Ferrofluid Samples Research Articles

Electric partial discharges in biodegradable oil-based ferrofluids: A study on effects of magnetic field and nanoparticle concentration

This paper presents an experimental study of partial discharge activity in ferrofluids based on biodegradable transformer oil and iron oxide nanoparticles. Three ferrofluid samples with low, medium and high nanoparticle concentrations are employed in the research. The basic ferrofluid characterization is followed by a partial discharge experiment exposing the ferrofluids to a high voltage in a needle-plate electrode configuration. The analysis confirms that the apparent charge and number of discharges decrease with increasing nanoparticle concentration. These findings are interpreted with reference to the well-recognised electro-hydrodynamic streamer model. The charge trapping by nanoparticles hinders the ionization and discharge development. The study also focuses on the partial discharge activity in the ferrofluids under the action of a static magnetic field acting perpendicularly to the electric field. A decreasing trend in the number of discharges due to the magnetic field is revealed. A qualitative explanation is provided based on the field-induced cluster formation and charge mobility reduction. The presented experiment and the discussed findings may be valuable for practical application of the ferrofluid in high voltage equipment with a special need for partial discharge suppression.

Dielectric spectrum of a ferrofluid layer exposed to a gradient magnetic field.

A low-frequency dielectric response of a ferrofluid based on transformer oil and MnZn ferrite nanoparticles is investigated in a gradient magnetic field. Four ferrofluid samples of various nanoparticle concentrations were introduced into planar micro-capacitors located over a magnetized tip. The dielectric spectra were measured in the frequency range from 0.1Hz to 200kHz and in the local magnetic field up to 100 mT. The spectra exhibit a dielectric relaxation ascribed to nanoparticle interfacial polarization. The low-frequency spectrum of each ferrofluid decreases upon application of the magnetic field up to 20 mT. The decrease in dielectric permittivity is caused by a magnetic force acting on larger nanoparticles in the gradient magnetic field. It is assumed that the interfaces of the concentrated nanoparticles in the gradient field do not contribute to the effective dielectric response. This reduces the effective relaxation time and shifts the relaxation toward higher frequencies. The dielectric spectra are well described by a relaxation fit function consisting of one Havriliak-Negami and a conductivity term. The fitting confirms that the only effect of the gradient magnetic field on the dielectric spectra is the shift of the dielectric relaxation and the decrease of the amplitude in the imaginary permittivity. This behavior is evident from a master plot, where all dielectric relaxations are superimposed on a single line. The knowledge of the presented behavior of the ferrofluid may be valuable when applying a ferrofluid to sharply magnetized parts of various electrical equipment (wires, tips, screws, nails, edges) as a liquid dielectric medium.

DETERMINATION OF THE PHYSICO-CHEMICAL CHARACTERISTICS OF A MAGNETIC LIQUID IN THE IMPLEMENTATION OF THE METHOD BASED ON AN ELECTROMAGNETIC CONVERTER

A non-contact three-parameter electromagnetic method for measuring magnetic susceptibility κ, electrical conductivity χ, and temperature t of a ferrofluid sample is proposed. Theoretical principles of the operation of the inductive parametric electromagnetic transducer (IPET) with a sample of ferro-fluid are considered. As a result of studies of the universal functions of transformation of the IPET with a sample of the ferro-fluid, the theoretical provisions of the work of the IPET were further developed, which are related to the possibility of estimating the static parameters of the magnetic fluid. Because record of the effect of eddy currents leads to the need to determine three parameters of magnetic fluids with only one IPET, the necessity of using an inductive IPET switching circuit with a ferro-fluid sample, which provides for compensation of interfering external inductance L1 using a compensating capacitance P567 to improve the accuracy of measurements of the physical and chemical parameters of magnetic fluids, has been proved. The operation of the circuit is based on the fact that the eddy EMF excites a magnetic flux in the sample of the magnetic fluid under study, which is added geometrically with the excited magnetic flux from an external source, creating the resulting magnetic flux Ф2t in the sample of the magnetic fluid under study, while the resulting magnetic flux decreases in magnitude and shifts by the phase angle in relation to the excited magnetic flux, all this leads to a change in the components of the signals of the IPET, that is: inductance Lit and resistance Ω2t, which are associated with the physicochemical parameters κ, χ and t of the magnetic fluid sample. In further studies, in order to increase the efficiency of wastewater treatment of a mini-brewery, it is recommended to use a magnetic fluid in complex cleaning methods that involve the use of magnetic fluids in filters for post-treatment of wastewater from food production of acidic and alkaline composition.

