Dilute Magnetorheological Fluid Research Articles

Effect of Confinement on the Aggregation Kinetics of Dilute Magnetorheological Fluids

The goal of this study is to investigate the field-driven structuration of model magnetorheological (MR) fluids in narrow gaps (below 1 mm) for high shear applications. With this in mind, we study the influence of confinement in the aggregation kinetics of dilute carbonyl iron suspensions under strong fields in rectangular microchannels using video-microscopy, image analysis and particle level dynamic simulations. Channel widths studied are ranged in the interval [75–1000 μm]. In these particular suspensions the experimental and simulation time scales according to: where λ is the Lambda parameter, is the particle surface fraction and W* is the reduced microchannel width. We show that the effect of channel width is crucial in the dynamic exponent and in the saturation (long time) mean cluster size. In contrast, it has no effect in the onset of the tip-to-tip aggregation process.

Flow and heat transfer performances of dilute magnetorheological fluid flowing through hot micro channel

The thermal lattice Boltzmann method (TLBM) with doubled-population is used to analyze the dilute magnetorheological fluid (MRF) flow and heat transfer performances in two-dimensional hot micro channels, by regarding the magnetic particles (MPs) as a quasi fluid. A method for calculating external forces on MPs is established in order to improve the computational accuracy and efficiency. It is revealed that with the increasing volume fraction of MPs, on lattice node, both the Van der Waals force and the magnetic dipole force increase but the Brownian force only randomly varies within a range which is three orders of magnitude smaller than magnetic dipole force. Simulations are carried out under both constant temperature and heat flux boundary conditions. The effects of channel length, inlet velocity and volume fraction of MPs on heat transfer performances are investigated and the results indicate that the dilute MRF behaves a better heat transfer performance than water even without the effect of external magnetic field.

A micromechanical model for magnetorheological fluids under slow compression

We propose a micromechanical model for the behavior of dilute magnetorheological fluids under unidirectional slow-compression, constant-volume squeeze flow mode. In the linear magnetization regime, the model predicts a power law scaling of the normal stress with the particle volume fraction and magnetic field strength squared at low fields. The predictions are satisfactorily compared with experimental measurements for different particle loadings, sample volume, surface roughness, and initial gap distance.

Multifractality in dilute magnetorheological fluids under an oscillating magnetic field.

A study of the multifractal characteristics of the structure formed by magnetic particles in a dilute magnetorheological fluid is presented. A quasi-two-dimensional magnetorheological fluid sample is simultaneously subjected to a static magnetic field and a sinusoidal magnetic field transverse to each other. We analyzed the singularity spectrum f(α) and the generalized dimension D(q) of the whole structure to characterize the distribution of the aggregates under several conditions of particle concentration, magnetic field intensities, and liquid viscosity. We also obtained the fractal dimension D(g), calculated from the radius of gyration of the chains, to describe the internal distribution of the particles. We present a thermodynamic interpretation of the multifractal analysis, and based on this, we discussed the characteristics of the structure formed by the particles and its relation with previous studies of the average chain length. We have found that this method is useful to quantitatively describe the structure of magnetorheological fluids, especially in systems with high particle concentration where the aggregates are more complex than simple chains or columns.

Unexpected shear strength change in magnetorheological fluids

Smart materials of magnetorheological (MR) fluids could be turned from a liquid state into a solid state, which solidification extent or shear strength often increases monotonically with the applied magnetic field. In this study, the shear stress of a dilute MR fluid decreased with increasing applied magnetic field at a constant shear rate. The dynamic shear stress was significantly higher than the stable counterpart at medium magnetic fields. They are ascribed to the slow particle structure transformation. A higher shear rate and particle volume fraction could reduce the transient time and the shear strength difference.

Average particle magnetization as an experimental scaling parameter for the yield stress of dilute magnetorheological fluids

We propose an experimental parameter for the scaling of the yield stress (τy) of magnetorheological (MR) fluids: the average particle magnetization ⟨Mp⟩ as estimated from magnetization curves of the MR suspensions. When τy was expressed as a function of this scaling parameter, the curves for MR suspensions prepared with particles of different saturation magnetization and even different morphology collapsed together. In addition, the collapse worked reasonably well for a wide range of magnetic fields: from weak fields below which the sensitivity of our magnetorheometer could not detect the τy, to fields close to particle saturation. The collapse failed for particles of highly anisotropic morphology, which must be indicative of non-magnetostatic contributions to the yield stress.

Advanced Fluid Information. Magnetorheological Fluids: From Basic Physics to Application.

Magnetorheological (MR) fluids are suspensions of magnetizable micrometer particles in a liquid. The rheological properties of the fluids can be changed rapidly, reversibly and repeatedly from that of a liquid to that of a solid when an external magnetic field is applied. This tunability of the fluids comes from the microscopic structure of the particles. Different microscopic structures are observed, quantified and correlated with rheological properties. The yield stress and viscosity of MR fluids depends on the particle size and volume fraction, sample cell geometry, magnetic field strength and application rate. The application of MR fluids in cancer therapy is discussed. In-vitro experiments show the blood flow can be blocked leading to a tumor by injecting a dilute MR fluid in a blood vessel and placing a magnet on top of the tumor. Without blood supply, tumor necrosis occurs shortly.

Spontaneous chirality in simple systems

Two simple examples of spontaneous chiral symmetry breaking are presented. The first is close-packed cylindrically confined spheres. As the cylinder diameter is varied, one obtains a variety of chiral phases. The second example involves unconfined dipolar particles with an isotropic attraction, which also exhibits chiral ground states. We speculate that a dilute magnetorheological fluid film, with the addition of smaller particles to provide an attractive entropic interaction, will exhibit a chiral columnar ground state.

Chain formation and chain dynamics in a dilute magnetorheological fluid

Magnetorheological fluids are suspensions of magnetizable particles that reversibly change from liquid to solid when subjected to a magnetic field. A field-induced structure of dipolar chains is responsible for these changes. Our work aimed at understanding chain dynamics and the kinetics of chain formation by using dynamic light scattering. Chain length is determined by measurement of the diffusion coefficient. Chain-length growth shows a Smoluchowski behavior.

Role of nucleation in the structure evolution of a magnetorheological fluid.

A phenomenological model of nucleation-controlled structure evolution in dilute magnetorheological fluid is suggested. We find that the critical size of column nuclei is related to the magnetic field H, the increase of permeability \ensuremath{\delta}\ensuremath{\mu} of the nuclei, and to the nucleus surface tension \ensuremath{\sigma}, by the relation ${\mathit{D}}_{\mathit{c}}$\ensuremath{\sim}16\ensuremath{\pi}\ensuremath{\sigma}/(${\mathit{H}}^{2}$-${\mathit{H}}_{\mathit{c}}^{2}$)\ensuremath{\delta}\ensuremath{\mu}. The growth rate of columnar structure, and the field and volume fraction dependences of the separation between the columns are calculated, and are found to be given, respectively, by d\ensuremath{\sim}${\mathit{t}}^{2/7}$, d\ensuremath{\sim}${\mathit{H}}^{3/14}$, and d\ensuremath{\sim}${\mathrm{\ensuremath{\varphi}}}^{1/7}$. These results fit the experimental data in ferrofluid emulsion systems very well. \textcopyright{} 1996 The American Physical Society.