Magnetoviscous Effect Research Articles

Role of Fibrillation on the Magnetorheological and Viscoelastic Effects in Fe, Ni, and Co Nanocolloids

Magnetic metallic nanoparticle (NP)-based colloidal systems have the ability to show highly augmented magnetorheological (MR) and magnetoviscoelastic responses under the influence of magnetic fields. Magnetic metal NPs, viz., iron (Fe), nickel (Ni), and cobalt (Co) have been employed to formulate oleo magnetic-nanocolloids. The magnetocolloids have been characterized for rheological behavior and have been observed to demonstrate superior magnetoviscous effects under magnetic field due to fibrillation by the particles. The magnitude of maximum dynamic yield stress and viscosity of Fe magnetocolloids attained is 18 kPa and 4 kPa.s, respectively, for 15 wt% particle concentration and at ~1.2 T magnetic field intensity. Furthermore, Fe-based colloids are found to be the best candidate among the three types of metallic colloids in terms of high MR effect under magnetic field. The same has been observed for magnetoviscoelastic effects in terms of the enhanced linear viscoelastic range, thixotropy, and strain creep behavior. The experimental observations also show that formulated colloids have highly stable structure under magnetic field even at high shear loads and demonstrate reversible behavior when subjected to rise and decay of magnetic fields.

Magneto-viscous effect on thermal convection thresholds in an Oldroyd magnetic fluid

In magnetic fluids the viscosity can depend on an external magnetic field. We theoretically investigate the influence of this magneto-viscous effect on the thermal convection thresholds for viscoelastic ferrofluids, which are described by a linear Oldroyd model. Such a system is influenced by a static magnetic field not only via the Kelvin force, but also through the magneto-viscous effect. In particular, we find that these two contributions compete oppositely when the threshold for the oscillatory instability is considered. While the Kelvin force tends to decrease the critical Rayleigh number, the magneto-viscous effect increases it. The critical properties at the onset of the oscillatory instability are discussed as a function of the viscoelastic parameters, the external field strength, and the magneto-viscous coefficient. The transition between the stationary and the oscillatory instability is only slightly affected by the magneto-viscous effect. Examples for codimension-2 lines are given.

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.

Influence of Viscosity and Magnetoviscous Effect on the Performance of a Magnetic Fluid Seal in a Water Environment

ABSTRACTThe magnetic fluid seal is one of the most successful applications of magnetic fluids. The theory of magnetic fluid seals in liquid environments has not been developed. This work mainly presents an experimental study of the influence of viscosity and magnetoviscous effects on the performance of a magnetic fluid seal in a water environment. Three engine oil–based magnetic fluids of different viscosities and similar saturation magnetization values were prepared and a multistage magnetic seal structure was designed. The magnetoviscous effect of the magnetic fluids under different working conditions was measured with an advanced rotational rheometer. An experimental platform and a multistage magnetic seal structure were designed for the critical pressure value and durability tests. The experimental results show that the viscosity of a magnetic fluid is a decisive factor in its sealing performance under a water environment and a discussion is presented that can explain the experimental results qualitatively.

Labyrinthine instabilities of miscible magnetic fluids in a rotating Hele-Shaw cell

This study presents the first experimental results of confining miscible magnetic fluids in a rotating Hele-Shaw cell. Variations in the prominence of labyrinthine instabilities are observed under a range of experimental conditions, with different magnetic field strengths, gap depths, and rotation speeds. These instabilities are characterized by two modified Péclect numbers, namely, Pem (the ratio of the characteristic magnetic advection rate and the diffusion rate) and Pec (the ratio of characteristic rotation advection and the diffusion rate). The magnetic effect is characterized by dipolar repulsion, which triggers a distinctive fingering pattern differing from the progressive diffusion pattern that occurs without magnetic fields or rotation. Under the same rotation speed, the magnetoviscous effect will hinder the growth rate of the magnetic drops at the later stage. However, both the rotation effect and the gap depth greatly enhance the growth rate of the magnetic drops, as these conditions help to intensify the labyrinthine instabilities. In contrast, the countering pressure gradient produces an opposite force that constrains the trend toward expansion. Two major phases in the growth of instabilities are defined: a magnetization phase and a rotation phase, which are dominated by the magnetic and the rotation effect, respectively. The significance of the rotation effect is confirmed by the linear regression between the rotation growth rate and Pec. Finally, main fingering structures that evolve from the secondary waves are verified as having a wavelength λ to gap depth h relation of λ≈(7±1)h.

