Suspensions Of Magnetic Particles Research Articles

Optical visualization of the formation behavior of magnetic particle clusters in a forced convection field

Temperature-sensitive magnetic fluids are expected to achieve more efficient heat transfer by controlling the internal flow of suspended magnetic particles, in addition to improving the heat transfer coefficient of the fluid itself. Magnetic clusters are considered to be generated inside the channel under the application of a magnetic field, but their measurement is difficult experimentally, and various assumptions have been used for calculation because of the complicated flow. Therefore, in this study, fluorescent magnetic microcapsules were created to visualize cluster formation and collapse behavior at the magnetic field application position in the forced convection field. Overall, shorter clusters tended to form at larger flow velocities owing to the fluid shear force. When the fluid shear force applied exceeded a certain level, the magnetic clusters collapsed on the wall surface and stopped growing. The average length of the cluster became unstable in the region immediately before the critical shear force. The shear stress at the boundary where the cluster collapsed was correlated with the applied magnetic flux. The cluster formation behavior can be controlled by the flow velocity, magnetic flux density, and particle magnetization. The observations that would aid the achievement of a more efficient forced convection heat transfer by magnetic fluids are presented in this study.

The Influence of the Temperature on the Dynamic Behaviors of Magnetorheological Gel

Magnetorheological (MR) gel is a new generation of MR material, which can overcome disadvantages attaching MR fluid such as sedimentation. MR gel is formed by the soft magnetic particle suspension in the gel‐like matrix and its rheological properties are mainly controlled by the following two factors, i.e., magnetic field and temperature. Herein, MR gel based on polyurethane gel with a 60% weight fraction of carbonyl iron is prepared. The influence of temperature on the rheological characteristics of the MR gel sample is systematically investigated. Oscillatory shear test is used to investigate the impact of temperature on the modulus of the sample. It is observed that temperature has a significant effect on the viscoelastic properties of MR gel. The hysteresis responses activated by harmonic strain signal with frequencies of 0.1, 5, and 15 Hz and amplitude of 10% and 50% under five temperature levels are measured and employed to analyze the viscous and elastic properties of MR gel and the viscoelastic–plastic model is employed to capture the nonlinear hysteresis characteristics. From the analysis results, the viscoelastic–plastic model can predict the hysteresis properties of MR gel accuracy under various temperatures.

Quasi-two-dimensional Brownian dynamics simulations on the local internal structure of a cubic magnetic particle suspension in an alternating magnetic field

Quasi-two-dimensional Brownian dynamics simulations on the local internal structure of a cubic magnetic particle suspension in an alternating magnetic field

Elucidation of the relationship between aggregate structures and magnetorheological properties of a magnetic cubic particle suspension by means of Brownian dynamics simulations

We have developed a Brownian dynamics simulation technique for a cubic magnetic particle suspension in a simple shear flow in order to elucidate the relationship between the particle aggregates and the magnetorheological characteristics. A magnetic field is applied in the direction normal to the shearing plane. In a weak applied magnetic field, if the magnetic particle–particle interaction strength is sufficiently large, the particles aggregate to form closely-packed clusters even when subject to the influence of the shear flow. As the magnetic field strength is increased, the closely-packed aggregate structures are transformed into chain-like structures. The net viscosity is increased because the chain-like clusters give rise to a larger resistance to the flow field. As the magnetic field strength is further increased, the chain-like clusters grow into wall-like clusters aligned in the direction of the magnetic field. If the wall-like clusters are the predominant clusters, a magnetic force arises due to a characteristic of the particle arrangement in the wall-like clusters that tends to accelerate the flow field and, as a consequence, decrease the net viscosity. From these results, it may be suggested that under certain conditions a magnetic cubic particle suspension may exhibit a negative contribution to the magnetorheological characteristic. Highlights of the present study We have developed a Brownian dynamics (BD) simulation technique for a magnetic cubic particle suspension. BD simulations have been performed in order to investigate the relationship between particle aggregates and magnetorheological properties. The net viscosity exhibits a complex dependence on the regime of particle aggregates. It is suggested that under certain conditions a magnetic cubic particle suspension may exhibit a negative contribution to the magnetorheological characteristics. An increase in the shear rate causes instability in the face-to-face configuration and leads to the collapse of closely-packed clusters.

