Behavior Of Ferrofluids Research Articles

Study on the yielding behaviors of ferrofluids: a very shear thinning phenomenon.

The yielding behaviors of the ferrofluids are vital for many applications. However, previous studies have mainly focused on the magnetoviscous effect under relatively high shear rates and rarely involved the yielding process of ferrofluids under very low shear rates. In this study, ferrofluid samples of different particle volume concentrations were prepared and their shear thinning behaviors within a wide shear stress range were systematically studied under various magnetic field strengths and temperatures. The very shear thinning phenomenon of ferrofluids was first observed and its microscopic mechanism was analyzed. A precipitous fall-off stage as the mark of yielding appeared between the low shear and high shear plateaus in the viscosity curves of ferrofluids. The precipitous fall-off stage in the viscosity curves became steeper with the increase of the magnetic field strength or decrease in the temperature. For ferrofluids with relatively low particle volume concentrations, the high viscosity limit under the low shear region disappeared when temperature exceeded a certain value and was interpreted as the disappearance of the equilibrium columnar structures under high Brownian thermal interaction level. A composite Ellis model was proved to be suitable for the fitting of different types of yield stresses and a structural number, Sn was proposed for the dimensionless analysis of the shear thinning behaviors of ferrofluids. The findings in this study contribute to a better understanding of the microscopic mechanism of yielding behaviors of ferrofluids and also provide guidance for many practical applications.

Dynamic study of ferrodroplet and bubbles merging in ferrofluid by a simplified multiphase lattice Boltzmann method

In this article, the dynamics of ferrodroplet deformation and bubbles merging within the ferrofluid under a uniform magnetic field are studied through numerical simulations. A recently developed simplified multiphase lattice Boltzmann method (SMLBM) is used to capture the hydrodynamic behavior of ferrofluids, the Cahn-Hilliard (CH) equation is employed as the interface tracking algorithm, whose solution is also obtained within the lattice Boltzmann framework and a self-correcting procedure is used to solve the magnetic field along with conjugate boundary conditions for a smooth transition of magnetic permeability at the interface. A series of numerical simulations are performed for a range of magnetic field strength, magnetic susceptibility and Ohnesorge number (Oh), along with a comparison between obtained results and previous experimental and numerical results. The dynamical behavior of ferrodroplets and bubbles merging in ferrofluids is analyzed through the distribution of magnetic field and magnetic flux density, velocity contours, pressure variations, the magnetic energy density and the effect of inertia. Findings of this study may help to explain the mechanism of ferrodroplet deformation and merging of bubbles within the ferrofluid and provide a useful insight into the distribution of flow variables during these deformations and merging phenomena.

Numerical Investigation of Ferrofluid Sloshing by Applying MHD Magnetic Field: Using Level Set Method

The sloshing phenomenon has exceptional significance due to its occurrence in various processes as well as its application. This phenomenon occurs when a vessel is partly filled with a fluid and under the influence of external forces the free surface of the liquid moves and exchanges forces with the wall of the vessel. In this research, numerical modeling is used to study the behavior of ferrofluid in sloshing phenomenon in a rectangular container with a specified length and width of 10 cm × 5 cm respectively. The force that moves the vessel is the oscillatory motion in the x-axis direction. Applying a uniform magnetic force, which creates additional modules in the governing equations, such as the momentum equation, has effects on this phenomenon and fluid motion. The main aim of this research is to study the effects of the uniform MHD field in different directions and angles on the ferrofluid sloshing. By studying the results of some factors (such as; the pressure of the ferrofluid to the specific points on the vessel wall, the maximum surface at any time, and the analysis of the surface situation at different times) the impact of the magnetic field with different angles has been identified on the ferrofluid sloshing. The results showed that in the absence of an external magnetic field, the sloshing behavior of water and ferrofluid were approximately the same. Applying the MHDmagnetic field caused a 14.5%, 25% and 36% decrease in the maximum height of the fluid level at angles 0◦, 45◦ and 90◦ of magnetic field respectively. Therefore, these results indicate the influence of the magnetic field direction on the behavior of the ferrofluid sloshing.

Heat transfer in a permeable cavity filled with a ferrofluid under electric force and radiation effects

The behavior of ferrofluid inside a porous space due to electric field has been investigated through an innovative approach. The coupled equations were solved with Control volume finite element method. Properties of Fe3O4- Ethylene glycol nanofluid are functions of electric field and nanoparticles’ shape. Radiative term has been involved in energy equation. Impacts of nanoparticles’ shape with various relevant parameters on nanofluid thermal behavior have been depicted.

