally observed types of these structures are considered. The first one is the direct domain structure, presenting a system of dense cylindrical-like domains in a relatively dilute surrounding ferrofluid. The second one is the inverse structure, i.e., the system of dilute cylindrical holes in the dense simply connected environment. We estimate dependencies of diameter of the domains on applied magnetic field, on the gap thickness and other parameters of the system. The diagrams, corresponding to the transition from the direct to inverse structure, are calculated.

A strategy for the assembly of three-dimensional mesoscopic structures using a ferrofluid

A novel technique for the self-assembly of three-dimensional mesoscopic structures in a forced fluid flow by employing a magnetic field is described. There are advantages of using magnetic fields for this purpose: unlike many other forces, a magnetic force is effective even from a distance, permitting “action at a distance,” it is also localized, and competition between the magnetic force and fluid shear enables unique self-assembled ferrofluid structures. Herein, a simulation provides insight into the possibility of using magnetic field to assemble colloidal nanoparticles into aggregates. Subsequently, a demonstration experiment is conducted to characterize the development and decay of such aggregates. The analysis provides a basis for developing effective self-assembly techniques for various engineering applications.

Mesoscale simulation of ferrofluid structure

By means of the lattice-Boltzmann (LB) method, the mesoscaled structure of ferrofluids consisting of magnetic nanoparticles and a carrier fluid as well as some surfactant is investigated. The ferrofluid is a complicated system and its morphology is affected by a number of internal and external forces including gravitational force, Brownian force, van der Waals attraction potential, and dipole–dipole interaction potential. All these factors are included in the lattice-Boltzmann model. The distribution of suspended magnetic nanoparticles and morphology of the ferrofluid are simulated in both cases of the absence and the presence of an external magnetic field. The effects of the dipole–dipole interaction energy and the thermal energy on the aggregation structures of the magnetic nanoparticles are discussed.

Ferrofluid pipe flow in an oscillating magnetic field

Ferrofluid pipe flow in an oscillating magnetic field along the pipe axis is studied theoretically in a wide range of the flow rate. The field-dependent part of viscosity (it can be positive or negative) reveals significant dependence on the flow vorticity, i.e., ferrofluids exhibit non-Newtonian behavior. This is manifested in an alteration of the velocity profile—it ceases to be parabolic—and deviation of the flow rate from the value prescribed by Poiseuille’s formula. The presented model based on the conventional ferrohydrodynamic equations and an assumption of the ferrofluid structure fits well experimental data recently obtained by Schumacher, Sellien, Konke, Cader, and Finlayson [“Experiment and simulation of laminar and turbulent ferrofluid pipe flow in an oscillating magnetic field,” Phys. Rev. E 67, 026308 (2003)].

The effect of temperature on the anisotropy of ultrasound attenuation in a ferrofluid

This paper reports results of a study on the acoustic properties of a ferrofluid, APG-513A. The anisotropy of the ultrasonic wave absorption coefficient was determined in a sample subjected to an external magnetic field. The measurements were performed in a strong magnetic field HDC = 125 kA m−1, for temperatures of 1–60°C. The dependence of the absorption coefficient of the ultrasonic wave (f = 3.6 MHz) on the angle between the direction of measurement and that of the magnetic field provided important information on the ferrofluid structure in a magnetic field. A comparison of the experimental results with the predictions of the Taketomi theory allowed a determination of the cluster radius and the number density of the colloidal particles. These data allowed us to draw conclusions about the changes in the parameters describing the structure of the ferrofluid at different temperatures. The per cent contribution of the magnetic particles in the cluster structures was estimated. On the basis of the measurements performed, it was possible to establish the proportion of the ultrasonic wave used for excitation of the translational and rotational degrees of freedom. The results revealed a significant effect of temperature on the translational and rotational components of the absorption coefficient. Moreover, the magnetic susceptibility was measured versus the external magnetic field intensity.