PH-dependent phase transitions in ferrofluids: A Monte Carlo simulation study using an extended DLVO model

The present survey focuses on the pH-dependent phase behavior of acidic ferrofluids using Monte Carlo simulations. Samples based on CoFe2O4@γ-Fe2O3 core@shell nanoparticles with five different mean sizes were investigated. The interparticle interactions were modeled employing an extended DLVO theory, which includes the anisotropic magnetic dipolar interaction. The electrostatic repulsive component considers the dependence of the surface charge density with pH and nanoparticle mean size. The simulations highlight the effect of pH on the phase behavior of the samples, where the pH increase towards neutral induces an increase in the number of agglomerates, while also reducing first-neighbor distances, especially for samples based on larger nanoparticles. The results obtained in these simulations agree with experimental observations carried out on acidic ferrofluid samples and allow a reliable characterization of the stability domains of these systems. Ultimately, the general pH-dependent phase behavior is confirmed through a detailed analysis of the results of the simulated samples.

Effect of Magnetic Interactions on Magnetic Remanence in a Fine Particle System

The magnetic field-dependent remanent magnetization and demagnetization of magnetite nanoparticles dispersed in a frozen isoparaffin oil have been measured as a function of magnetite concentration. Frozen ferrofluid samples with magnetite volume fractions between 7.1% and 0.03% were demagnetized using either thermal or dc demagnetization and were kept frozen at 77 or 4 K during measurement to restrict mechanical rotation and translation of the particles. The magnetization and demagnetization remanences of the frozen ferrofluid were measured and plotted parametrically in the magnetic field magnitude to produce Henkel and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta M$ </tex-math></inline-formula> plots for comparison with the Wohlfarth model. The Henkel and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathrm {\Delta }M$ </tex-math></inline-formula> plots of the ferrofluid samples with higher concentrations of magnetite deviated from the predictions of the Wohlfarth non-interacting model, consistent with demagnetizing interactions in the system. The magnitude of this deviation decreased with decreasing magnetite concentration. A simple model is presented that quantitatively explains the deviations from the expected relations and can be used in other systems to characterize magnetic interactions.

Control of Magnetic Particle Size in Ferrofluid and Its Effect on Rheological Properties

Rheological properties are the theoretical basis and the key to common problems in ferrofluid applications, therefore they are expected to be adjustable to satisfy different technical requirements through altering the microstructure of ferrofluid during the process of preparation. In this paper, Four ferrofluid samples with different magnetic particle size were prepared by controlling the concentration of precursor solution during co-precipitation process and the rheological properties of these samples were investigated. These samples exhibited field-controlled rheological properties. Eternal magnetic field would enhance the formation of microstructures, resulting in an increase of viscosity. While with the increase of shear rate, microstructures tended to be destroyed, causing viscosity to decrease. There were two opposing mechanisms of the influence of precursor solution concentration. On one hand, the reduction of the precursor solution concentration would produce primary magnetic particles of smaller size. But on the other hand, the surfactant became insufficient to completely coat the magnetic particles because of an increased specific surface area, causing the magnetic particles to aggregate and form secondary clustering structures which strongly enhanced the magnetoviscous effect and weakened the viscoelastic effect.

Nonlinear behavior of the impedance spectrum of a kerosene based ferrofluid

We investigate the nonlinear behavior of the electric impedance of a kerosene-based ferrofluid (FF) sample subjected to an ac electric voltage of amplitude ranging from 10 mV to 3 V in the frequency range 6.3 mHz, 100 kHz. The FF sample was inserted between two parallel gold electrodes separated by 127 μm distance. The results show that even a sinusoidal voltage of amplitude low as 80 mV can give origin to nonlinear effects for frequency of the applied voltage smaller than 100 mHz. Our experimental data confirm the results obtained by solving numerically the equations of the Poisson-Nernst-Planck model. From this agreement it follows that the model based on the equation of continuity for the mobile ions, and the equation of Poisson for the actual potential across the sample, works well also in its non-linear version.

A numerical study on the interplay between the intra-particle and interparticle characteristics in bimagnetic soft/soft and hard/soft ultrasmall nanoparticle assemblies.