Structure and viscosity of a transformer oil-based ferrofluid under an external electric field

Various structural changes of ferrofluids have been intensively studied under external magnetic fields. In this work we present an experimental evidence of similar changes induced by an electric field. In the context of the electric field effect on ferrofluids structure, we studied a simple ferrofluid consisting of iron oxide nanoparticles coated with oleic acid and dispersed in transformer oil. The structural changes have been observed both on macroscopic and microscopic scale. We also demonstrate a remarkable impact of the electric field on the ferrofluid viscosity in relation to the reported structural changes. It was found that the electric field induced viscosity changes are analogous to the magnetoviscous effect. These changes and the electroviscous effect are believed to stem from the dielectric permittivity contrast between the iron oxide nanoparticles and transformer oil, giving rise to the effective electric polarization of the nanoparticles. It is highlighted that this electrorheological effect should be considered in studies of ferrofluids for high voltage engineering applications, as it can have impact on the thermomagnetic convection or the dielectric breakdown performance.

Research on the rheological properties of a perfluoropolyether based ferrofluid

A perfluoropolyether based ferrofluid was prepared using co-precipitation method and the rheological properties of the ferrofluid were studied by a rotational rheometer. A series of experiments were designed to study the influence of magnetic field, shear rate and temperature on the magnetoviscous effect of the perfluoropolyether based ferrofluid. Consecutive measurements of the megnetoviscous parameter with the temperature-increasing process were made and totally different tendency of the curves was observed under a range of shear rates. The magnetic field strength influence on the observed temperature dependencies was also studied experimentally. A discussion on the different mechanisms of the influence of temperature on magnetoviscous effect is presented based on the chain model of magnetic particles and the viscosity-temperature characteristics of the base carrier liquid.

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.

Ferrofluid-based magnetorheological fluids: tuning the properties by varying the composition at two hierarchical levels

The magnetorheological behavior of three sets of ferrofluid-based magnetorheological (MR) fluids is investigated to determine the dependence of the static and dynamic yield stresses, as well as of the magnetoviscous effect on the volume fraction of magnetite nanoparticles φ and of the added Fe particles Φ Fe , at different values of the magnetic induction B in the MR cell. A wide range of the magnetic particle volume fractions were considered at the two hierarchical levels involved: at the nanometer level, φ = 2.75, 11.67, and 22.90 %, and at the micrometer level, Φ Fe = 5–40 %. Taking into account also the oleic acid monolayer coating of the magnetite nanoparticles, the hydrodynamic volume fraction of the most concentrated ferrofluid results to be 65 % and, consequently, the highest value of the effective total volume fraction of the investigated extremely bidisperse MR fluids attains 85 %. The diagrams of the static and dynamic (Bingham) yield stresses and of the magnetoviscous effect offer an adequate choice of the nanometer and micrometer range particle volume fractions to fulfill the specific requirements of high-pressure rotating seals and of various MR devices.

A capillary viscometer designed for the characterization of biocompatible ferrofluids

Suspensions of magnetic nanoparticles are receiving a growing interest in biomedical research. These ferrofluids can, e.g., be used for the treatment of cancer, making use of the drug targeting principle or using an artificially induced heating. To enable a safe application the basic properties of the ferrofluids have to be well understood, including the viscosity of the fluids if an external magnetic field is applied. It is well known that the viscosity of ferrofluids rises if a magnetic field is applied, where the rise depends on shear rate and magnetic field strength. In case of biocompatible ferrofluids such investigations proved to be rather complicated as the experimental setup should be close to the actual application to allow justified predictions of the effects which have to be expected. Thus a capillary viscometer, providing a flow situation comparable to the flow in a blood vessel, has been designed. The glass capillary is exchangeable and different inner diameters can be used. The range of the shear rates has been adapted to the range found in the human organism. The application of an external magnetic field is enabled with two different coil setups covering the ranges of magnetic field strengths required on the one hand for a theoretical understanding of particle interaction and resulting changes in viscosity and on the other hand for values necessary for a potential biomedical application.The results show that the newly designed capillary viscometer is suitable to measure the magnetoviscous effect in biocompatible ferrofluids and that the results appear to be consistent with data measured with rotational rheometry. In addition, a strong change of the flow behaviour of a biocompatible ferrofluid was proven for ranges of the shear rate and the magnetic field strength expected for a potential biomedical application.

Nano-micro composite magnetic fluids: Magnetic and magnetorheological evaluation for rotating seal and vibration damper applications