Modeling and Simulation of the Magnetorheological Fluid Sleeve Valve

Magnetorheological fluids contain suspended magnetic particles that arrange in chains in the presence of a magnetic field, causing the conversion of the fluid from a liquid state to a quasi-solid state. These fluids can be used in valves as a tool for pressure drop and flow interruption. This research aims to investigate the feasibility of using magnetorheological fluid (MRF) in industrial valves. The rheological properties of the MRF sample were measured with the MCR300 rheometer in the presence of a magnetic field. In this connection, the Bingham plastic continuous model was used to predict fluid behavior, and model coefficients were obtained using MATLAB software. Then, the model's coefficients were used to simulate the behavior of the magnetorheological fluid in the presence of the magnetic field in the valve. The geometry and dimensions of the valve were designed according to the dimensions of industrial samples. Then the CFD simulation with Fluent software was done by using the Bingham model and fluid characteristics obtained from experimental results. The results showed that the pressure increased by increasing the magnetic field at the center of the sleeve. The magnetic field up to 0.5 Tesla, increases pressure and decreases amplitude. Therefore, as the magnetic field increase, the amplitude of the maximum pressure on the sleeve was significantly reduced.

Synthesis and rheological properties of 3D structured self-healing magnetic hydrogels

Hydrogels with self-healing properties constitute one of the most versatile three-dimensional (3D) materials for application in biomedicine due to the possibility of modulating their characteristics and incorporating specific elements such as magnetic particles (MPs). Herein, we describe a simple approach on the fabrication of self-healing magnetic hydrogels with anisotropic characteristics. Magnetic hydrogels are prepared by means of Schiff's base reaction starting from a suspension of MPs in a polysaccharide's solution. The MPs within the hydrogels are self-assembled by the superposition of an external magnetic field (from 5 to 240 kA/m). The synthesized hydrogels are then characterized using infrared spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy and rheometry. The results demonstrate a remarkable influence of the magnetic field strength on the MPs structuration and mechanical properties of the material. The manuscript envisages the possibility to manufacture more complex 3D materials that can be used as multidimensional platforms with potential applications in biomedicine.

Reinforcing Magnetorheological Fluids with Highly Anisotropic 2D Materials.

Magnetorheological fluids (MRF) are suspensions of magnetic particles that solidify in the presence of a magnetic field. While non-magnetic additives could improve MRF performance, explorations into such additives have not coalesced into an understanding of their influence, and particularly the role of additive morphology. Here, we explore α-Ni(OH)2 2D sheets, with aspect ratios of ∼25,000, as highly anisotropic MRF additives. Experiments studying pressure-driven flow of an MRF with and without these sheets show that their addition can increase the saturation pressure by as much as 46 %. However, shear-mode rheology reveals that they can also weaken the MRF by inhibiting the chaining of the iron particles at low field strengths and have no effect at higher field strengths. In order to reconcile the strikingly different results, we propose that 2D materials introduce a non-Newtonian handle to modify smart fluids in a manner that depends on the curvature of the shearing strain rate profile. Specifically, we identify a modification to the Buckingham-Reiner model of pressure-driven flow for a Bingham plastic in which the sheets widen the solidified plug. This work highlights the subtle interaction between particles in smart fluids and flows while emphasizing the opportunity for using anisotropy to tune this interaction.

Self-assembly of millimeter-scale magnetic particles in suspension.