A mesoscopic scale approach and the Monte Carlo (MC) method have been employed to study the exchange bias behaviour of MnFe2O4 (soft)/maghemite (soft) and CoFe2O4 (hard)/maghemite (soft) nanoparticles (NPs) of size ∼ 3 nm in dense and diluted assemblies at low temperatures. The analysis of our MC results clearly shows that in the powder samples the contribution to the exchange bias field (Hex) and the coercivity (Hc) comes mainly from the intraparticle core/shell structure in the hard/soft sample and that the interplay between the internal characteristics and the interparticle interactions is more important in the soft/soft samples where the dipolar strength is enhanced. In the diluted frozen ferrofluid samples where interparticle exchange interactions are absent and the role of the dipolar interactions is not significant the exchange bias effects are reduced, and they come from the intra particle structure. The variation of Hex and Hc with the applied cooling field well reproduces the experimental findings and sheds light on the key mechanisms of the observed magnetic behaviour. Our results demonstrate the possibility to control the magnetic behaviour of nanostructures by using properly chosen core/shell bimagnetic nanoparticles.

The effect of a polarizing magnetic field on the dynamic properties and the specific absorption rate of a ferrofluid in the microwave range

ABSTRACT Measurements are presented of the frequency and field dependent, complex magnetic permeability, μ(f, H) = μ′(f, H)-i μ″(f, H), of a kerosene-based ferrofluid sample with magnetite particles, over the frequency range, f, of 0.4–6 GHz, and polarizing field, H, range of 0–102 kA/m. In this frequency range, both the ferromagnetic resonance at a frequency, f res , and the corresponding maximum absorption at a frequency, f max , were determined. From the H dependence of both f res and the ratio of f max /f res , we determined the anisotropy field, H A , the anisotropy constant K eff , the gyromagnetic ratio, , the damping parameter, , the spectroscopic splitting factor, g, and the internal magnetic viscosity, . Also, the theoretical Néel relaxation time, , was evaluated. Furthermore, the measurements of, μ(f), enabled one to study the effect of, H, on the the specific absorption rate, SAR, and the time dependence of the variation in temperature, ΔT, of the investigated ferrofluid sample. The SAR was calculated, using a new equation for the calculation of the SAR of ferrofluids. This equation offers a more precise computation of the SAR in that it takes into account the density of the ferrofluid ρ F , as opposed to using just the density of the dispersed solid particles, ρ S . These results illustrate how control of the SAR and the variation in ΔT, of the ferrofluid sample, through the variation of H, and has applications in the treatment of cancer by magnetic hyperthermia, for modeling bio-heat transfer phenomena, microwave heating or the possibility of use of the ferrofluid as a shield material against microwave electromagnetic radiation. Also, knowledge of the magnetic parameters of ferrofluids, is useful in the design and manufacture of some microwave devices.

Numerical Investigation of Ferrofluid Preparation during In-Vitro Culture of Cancer Therapy for Magnetic Nanoparticle Hyperthermia.

Recently, in-vitro studies of magnetic nanoparticle (MNP) hyperthermia have attracted significant attention because of the severity of this cancer therapy for in-vivo culture. Accurate temperature evaluation is one of the key challenges of MNP hyperthermia. Hence, numerical studies play a crucial role in evaluating the thermal behavior of ferrofluids. As a result, the optimum therapeutic conditions can be achieved. The presented research work aims to develop a comprehensive numerical model that directly correlates the MNP hyperthermia parameters to the thermal response of the in-vitro model using optimization through linear response theory (LRT). For that purpose, the ferrofluid solution is evaluated based on various parameters, and the temperature distribution of the system is estimated in space and time. Consequently, the optimum conditions for the ferrofluid preparation are estimated based on experimental and mathematical findings. The reliability of the presented model is evaluated via the correlation analysis between magnetic and calorimetric methods for the specific loss power (SLP) and intrinsic loss power (ILP) calculations. Besides, the presented numerical model is verified with our experimental setup. In summary, the proposed model offers a novel approach to investigate the thermal diffusion of a non-adiabatic ferrofluid sample intended for MNP hyperthermia in cancer treatment.