Influence of the Carrier Fluid Viscosity on the Rotational and Oscillatory Rheological Properties of Ferrofluids.

This work mainly presents a comprehensive experimental study on the magnetorheological behavior of ferrofluids with carrier fluids of different viscosities. Three lubrication oil based ferrofluids of different viscosities and similar saturation magnetization values were prepared and characterized. Static and dynamic rheological tests of the three ferrofluids were performed using an advanced rotational rheometer with a magnetic field generating module. According to the experimental results, the magneto viscous effect, yield stress and storage modulus increase with the viscosity of carrier fluid, indicating that the structures of ferrrofluids tend to be larger and more stable in a carrier fluid with larger viscosity. The emergence and growth of "crossover" region with the increase in carrier fluid viscosity was observed using the strain-rate frequency superposition method, which were explained according to the microscopic mechanism of magnetorheology. These findings contribute to a better understanding of the microscopic mechanism of magnetorheology and provide guidance for practical applications.

Thixotropic yielding behaviors of ferrofluids

The thixotropic yield behaviors of ferrofluids with different particle volume concentrations were studied through a series tests under constant stresses for sufficiently long times. A lubrication oil based ferrofluid was prepared and a diluted sample was acquired by mixing the initial ferrofluid sample with carrier fluid according to a ratio of 1:3. The particle volume concentration of the samples was estimated through the magnetization curves as 7.8% and 1.9%. A critical bifurcation stress can be observed through the time evolution of the initial ferrofluid sample under moderate magnetic fields, over which the viscosity curves tend to infinity. For the diluted ferrofluid sample, no viscosity bifurcation can be observed even under the minimum stress values. Similar viscosity bifurcation phenomenon for initial samples can be seen from the curves under different magnetic field strengths or temperatures. The experimental results indicate that the yield process of ferrofluids is thixotropic and significantly influenced by the shear history. The microscopic mechanism behind the thixotropic yield behaviors of ferrofluids is discussed and analyzed. A non-linear structural model was presented to describe the time evolution of viscosity in ferrofluids under different magnetic field strength. The comparison between the fitting results and experimental data indicates that the thixotropic yielding behaviors of ferrofluids can be predicted in some extent. These findings provide a new perspective on the yielding behaviors of ferrofluids which takes the thixotropic effect into consideration.

The influence of dipolar particle interactions on the magnetization and the rotational viscosity of ferrofluids

The effect of the dipolar particle interactions on the behavior of ferrofluids under a shear flow is not yet well understood. The equilibrium magnetization in the absence of flow is studied in Paper I [A. P. Rosa, G. C. Abade, and F. R. Cunha, “Computer simulation of equilibrium magnetization and microstructure in magnetic fluids,” Phys. Fluids 29(9), 092006 (2017)]. In this paper, we present the results of magnetization and rheology in terms of a rotational viscosity obtained by applying Brownian dynamics simulations for a periodic magnetic suspension, where the many body long-range dipole-dipole interactions are calculated by the Ewald summation technique. The dependence of these macroscopic properties on the dipolar interactions is explored in ferrofluids undergoing both weak and strong shear flows in the presence of a uniform magnetic field. Through the simulations, the suspension microstructure is also analyzed in order to characterize the interplay between the structure and the investigated macroscopic properties. We show that for weak shear flows the dipole-dipole interactions produces a magnetization increasing. In contrast, a decrease in the ferrofluid magnetization with the shear rate is substantially intensified as the dipolar interactions are accounted for. Therefore, for strong shear flows, the dipolar interactions always have an effect of decreasing magnetization. In addition, while the dipolar particle interactions produce an increase in the rotational viscosity for weak flows, variations in the same property are not perceptible under the condition of strong flows. The numerical simulations show chain-structure formation oriented in the direction of the magnetic field (i.e., perpendicular to the direction of the shear) for weak flows, which explains the remarkable increasing of the suspension rotational viscosity as a function of the applied magnetic field and of the dipolar interactions parameters. A detailed comparison shows that our simulation results of magnetization and the rotational viscosity are in excellent agreement with approximate theoretical predictions reported in the literature for the case of noninteracting particles.

In-Field Orientation and Dynamics of Ferrofluids Studied by Mössbauer Spectroscopy.