Self-assembly is ubiquitous at all scales in nature. Most studies have focused on the self-assembly of micron-scale and nano-scale components. In this study, we explore the self-assembly of millimeter-scale magnetic particles in a bubble-column reactor to form 9 different structures. Two component systems (N-N and S-S particles) assemble faster than one-component systems (all particles have N-S poles) because they have more numerous bonding pathways. In addition, two-components add control to process initiation and evolution, and enable the formation of complex structures such as squares, tetrahedra and cubes. Self-assembly is collision-limited, thus, the formation time increases with the total number of bonds required to form the structure and the injected power. The dimensionless Mason number captures the interplay between hydrodynamic forces and magnetic interactions: self-assembly is most efficient at intermediate Mason numbers (the system is quasi-static at low Mason numbers with limited chances for particle interaction; on the other hand, hydrodynamic forces prevail over dipole-dipole interactions and hinder bonding at high Mason numbers). Two strategies to improve yield involve (1) the inclusion of pre-assembled nucleation templates to prevent the formation of incorrect initial structures that lead to kinetic traps, and (2) the presence of boundaries to geometrically filter unwanted configurations and to overcome kinetic traps through particle-wall collisions. Yield maximization involves system operation at an optimal Mason number, the inclusion of nucleation templates and the use of engineered boundaries (size and shape).

Quasi-two-dimensional Brownian dynamics simulations on the aggregation phenomena of a cubic magnetic particle suspension in a rotating magnetic field

Quasi-two-dimensional Brownian dynamics simulations on the aggregation phenomena of a cubic magnetic particle suspension in a rotating magnetic field

Elucidation of the formation mechanism of the aggregate structures of a magnetic cubic particle suspension in a rotating magnetic field by means of Brownian dynamics simulations

Elucidation of the formation mechanism of the aggregate structures of a magnetic cubic particle suspension in a rotating magnetic field by means of Brownian dynamics simulations

Hierarchically Structured Fe3O4 Nanoparticles for High-Performance Magnetorheological Fluids with Long-Term Stability

The magnetic Fe3O4 nanoparticles were produced by chemical precipitation from ferric chloride (FeCl3) and ferrous chloride (FeCl2) in an alkaline medium (ammonia hydroxide). The magnetic particle suspension was stabilized with citric acid. Hierarchically structured Fe3O4 (HS-Fe3O4) particles of submicrometer size were successfully produced from citric acid-capped Fe3O4 solution by using a simple electrospray (ES) process and applied for magnetorheolgical (MR) fluids. Submicrometer-sized spherical HS-Fe3O4 particles were formed by evaporation of the solvent and agglomeration of capped Fe3O4 particles during the ES process. With formation of hierarchically structured particles, the density of HS-Fe3O4 particles (ρ = 3.32 g/cm3) was noticeably decreased from that of the capped Fe3O4 (ρ = 4.29 g/cm3) without use of light materials, while the saturation magnetization was comparable to primary capped Fe3O4 (Ms = 73 emu/g). The MR fluid based on the submicrometer HS-Fe3O4 particles showed better MR performance with a large static yield stress (337 Pa), which was 300% greater than that of the hollow polymer/Fe3O4 suspension. Reduced density mismatch between the particles and the silicon oil medium significantly improved the stability of the HS-Fe3O4 suspension compared to the pure Fe3O4 suspension. The high MR performance and the improved sedimentation stability showed a possibility of the practical application of the submicrometer HS-Fe3O4 particle suspensions to microfluidic devices.

Theoretical study of the blood flow in arteries in the presence of magnetic particles and under periodic body acceleration

Abstract In this article, the dynamics of blood with suspended magnetic particles in coronal and femoral arteries are investigated. The flow of blood is examined in the presence of external magnetic field, periodic body acceleration and a pressure gradient of an oscillating type. Expressions for the velocity of blood and velocity of magnetic particles will be yielded by employing integral transforms. The analytical results will be expressed in terms of steady-state and transient parts. Moreover, to get insight of the control of the material parameters such as amplitude, the lead angle, frequency of body acceleration, magnetic field and particles’ concentration parameter, numerical simulations and graphical illustrations will be used and useful consequences will be summarized.