Analysis of the ferrofluid microstructure based on the static magnetic measurements

In this work, we applied regularized inversion method [New J. Phys. 19, 073012 (2017)] for microstructure analysis of 10 ferrofluid samples with different ferroparticle concentrations, which were obtained by diluting of the first one. The experimental measurements of the magnetization M(H) of all samples were subjected to numerical inversion using 120 ferroparticle fractions. Since volume concentration of ferrofluid samples does not exceed 3%, the properties of each fraction were described using Langevin theory. The magnetogranulomertic analysis confirmed that all 10 samples have the same magnetic moment distribution function, which has two peaks. The first (small) peak in the magnetic moment distribution function demonstrates the presence of a certain number of aggregates or correlated ferroparticle structures in the samples, whose magnetic moments have been compensated. The second peak corresponds to ordinary single particles distributed in the sample. The main advantage of using numerical inversion method is the absence of an a priori fixed form of the distribution function instead of a standard log-normal or Γ-distribution.

Microscopic understanding of particle-matrix interaction in magnetic hybrid materials by element-specific spectroscopy

Abstract Mössbauer spectroscopy is a well-known technique to study complex magnetic structures, due to its sensitivity to electronic and magnetic interactions of the probed nucleus with its electronic surrounding. It has also been applied to the emerging fields of magnetic hybrid materials as well as to ferrofluids, as information on the magnetic alignment and the velocity of the probed nucleus, i.e. of the particle it is embedded in, can be inferred from the spectra in addition to the above-mentioned quantities. Considering the wide range of preparation methods and sample properties, including fluids, particle powders, sintered pellets, polymer matrices and viscoelastic hydrogels, a considerable advantage of Mössbauer spectroscopy is the usage of γ-photons. This allows measurements on opaque samples, for which optical experiments are usually not feasible, also making the technique relatively independent of specific sample geometries or preparation. Using iron oxide nanoparticles in glycerol solution as an exemplary material here, the variety of system parameters simultaneously accessible via Mössbauer spectroscopy can be demonstrated: Spectra recorded for particles of different sizes provided information on the particles’ Brownian dynamics, including the effect of the shell thickness on their hydrodynamic diameter, the presence (or absence) and ballpark frequency of Néel superspin relaxation as well as the particles’ average magnetic orientation in external magnetic fields. For single-core particles, this resulted in the observation of standard Langevin-type alignment behavior. Mössbauer spectra additionally provide information on the absolute degree of spin alignment, also allowing the determination of the degree of surface spin canting, which limits the maximum magnetization of ferrofluid samples. Analyzing the alignment behavior of agglomerated particles for comparison, we found a completely different trend, in which spin alignment was further hindered by the competition of easy magnetic directions. More complex particle dynamics are observed when going from ferrofluids to hybrid materials, where the particle mobility and alignability depends not only on the particles’ shape and material, but also on the matrices’ inner structure and the acting force-transfer mechanism between particles and the surrounding medium. In ferrohydrogels for example, particle mobility in terms of Mössbauer spectroscopy was probed for different crosslinker concentrations, resulting in widely different mesh-sizes of the polymer network and different degrees of freedom. While a decrease in particle dynamics is clearly visible in Mössbauer spectroscopy upon rising crosslinker density, complementary AC-susceptometry experiments indicated no Brownian motion on the expected timescales. This apparent contradiction could, however, be explained by the different timescales of the experiments, probing either the relatively free Brownian motion on ultrashort timescales or the more bound state preventing extensive particle motion by interaction with the trapping mesh walls in the millisecond regime. However, it should also be considered that the effect of the surroundings on particle rotation in AC-susceptometry may also differ from the variation in translational motion, probed by Mössbauer spectroscopy. Being sensitive mainly to translational motion also results in a wide range of particles to be accessible for studies via Mössbauer spectroscopy, including larger agglomerates embedded in polymers, intended for remote-controlled heating. Despite the agglomerates’ wide distribution in effective diameters, information on particle motion was found to be in good agreement with AC-susceptometry experiments at ultralow frequencies in and above the polymer melting region, while additionally giving insight into Néel relaxation of the individual nanoparticles and their magnetic structure.

Efficient Pb (II) removal from wastewater by TEG coated Fe3O4 ferrofluid

Tri-ethylene glycol (TEG) coated Fe3O4 nanoparticles ferrofluid were used for Pb (II) removal from simulated wastewater. The samples were synthesized using a modified co-precipitation method. The prepared samples were characterized by different techniques including X-ray diffraction, Rietveld method, FTIR, FESEM, TEM, VSM, TGA, BET and atomic adsorption experiments. The crystallinity of nanoparticles with a cubic spinel ferrite structure and absence of impurity phases were verified using X-ray diffraction and Rietveld method. The presence of TEG was approved by FTIR and thermogravimetric analysis. The VSM results showed that the bonding between the TEG molecules and ferrite nanoparticles, reduces the surface spin disorder, influences the morphology and magnetization, and consequently increases the Pb (II) removal efficiency to a high value of 97%. The obtained high value of adsorption capacity of q=363.4 mg.g-1 with R= 91 % and q=129.4 mg.g-1 with R=97 %shows effective role of TEG coating on Pb (II) adsorption. The interesting results of this study imply that the TEG coated ferrofluid sample is suitable candidate for practical applications.