By studying the response behavior of ferrofluids of 6-22 nm maghemite nanoparticles in glycerol solution exposed to external magnetic fields, we demonstrate the ability of Mössbauer spectroscopy to access a variety of particle dynamics and static magnetic particle characteristics at the same time, offering an extensive characterization of ferrofluids for in-field applications; field-dependent particle alignment and particle mobility in terms of Brownian motion have been extracted simultaneously from a series of Mössbauer spectra for single-core particles as well as for particle agglomerates. Additionally, information on Néel superspin relaxation and surface spin frustration could be directly inferred from this analysis. Parameters regarding Brownian particle dynamics, as well as Néel-type relaxation behavior, obtained via Mössbauer spectroscopy, have been verified by complementary AC-susceptometry experiments, modulating the AC-field amplitude, and using an extended frequency range of 10-1 to 106 Hz, while field-dependent particle alignment has been cross-checked via magnetometry.

Ferrofluids for Active Shock Absorbers

This paper deals with synthesis and characterization of a ferrofluid and examining its suitability for developing a damper whose damping behavior can be controlled by applying magnetic field. In particular, ferrofluids, consisting of oleic acid coated Fe3O4 nanoparticles dispersed in kerosene, have been synthesized and characterized. Oleic acid coated nanoparticles of Fe3O4 have been characterized using XRD, TEM and FTIR. These studies show that sizes of particles are in range of 6 to17 nm and oleic acid is adsorbed on the surface of above particles. The role of damping medium on the damping behavior of a damper has been studied by carrying out studies with several different liquids. The effect of magnetic field on the damping behavior of ferrofluid based damper has also been studied. It is seen that damping ratio depends on the viscosity of damping medium. In particular, it is seen that damping ratio of a ferrofluid based damper increases on application of magnetic field. However, the sensitivity of damping ratio to the magnetic field has been found to be different for different ferrofluids. It seems, magnetic moment or size of the particle plays an important role in deciding the sensitivity of the damper. No attempt has been made to improve the sensitivity of the damper by optimizing various parameters. All the same, present studies provide a proof of concept for smart shock absorber.

Field induced deformation of sessile ferrofluid droplets: Effect of particle size distribution on magnetowetting

The field tunable thermal, rheological and optical properties of ferrofluids have been studied extensively and have been exploited in numerous technological applications. However, there are very few studies on the field assisted wetting behavior of ferrofluids, though it is important from fundamental and practical point of view. In this study, we probe the effect of magnetic field induced aggregation on ‘magnetowetting’ in three different well characterized oil based ferrofluids of similar crystallite sizes (∼10 nm) but with different particle size distributions and polydispersity index, through contact angle measurements of sessile droplets. When exposed to a uniform external magnetic field, ferrofluid droplets of all three samples showed elongation along the magnetic field direction, adopting a prolate ellipsoidal meniscus in an effort to balance magnetic force and surface tension. However, the extent of deformation depends on the aggregation kinetics of ferrofluid system as the field induced aggregates are responsible for the meniscus deformation. Further, our studies indicate that at low field strengths, droplet deformation is not significant for systems that displayed rapid aggregation with micron sized thick and long columnar aggregates. The decrease in base fluid-aggregate interaction, brought about by rapid aggregation initiated by significant quantities of larger sized particles, is believed to be the cause for the decreased deformation of meniscus. For magnetic fluids with polydispersity index (PDI) values of 0.79, 0.23 and 0.22, the zero field contact angles were 59 ± 1.5, 63 ± 1.3 and 64 ± 1.2°, respectively. On increasing field strength from 0 to 160 G, the sample with the highest PDI showed a marginal decrease in contact angle from 59 to 54° where the system with minimum PDI showed a significant contact angle change from 64 to 44°. These results will be useful in better control on tunable ‘magnetowetting’ for practical applications.

Study of the thixotropic behaviors of ferrofluids.

The thixotropic behaviors of ferrofluid samples of different particle concentration were studied using different measurement methods, including the three interval thixotropic test and the hysteresis loop test. The experimental results demonstrated that ferrofluids exhibit significant thixotropic behaviors and the microstructural evolution in ferrofluids behind the macroscopic rheological mechanics is discussed. The influence of magnetic field strength, particle concentration and temperature on the thixotropy of ferrofluids was also analyzed. Microscopic ferrofluid theory was adopted to study the thixotropic behaviors of ferrofluids under different shearing conditions, indicating that different thixotropic behaviors of ferrofluids can be induced by the presence and evolution of different kinds of microstructures, such as linear chain-like and dense drop-like structures. Furthermore, a phenomenal thixotropic model was employed to analyze the experimental results, indicating that a more specific model for ferrofluids is needed. These findings contribute to a better understanding of the microscopic mechanism of the complex rheological behaviors of ferrofluids.