New insights on boundary layer control using magnetic fluids: A numerical study

This work investigates the effects of an applied magnetic field on the laminar flow of a ferrofluid over a backward-facing step. Both constitutive equation and global magnetization equation for a ferrofluid are considered. The resulting formulation consists in a coupled magnetic-hydrodynamic problem. Computational simulations are carried out in order to explore the physics of the flow and the consistency of theoretical aspects of our formulation. The unidirectional sudden expansion in a ferrofluid flow is investigated numerically under the perspective of Ferrohydrodynamics in a two-dimensional domain using a Finite Volumes Method. The boundary layer detachment induced by the sudden expansion results in a recirculating zone, which has been extensively studied in purely hydrodynamic problems for a wide range of Reynolds numbers. Similar investigations can be found in literature regarding the sudden expansion under the Magnetohydrodynamics framework, but none considering a colloidal suspension of magnetic particles out of the superparamagnetic regime in the framework of Ferrohydrodynamics. The vorticity-stream function formulation is implemented. Our simulations show a clear coupling between the flow vorticity and the magnetization field. We observe a systematic decay on the length of the recirculating zone as we increase the magnetic parameters of the flow, such as the intensity of the applied field and the volume fraction of particles. The results are discussed from a physical perspective in terms of the dynamical non-dimensional parameters. We argue that the reduction of the recirculating region is a direct consequence of the magnetic torque balancing the action of the torque produced by viscous and inertial forces of the flow. For the limiting case of small Reynolds and magnetic Reynolds numbers, the diffusion of vorticity balances the diffusion of the magnetic torque on the flow. This mechanism controls the growth of the recirculating region.

Shear thickening prevents slip in magnetorheological fluids

Magnetorheological (MR) fluids are smart fluids that solidify upon the application of a magnetic field by virtue of suspended magnetic particles forming chains. Here, the effect of shear thickening additives on the performance of MR fluids is explored and found to increase yield stress by 60% in fluids that have the same iron content. In order to explore the origin of this improvement, a series of flow- and oscillation-based rheology experiments were performed at various plate separations and the results were compared with a discrete dipole model of chain formation and breakage. Ultimately, conventional MR fluids, which are typically shear thinning, are found to yield at small strains that are consistent with slip failure where chains shear apart at one particle-particle joint. In contrast, shear thickening fluids reliably yield at high strain in a manner consistent with affine failure of the chain where each joint is pulled apart equally. The prevention of slip failure is attributed to the shear thickening additive locally increasing the effective viscosity as slip begins, thus preventing it from leading to yield. In addition to motivating the use of shear thickening agents broadly in MR fluids, these findings highlight the importance of additive choice in tuning rheological properties of MR fluids and the benefit of considering the microstructural and bulk behavior of the fluid simultaneously when synthesizing smart fluids.

Graphical Magnetogranulometry of EMG909

The magnetization curve of the commercially available ferrofluid EMG909 is measured. It can adequately be described by a superposition of four Langevin terms. The effective dipole strength of the magnetic particles in this fluid is subsequently obtained by a graphical rectification of the magnetization curve based on the inverse Langevin function. The method yields the arithmetic and the harmonic mean of the magnetic moment distribution function, and a guess for the geometric mean and the relative standard deviation. It has the advantage that it does not require a prejudiced guess of the distribution function of the poly-disperse suspension of magnetic particles.

Manipulation of Taylor bubble flow in a magneto-fluidic system

Transport characteristics of gas bubbles, suspended in magnetically activated ferrofluids, such as their shape and size, formation frequency and superficial velocities, can be effectively manipulated by strategically imposing an external magnetic field in the flow domain. We demonstrate and analyse such a flow manipulation technique on an air-ferrofluid Taylor slug-bubble flow, formed in a T-junction mini-channel fluidic device having a square cross-section of sides 2 mm. The flow transport characteristics get affected by an interplay between strength of the externally applied magnetic field, local magnetic pressure barrier due to the induced magnetic force, the concentration of suspended magnetic particles and the flow Reynolds number. The applied magnetic field, depending on its location and strength with respect to the T-junction, tends to cause a hump-shaped agglomerate or a flocculate of the suspended nano-particles in the flow. This alters the dynamics of the resulting Taylor bubbles getting generated at the T-junction, providing the required means of flow manipulation and control. Taylor bubble formation at the T-junction and the effect of magnetic field on its dynamics is analysed. A parametric investigation is also performed to study the effect of air-ferrofluid flow rate ratio and magnetic force on the resulting hydrodynamics of the Taylor bubble flow. Such magnetic manipulation of air-ferrofluid flow can be effectively utilized in technological applications such as chemical reactors for mass transfer augmentation, pulsating heat pipes and lab-on-chip devices, to name a few.