Colloidal Stability and Concentration Effects on Nanoparticle Heat Delivery for Magnetic Fluid Hyperthermia.

The heat produced by magnetic nanoparticles, when they are submitted to a time-varying magnetic field, has been used in many auspicious biotechnological applications. In the search for better performance in terms of the specific power absorption (SPA) index, researchers have studied the influence of the chemical composition, size and dispersion, shape, and exchange stiffness in morphochemical structures. Monodisperse assemblies of magnetic nanoparticles have been produced using elaborate synthetic procedures, where the product is generally dispersed in organic solvents. However, the colloidal stability of these rough dispersions has not received much attention in these studies, hampering experimental determination of the SPA. To investigate the influence of colloidal stability on the heating response of ferrofluids, we produced bimagnetic core@shell NPs chemically composed of a ZnMn mixed ferrite core covered by a maghemite shell. Aqueous ferrofluids were prepared with these samples using the electric double layer (EDL) as a strategy to maintain colloidal stability. By starting from a proper sample, ultrastable concentrated ferrofluids were achieved by both tuning the ion/counterion ratio and controlling the water content. As the colloidal stability mainly depends on the ion configuration on the surface of the magnetic nanoparticles, different levels of nanoparticle clustering are achieved by changing the ionic force and pH of the medium. Thus, the samples were submitted to two procedures of EDL destabilization, which involved dilution with an alkaline solution and a neutral pH viscous medium. The SPA results of all prepared ferrofluid samples show a reduction of up to half the efficiency of the standard sample when the ferrofluids are in a neutral pH or concentrated regime. Such results are explained in terms of magnetic dipolar interactions. Our results point to the importance of ferrofluid colloidal stability in a more reliable experimental determination of the NP heat generation performance.

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.

Magnetorheological properties of a kerosene-based ferrofluid with magnetite particles hydrophobized in the absence of the dispersion medium

A kerosene-based ferrofluid sample was prepared by a method in which the magnetite nanoparticles were stabilized by hydrophobization in the absence of the dispersion medium. This method ensures formation of aggregates that in the presence of a static magnetic field transform into large drop-like particle agglomerations, which disappear when the field vanishes.The Brownian relaxation measurements and the magneto-optical microscopy image demonstrate that the obtained sample exhibits magnetic field induced particle agglomerations.The magnetorheological investigations (i.e. the viscosity curves at different values of the static magnetic field, H, between (0–520) kA/m, the magnetoviscous effect, the temperature dependence of viscosity, the apparent viscosity dependence on Mason number and the thixotropic analysis) demonstrate that the ferrofluids with drop-like particle aggregation induced by the magnetic field exhibit significant magnetorheological effect, recommending them in electro-mechanical devices that operates at micrometric or sub-micrometric scale, where magnetorheological fluids cannot be used.

FREQUENCY AND TEMPERATURE ANALYSIS OF THE CLAUSIUSMOSSOTTI FACTOR OF A KEROSENE-BASED FERROFLUID IN LOW FREQUENCY FIELD

Based on the complex dielectric susceptibility measurements, over the frequency range 10 kHz to 2 MHz and at different temperatures ranging from 25 0C to 80 0C, for a kerosenebased ferrofluid sample, the Claussius-Mossotti complex factor, K, was analyzed. Using the K obtained values and the Schwarz theoretical model, we have proposed an original method, very useful for determining the activation energy Ea of the Schwarz relaxation process from ferrofluid. The results show that the Ea decreases with the increase of temperature for all investigated frequencies. This behavior can be correlated with the deformation of the atmosphere of counterions from the surface of nanoparticles, which determines the polarization of ferrofluid in the low frequency field.