In this paper, static magnetic properties and magnetorheological behavior of a set of 12 nano-micro composite magnetic fluids (CMFs) were studied. The samples with a ferromagnetic particle volume fraction ranging in a large interval φFe=(1÷44)% were prepared by adding carbonyl iron powder in a highly concentrated transformer oil-based ferrofluid (FF). The ferrofluid has the magnetite volume fraction of φFe3O4=22.90% and saturation magnetization of Ms=74kA/m (930 Gs). No further additives were used in order to prevent sedimentation. It was noticed an increase of the static yield stress, of about 3 orders of magnitude, with the increase of the total solid volume fraction of samples and with the increase of the magnetic field, which varied between 0 kA/m and 950 kA/m. The dynamic yield stress (Herschel-Bulkley model) τHB of the samples strongly increases with the magnetic field and shows a slight tendency of saturation for higher intensities of the magnetic field. There is a less pronounced increase of τHB, about an order of magnitude with the increasing volume fraction of the iron particles. The relative viscosity increase induced by the magnetic field reaches a maximum for both considered shear rates: γ⋅=7.85s-1 and γ⋅=88.41s-1 and it was revealed an optimal volume fraction of Fe particles, φFe=20%, corresponding to a total volume fraction of φtot≈38%, at which the magnetoviscous effect has its maximum value. The magnetic properties and also the magnetorheological and the magnetoviscous behavior of highly concentrated ferrofluid-based CMFs can be controlled by the addition of iron microparticles in order to attain the optimal concentration for the envisaged engineering applications, rotating seals and magnetorheological vibration dampers.

Numerical modeling of non-Newtonian biomagnetic fluid flow

Blood flow dynamics have an integral role in the formation and evolution of cardiovascular diseases. Simulation of blood flow has been widely used in recent decades for better understanding the symptomatic spectrum of various diseases, in order to improve already existing or develop new therapeutic techniques. The mathematical model describing blood rheology is an important component of computational hemodynamics. Blood as a multiphase system can yield significant non-Newtonian effects thus the Newtonian assumption, usually adopted in the literature, is not always valid. To this end, we extend and validate the pressure correction scheme with discontinuous velocity and continuous pressure, recently introduced by Botti and Di Pietro for Newtonian fluids, to non-Newtonian incompressible flows. This numerical scheme has been shown to be both accurate and efficient and is thus well suited for blood flow simulations in various computational domains. In order to account for varying viscosity, the symmetric weighted interior penalty (SWIP) formulation is employed for the discretization of the viscous stress tensor. We disregard the dependency of the viscosity on spatial derivatives of the velocity in the Jacobian computation. Even though this strategy yields an approximated Jacobian, the convergence rate of the Newton iteration is not significantly affected, thus computational efficiency is preserved. Numerical accuracy is assessed through analytical test cases, and the method is applied to demonstrate the effects of magnetic fields on biomagnetic fluid flow. Magnetoviscous effects are taken into account through the generated additive viscosity of the fluid and are found to be important. The steady and transient flow behavior of blood modeled as a Herschel-Bulkley fluid in the presence of an external magnetic field, is compared to its Newtonian counterpart in a straight rigid tube with a 60% axisymmetric stenosis. A break in flow symmetry and marked alterations in WSS distribution are noted.

Magnetically controllable generation of ferrofluid droplets

This paper reports the manipulation of ferrofluid droplets by using a microfluidic flow-focusing device equipped with a magnetic tweezer. Besides the traditional flow rate controlling method, the magnetic field also can be applied to control the size of the droplets. Two major effects in magnetic manipulation process: magnetoviscous effect and magnetic drag effect, were studied. Under a fixed flow rate (CP = 1 mL/h, DP = 0.2 mL/h), the average sizes of ferrofluid droplets were tunable from 135 to 95 μm by varying the magnetic field from 0 to 60 mT. Moreover, square wave magnetic field can be used to periodically generate droplets with different sizes. These results are helpful to understand the generation mechanism of the ferrofluid droplet and supply a novel method for manipulating droplets with a predetermined size and distribution.

Anisotropy of the magnetoviscous effect in a cobalt ferrofluid with strong interparticle interaction

The anisotropy of the magnetoviscous effect (MVE) of a cobalt ferrofluid has been studied in a slit die viscometer for three orientations of the applied magnetic field: in the direction of the fluid flow (Δη1), the velocity gradient (Δη2), and the vorticity (Δη3). The majority of the cobalt particles in the ferrofluid exhibit a strong dipole–dipole interaction, which corresponds to a weighted interaction parameter of λw≈10.6. Thus the particles form extended microstructures inside the fluid which lead to enhanced MVE ratios Δη2/Δη1>3 and Δη3/Δη1>0.3 even for strong shearing and weak magnetic fields compared to fluids which contain non-interacting spherical particles with Δη2/Δη1≈1 and Δη3/Δη1=0. Furthermore, a non-monotonic increase has been observed in the shear thinning behavior of Δη2 for weak magnetic fields <10kA/m, which cannot be explained solely by the magnetization of individual particles and the formation and disintegration of linear particle chains but indicates the presence of heterophase structures.