Effect of a Low-Frequency Magnetic Field on the Release of Heat by Magnetic Nanoparticles of Different Shapes

Cubic nanoparticles of magnetite are synthesized and a comparative analysis is performed of the release of heat by needle-shaped and spherical nanoparticles inside a low-frequency magnetic field. The sizes, shapes, and magnetic properties of the investigated samples are determined. The effect concentration, temperature, and the frequency and intensity of the magnetic field have on the release of heat is investigated. A suspension of magnetic particles is obtained and found to have a high coefficient of heat release (160 W/g) inside a low-frequency electromagnetic field with a frequency of 5.2 kHz.

Feasibility of the multi-particle collision dynamics method as a simulation technique for a magnetic particle suspension

ABSTRACTWe have elucidated the feasibility of the multi-particle collision dynamics (MPCD) method as a technique for the simulation of a magnetic particle suspension, by addressing the dependence of the translational and rotational Brownian motion of the magnetic particles on the parameters that characterise a MPCD simulation. A reasonable level of activation of the Brownian motion is indispensable for simulating the aggregate structures of the magnetic particles because their internal structure is strongly dependent on particle Brownian motion. In the present study, we have employed a diffuse reflection model for treating the interaction between the fluid and the magnetic particles. The diffuse reflection model gives rise to the tendency that the translational Brownian motion of the magnetic particles is more significantly activated than the rotational Brownian motion. If a scaling coefficient is introduced for modifying the interaction between the fluid and the magnetic particles, a more accurate solution may be obtained in regard to the aggregate structures of the magnetic particles. We may conclude that the MPCD method with the diffuse reflection model is a promising simulation technique for analysing the behaviour of magnetic particles in a suspension.

3D printing of polymer-bonded magnets from highly concentrated, plate-like particle suspensions

This paper reports the 3D printing of polymer-bonded magnets using highly concentrated suspensions of non-spherical magnetic particles. In a previous study, magnets of arbitrary shapes have been successfully fabricated using the UV-Assisted Direct Write (UADW) method. The magnetic remanence (Br) of the UADW magnets was limited by the type of magnetic particles used and the highest printable particle loading. Magnetic particles produced from melt spinning have better intrinsic magnetic properties, but their plate-like shape has resulted in a higher working viscosity, posing a major challenge in 3D printing with UADW. Inspired by the “Farris effect” in rheology, we mixed the plate-like particles of two different sizes to increase the polydispersity and reduce the overall viscosity of the mixture as the smaller particles can now fill the interstitial space between the larger ones. Using this rheological technique, a particle loading of as high as 65% by volume, or 93% by weight, was 3D printed. The resulting magnet has a density of 5.2 g/cm3, an intrinsic coercivity (Hci) of 9.39 kOe, a remanence (Br) of 5.88 kG, and an energy product ((BH)max) of 7.26 MGOe, marking the highest values reported for 3D printed polymer-bonded magnets.

Influence of the cluster formation in a magnetic particle suspension on heat production effect in an alternating magnetic field

In the present study, we concentrate on the results of the unexpected characteristic that the stable particle cluster formation induces a decrease in the degree of heating effect in an alternating magnetic field, by means of Brownian dynamics simulations. If chain-like clusters are stably formed in the system, whether or not a large heating effect is obtained is dependent on the magnitude relationship between the magnetic particle-particle and the magnetic particle-field interaction strengths. As the magnetic particle-particle interaction strength increases and becomes more dominant than the influence of the magnetic field, the magnetic moments of stable chain-like clusters do not have a sufficient tendency to inline in the field direction. This leads to a smaller area of the hysteresis loop, and therefore the stable cluster formation induces a significant decrease in the heating effect.