FREQUENCY AND TEMPERATURE ANALYSIS OF THE CLAUSIUSMOSSOTTI FACTOR OF A KEROSENE-BASED FERROFLUID IN LOW FREQUENCY FIELD

Based on the complex dielectric susceptibility measurements, over the frequency range 10 kHz to 2 MHz and at different temperatures ranging from 25 0C to 80 0C, for a kerosenebased ferrofluid sample, the Claussius-Mossotti complex factor, K, was analyzed. Using the K obtained values and the Schwarz theoretical model, we have proposed an original method, very useful for determining the activation energy Ea of the Schwarz relaxation process from ferrofluid. The results show that the Ea decreases with the increase of temperature for all investigated frequencies. This behavior can be correlated with the deformation of the atmosphere of counterions from the surface of nanoparticles, which determines the polarization of ferrofluid in the low frequency field.

From high colloidal stability ferrofluids to magnetorheological fluids: tuning the flow behavior by magnetite nanoclusters

The flow behavior of practically structureless ferrofluids was tuned over a large range by adding well-defined amounts of magnetite nanoparticle clusters. Magnetite nanoparticle clusters with hydrophobic coating were dispersed in high colloidal stability transformer oil based ferrofluid samples, initially free of clusters. The resulting ferrofluid composition consists both of single (approx. 7 nm mean size) and clustered (280 nm average size) magnetite nanoparticles, each of them in a prescribed amount. This new approach allows a precise evaluation of the role of clusters on the ferrofluid flow behavior. The magnetite nanoparticle clusters obtained by a solvothermal method have a saturation magnetization of 75 emu g−1 and a remnant magnetization of 0.2 emu g−1 due to the large size (∼25 nm) of the magnetite nanoparticles in their composition. Long bundles of thin elongated needle like aggregates of magnetic nanocluster particles are formed in a magnetic field and produce a significant magnetorheological response of the bidisperse magnetic suspensions observed already for moderate applied magnetic field values. The dynamic yield stress and magnetoviscous effect are analyzed and compared with previous results concerning suspensions of micrometer size iron particles in a ferrofluid carrier. The viscoelastic properties and the interactions between the components of ferrofluid based magnetite nanoparticle cluster suspensions were evaluated by creep tests in zero and non-zero applied magnetic field. The bidisperse magnetic nanocomposite suspensions investigated provide a promising new formulation of MR fluids with improved kinetic stability to be used in seismic dampers and flow control devices.

Dynamic susceptibility of a concentrated ferrofluid: The role of interparticle interactions.

The dynamic susceptibility of concentrated ferrofluids of magnetite-kerosene type is studied experimentally to clarify the effect of interparticle interactions on the magnetization reversal dynamics and the ferrofluid relaxation time spectrum. We synthesize six ferrofluid samples, four of which have the same wide particle size distribution with a high (more than 2kT) average energy of magnetic dipole interactions. These samples differ in particle concentration and dynamic viscosity. The two remaining samples have a lower content of large particles and a moderate energy of magnetic dipole interactions. For all samples, we measure the dynamic susceptibility in the weak probing field at frequencies up to 160kHz and the field amplitude dependence of the susceptibility at a frequency of 27kHz. The results show that the susceptibility dispersion at frequencies up to 10kHz is due to the rotational diffusion of colloidal particles and aggregates. Steric and hydrodynamic interparticle interactions are the main reason for the strong concentration dependence of the viscosity and so they also strongly influence the frequency dependence of the susceptibility. The influence of van der Waals and magnetic dipole interactions on the susceptibility is manifested indirectly, through the formation of multiparticle clusters. The contribution of clusters to the low-frequency susceptibility reaches 80%. Their large sizes (about 100 nm) shift the dispersion region to frequencies of 1-100Hz, depending on the temperature and particle concentration. Experiments at 27kHz demonstrate the increase in the dynamic susceptibility with increasing field amplitude. This growth is unexpected since all spectral amplitudes in the Debye function expansion of the dynamic susceptibility decrease monotonically with increasing field. To clarify the situation, the auxiliary problem of the magnetodynamics of a uniaxial particle in the alternating field is solved numerically. The Fokker-Planck-Brown rotational diffusion equation is used. It is shown that an increase in the field amplitude reduces the anisotropy barrier and the Néel relaxation time of particles and increases the dynamic susceptibility by one to two orders of magnitude compared to the weak-field limit. The calculation results are in qualitative agreement with the experimental data and allow us to propose a consistent interpretation of these data. We find that the increase in dynamic susceptibility with increasing amplitude is observed when two necessary conditions are met: (i) The suspension viscosity and the field frequency are high enough to cause the blocking of the rotational degrees of freedom of particles and aggregates and (ii) particles with a large magnetic anisotropy are present in the ferrofluid.