Study on the creep and recovery behaviors of ferrofluids

The creep and recovery behaviors of lubrication oil based ferrofluids of different particle concentration were systematically investigated to understand the viscoelasticity of ferrofluids. The influence of stress level, magnetic field strength and temperature on creep and recovery behaviors of ferrofliuids was studied experimentally and the microscopic mechanisms behind the rheological phenomenon were discussed. Linear viscoelasticity theory and generalized Burgers models were employed to analyze the experimental results. The experimental results demonstrate that the ferrofluids exhibits unique creep and recovery properties significantly different from other stimuli responsive materials both in the linear and nonlinear viscoelastic region. Furthermore, structures larger than single chains are supposed to be responsible for many experimental results, including the extended relaxation process in recovery phase and the nonlinear increasing trend of creep strain with magnetic field strength and temperature. These findings contribute to a better understanding of the microscopic mechanism of magnetorheology of ferrofluids and also provide guidance for many practical applications.

Magnetoviscous effect in ferrofluids diluted with sheep blood

Suspensions of magnetic nanoparticles in suitable carrier liquids, denoted as ferrofluids, are in the focus of current research in the biomedical area. Those fluids can be potentially used for the treatment of cancer by coupling chemotherapeutic agents and accumulating them in the diseased region with the help of external magnetic fields or by artificially local induced heating. Those applications rely on the help of external magnetic fields, which are well known to drastically influence the physical behaviour of ferrofluids. This study investigates the changing viscosity of a biocompatible ferrofluid in a flow situation close to the situation found in a biomedical application. For this purpose blood as diluting agent and thin capillaries have been utilised. The strong magnetoviscous effects found lead to the assumption of quite big changes of the microstructure due to the external magnetic fields, which was investigated and quantified using a microscopic setup. In the result an increases of the structure size as well as faster structure formation in the stronger magnetic fields were observed. Moreover, with increasing duration of the applied magnetic field the size of the structures increases too. The observed process of the structure formation is reversible.

Ferrofluid based composite fluids: Magnetorheological properties correlated by Mason and Casson numbers

Comprehensive flow and magnetization data obtained previously on the behavior of ferrofluid based magnetorheological fluids (FF-MRF) [Susan-Resiga and Vékás, Rheol. Acta 55(7), 581–595 (2016)] were correlated using a Mason number ( M n) defined for these nano-micro composite fluids and the corresponding Casson (Ca) number, instead of the Bingham number used for conventional magnetorheological (MR) fluids. Practically, agglomerate-free, high colloidal stability ferrofluids with Newtonian behavior in the absence of a magnetic field were used as carrier to keep the off-state viscosity as low as possible and to ensure the reproducibility of the MR response of the composite fluid. The apparent viscosities and magnetization values for samples with various nanosized magnetites and micrometer range iron particle volume fractions (from 2.75% to 22.90% and from 4% to 44%, respectively) at different values of magnetic induction and shear rate collapsed on a master curve by using the nondimensional M n and Ca numbers to correlate the measured data. By controlling the volume fraction both at nano- and microlevels, the magnetic and MR properties of the resulting composite fluids can be tailored to offer a high-performance lubricant for semiactive dampers and brakes, as well as for magnetofluidic rotating seals. The M n and Ca numbers offer a design metric based solely on the properties of FF-MRFs and can be reliably scaled to devices of different sizes and operating conditions.

Design, development and characterization of variable reluctance ferrofluid pump

The objective of this paper is to design and implement a ferrofluid pump actuated by switched magnetic field. The pump works on the principle of variable reluctance behavior of ferrofluid in an external magnetic field. A brief experiment to understand dynamic nature of magnetic flux density in a ferrofluid column is conducted. Variable reluctance phenomenon in the ferrofluid column is analyzed. Design and implementation details of two different structures of ferrofluid pump are discussed, namely single phase pump and two phase pump. Equivalent electrical circuit models of the pumps are developed based on hydraulic-electric analogy. Simulations and experiments are conducted and the results are compared. The designed pump is capable of pumping the ferrofluid at a rate of 84 A mu l/s with zero discharge head and a maximum flow rate of 56 A mu l/s is achieved with a discharge head of 120 mm.

Magnetoviscous effect in ferrofluids with different dispersion media

Ferrofluids based on magnetite nanoparticles dispersed in different carrier media (dialkyldiphenyl and polyethylsiloxane) have been synthesized using mixed surfactants (oleic acid, stearic acid and alkenyl succinic anhydride). Magnetic properties of the samples and a change of their shear viscosities in an applied magnetic field have been studied in order to evaluate an influence of the carrier medium on a magnetoviscous effect. A significance of the interaction of the carrier medium and surfactant with a consideration of the magnetic and rheological behavior of ferrofluids was demonstrated.