Viscosity Modulation by Magnetic Field for Colloidal Polyelectrolyte Brushes

Aqueous dispersions of magnetic spherical polyelectrolyte brushes (MSPB) are prepared by mini‐emulsion followed by photo‐emulsion polymerization. Their viscosity increases dramatically upon increasing the intensity of an external magnetic field, showing the “magnetoviscous effect.” Unlike general magnetic nano‐particles, the increase in viscosity of MSPB cannot be described by Shliomis's theory, probably due to the covering of a spherical polyelectrolyte brush layer. MSPB rheology is also tunable by pH, and the brushes can stay in atmospheric conditions for months without changes in particle size and size distribution, which should make them ideal candidates for a broad range of industrial applications. The future applications of our multifunctional magnetic responsive system are promising.

Anisotropy of magnetoviscous effect in structure-forming ferrofluids.

The magnetoviscous effect, change in viscosity with change in magnetic field strength, and the anisotropy of the magnetoviscous effect, change in viscosity with orientation of magnetic field, have been a focus of interest for four decades. A satisfactory understanding of the microscopic origin of anisotropy of the magnetoviscous effect in magnetic fluids is still a matter of debate and a field of intense research. Here, we present an extensive simulation study to understand the relation between the anisotropy of the magnetoviscous effect and the underlying change in microstructures of ferrofluids. Our results indicate that field-induced chainlike structures respond very differently depending on their orientation relative to the direction of an externally applied shear flow, which leads to a pronounced anisotropy of viscosity. In this work, we focus on three exemplary values of dipolar interaction strengths which correspond to weak, intermediate, and strong interactions between dipolar colloidal particles. We compare our simulation results with an experimental study on cobalt-based ferrofluids as well as with an existing theoretical model called the chain model. A nonmonotonic behavior in the anisotropy of the magnetoviscous effect is observed with increasing dipolar interaction strength and is explained in terms of microstructure formation.

The magnetoviscous effect of micellar solutions doped with water based ferrofluids

This work presents a magnetorheological study of micellar solutions of potassium laurate and water doped with magnetite nanoparticles, accompanied by auxiliary dynamic light scattering measurements. An increase in the viscosity of the samples under applied field was observed and, furthermore, a considerable magnetoviscous effect was revealed even at magnetic particles' concentrations as low as 0.005–0.01vol%. This indicates that the rheological behavior of the micelles is changed by the interaction of the magnetic particles with the applied field, leading to different microscopic arrangements in the micellar solutions.

Anisotropy of the magnetoviscous effect in a ferrofluid with weakly interacting magnetite nanoparticles

The anisotropy of the magnetoviscous effect of a ferrofluid has been studied in a specially designed slit die viscometer, which allows three distinct orientations of the magnetic field with respect to the fluid flow. The corresponding Miesowicz viscosity coefficients were determined in dependence of the shear rate and the magnetic field intensity to gain a comprehensive magnetorheological characterization of the fluid. The particles in the fluid have a mean diameter of 13 nm corresponding to an interaction parameter of λ ≈ 1.3 for magnetite. Thus, the fluid can be expected to show a transition from non-interacting individual particles to microstructures with chain-like associated particles when the magnetic field intensity is increased and the shear rate is decreased. The observed field and shear dependent anisotropy of the magnetoviscous effect is explained coherently in terms of these microstructural changes in the fluid.

Analytical calculation of chain length in ferrofluids

The response of a typical ferrofluid (FF) lies in its explicit property of chain formation of magnetic nanoparticles. The most significant magneto-optic (MO) and magneto-viscous (MV) effects of FF are attributed to chaining effect. In the present research, an effort was made to analytically justify the dependence of the structure evolution of FFs on different measurable parameters involved in MO and MV effects. The problem is treated with the help of dimensional analysis and an empirical relation is formulated relating the equilibrium chain length with Verdet coefficient (constant), particle diameter, viscosity of the carrier fluid, particle density, magnetization and shear rate. The formulated relation of chain length is supported by error analysis to yield the uncertainty in the result. The maximum uncertainty in four sets of data is found as ∼0.75.

Maneuvering the chain agglomerates of colloidal superparamagnetic nanoparticles by tunable magnetic fields

Magneto-viscous effect (MVE) provides an unique control over the rheological properties of magnetic nanofluids (MNFs) by externally applied magnetic field. In this letter, we report the factors affecting the MVE of surfactant coated magnetite (mean size ∼11.5 nm) dispersed water based nanofluid. We investigate the dependence of viscosity on the magnetic sweep carried out by increasing and decreasing the field in several consecutive cycles. We observe that the viscosity is considerably affected by the time interval between consecutive applications of the external field. The degree of hysteresis (quantified by the area of the hysteresis curve) decreases with the increase in time interval and the number of cycles of magnetic sweep. We also observe the excellent reversible switching properties of viscosity for MNF under pulses of applied magnetic field. The gap between the plates of the rheometer exhibits a profound influence in controlling the magnitude of viscosity. The lower the gap, the higher is the viscosity. All the above effects can be explained from the formation of aggregates of the nanoparticles and their relaxation by the application and withdrawal of the field, respectively. Visualization through microscopy supports the proposition.