Experimental investigation of a PVT system performance using nano ferrofluids

In this paper, the effects of ferrofluids as a coolant on the overall efficiency of a PVT (photovoltaic thermal unit) system are experimentally investigated. The fluids considered in the experiment are distilled water and a ferrofluid (Fe3O4-water) with 1% and 3% concentrations by weight (wt%). The experiments were performed in indoor conditions under two constant solar radiations (1100W/m2 and 600W/m2) using a solar simulator. Due to the unique characteristic behavior of ferrofluids, their rheological and thermophysical properties can be changed under an external magnetic field. The ferrofluids in this study were placed under constant and alternating magnetic fields in the cooling section in order to investigate the effect of both types of magnetic fields on the overall efficiency of a PVT system. The results show that by using a 3wt% ferrofluid, the overall efficiency of the system improved by 45% and when an alternating magnetic field with 50Hz frequency was applied, the overall efficiency increased to about 50% compared to that of the distilled water as coolant fluid. The overall exergy output of the system with and without ferrofluids, was also compared with that of the PV system with no collector. It was observed that by adding a thermal collector to a PV system and using a 3wt% ferrofluid under an alternating magnetic field, the total exergy can be increased as high as 48W.

The Onset of Normal Field Instability in a Ferrofluid in a Reduced Gravity Environment

A ferrofluid is a magnetic liquid comprised of nanoscale ferrous particles suspended in a low-viscosity carrier fluid. When subjected to a magnetic field, the surface of a ferrofluid deforms into peaks and valleys along the magnetic field lines. The onset of surface deformation is called the normal field instability. The theory describing the NFI identifies a critical magnetic field below which no magnetically driven surface deformations occur. The critical field depends on the gravitational acceleration and, according to the theory, should disappear as local gravitational acceleration approaches zero. While there have been previous demonstrations of the normal field instability in a reduced gravity environment, data has been inconclusive on the existence of a critical magnetic field in reduced gravity. For our work, we designed a sounding rocket payload for a suborbital rocket mission. Our experiment incorporates a ferrofluid sample and a uniform magnetic field which can be varied across a discrete range of values to observe surface deformations at different applied fields. During the microgravity portion of the rocket’s flight, we obtain video of the ferrofluid’s behavior and compare it to data taken in Earth’s gravity. A team of five undergraduate students designed and built the payload that will characterize the role of gravity in setting the critical magnetic field strength for the onset of the NFI. Data will be used to further understanding of ferrofluid behavior in a microgravity environment to advance its use in various space-based applications.

Volume fraction dependent magnetic behaviour of ferrofluids for rotating seal applications

Ferrofluid samples consisting of magnetite nanoparticles (NPs) coated with oleic acid and dispersed in a non-polar organic solvent have been synthesized by chemical routes. Different volume fractions, φ, of magnetic NPs were considered. The overall structural characterization of NPs has been performed by x-ray diffractometry, with lattice parameters and average coherence lengths evaluated via Rietveld refinements. The magnetic properties of different samples have been analysed by SQUID magnetometry and temperature-dependent Mössbauer spectroscopy and finally explained by adequate magnetic relaxation mechanisms. Zero field cooling–field cooling protocols provided useful information about specific volume fraction dependent magnetic relaxation and de-freezing processes, the lack of the Verwey transition and stronger dipolar interactions at higher volume fractions. Anisotropy energies as obtained by both temperature dependent Mössbauer spectroscopy and magnetometry data are compared and a new procedure for a quantitative characterization of the dipolar interactions is proposed.

The quasi-magnetic-hysteresis behavior of polydisperse ferrofluids with small coupling constant

The magnetization behaviors of ferrofluids based on γ-Fe 2 O 3 /Ni 2 O 3 composite nanoparticles of size about 11 nm have been investigated. The dipole coupling constant λ of these particles is so small (0.43) that they cannot form aggregates through magnetic interaction alone. Experimental results have shown that for a polydisperse ferrofluid with a particle volume fraction of ϕ V =2.4%, the magnetization curve exhibits quasi-magnetic-hysteresis behavior, i.e., the demagnetization curve lies above the magnetization curve in a high field. However, for a more dilute γ-Fe 2 O 3 /Ni 2 O 3 ferrofluid with ϕ V =0.94%, the magnetization curve does not show such behavior. According to the bidisperse model for polydisperse ferrofluids, these magnetization behaviors may be attributed to field-induced effects of self-assembled pre-existing chain-like aggregates. For such pre-existing chain-like aggregates, the orientation of the moments inside the particles is not co-linear, so that during the magnetization and demagnetization processes, their apparent magnetizations at the high-field limit are different. As a consequence, the magnetization curve of the ferrofluid with ϕ V =2.4% displays quasi-magnetic-hysteresis.