Yasuhiro Sakuda

Negative magneto-rheological effect of a dispersion composed of spindle-like hematite particles

We have experimentally investigated the negative magneto-rheological effect that is theoretically predicted to appear in a dispersion composed of spindle-like hematite (α-Fe2O3) particles. The spindle-like hematite particles were synthesized by aging a solution of FeCl3 and KH2PO4 for 72 h at 373 K. The particle size distribution was determined by digital image analysis from electronic microscope observation of the particles. In the present study we considered a glycerol–water-based dispersion in order to clarify the influence of the shear rate and the magnetic field strength on the negative magneto-rheological effect. Measurement of the viscosity was carried out using a rotational-type rheometer in an external magnetic field generated by Helmholtz coils. The main results obtained can be summarized as follows. The viscosity of a hematite/glycerol–water dispersion relative to that with no applied magnetic field decreases with increasing magnetic field strength: that is, we observed the negative viscosity effect that has previously been predicted from theoretical considerations. Moreover, the negative magneto-rheological effect is observed to decrease with increasing shear rate, which also agrees qualitatively with the theoretical prediction.

Negative viscosity due to magnetic properties of a non-dilute suspension composed of ferromagnetic rod-like particles with magnetic moment normal to the particle axis

The negative viscosity of a colloidal dispersion composed of ferromagnetic rod-like particles, which have a magnetic moment normal to the particle axis, have been investigated. A simple shear flow problem has been treated to clarify the particle orientational distribution and rheological properties of such a semi-dense dispersion, under circumstances of an external magnetic field applied in the direction normal to the shear plane of a simple shear flow. The results obtained here are summarized as follows. For the cases of a very strong magnetic field and magnetic interactions between particles, the magnetic moment of the rod-like particles is significantly restricted in the magnetic field direction, so that the particle approximately aligns in the shear flow direction. Also, the particle can easily rotate around the axis of the cluster almost freely even in a simple shear flow. Characteristic orientational properties of the particle cause negative viscosity, as in the previous study for a dilute dispersion. However, magnetic particle-particle interactions have a function to make such negative viscosity decrease.

Quasi-2D Monte Carlo simulations of phase transitions and internal aggregate structures in a highly dense suspension composed of magnetic plate-like particles

The phase transitions and the internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis have been investigated by means of the Monte Carlo method. The present study considered a quasi-2D system in order to clarify the influences of the volumetric fraction of particles and the magnetic field strength on particle aggregations and phase transitions. The internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation functions, orientational pair correlation functions, nematic and polar order parameters. The main results obtained here are summarized as follows. When the influence of the magnetic interaction between particles is of the same order of that of the perpendicular magnetic field strength, the particles form column-like clusters, and the internal structure of the suspension shows solid-like structures. For the case of a strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under the situation of a strong applied magnetic field, and the internal structure exhibits solid-like ones. The brick wall-like structures also appear for a relatively weak magnetic field applied along the in-plane direction despite a slightly smaller volumetric fraction compared with the case of the perpendicular applied magnetic field.

Quasi-2D Monte Carlo simulations of a colloidal dispersion composed of magnetic plate-like particles with magnetic moment normal to the particle axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modelled as disk-like particles with magnetic moment normal to the particle axis at the particle centre, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of the magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as the development of surface quality changing technology using such magnetic plate-like particles. A magnetic field is applied along the direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates are discussed quantitatively in terms of radial distribution and orientational pair correlation functions. For the case of strong magnetic interactions between particles, particles form long column-like clusters with their magnetic moments alternating in direction between the neighbouring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, particles incline towards the magnetic field direction, so that particles do not form such clusters.

Two-Dimensional Monte Carlo Simulations of Aggregation Phenomena in Ferromagnetic Colloidal Dispersions composed of Rod-like Hematite Particles

We have investigated aggregation phenomena of ferromagnetic colloidal dispersions composed of rod-like hematite particles which have a magnetic moment normal to the particle axis, by means of the cluster-moving Monte Carlo method. In concrete, we have treated a two-dimensional dispersion in order to clarify the influences of the particle aspect ratio, magnetic interactions between particles and the magnetic field strength on particle aggregations. The main results obtained here are summarized as follows. In the absence of an applied magnetic field, rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction as magnetic particle-particle interactions increase. However, shorter raft-like clusters are formed as the area fraction decreases. If a strong magnetic field is applied, the raft-like clusters tend to incline along the magnetic field direction, and this feature of the cluster formation is not significantly dependent on the particle length.

3D Monte Carlo Simulations of Aggregate Structures in a Magnetic Colloidal Suspension Composed of Plate-Like Particles With Magnetic Moment Normal to the Particle Axis

We have investigated the internal aggregate structures of a colloidal suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis by means of three-dimensional Monte Carlo simulations. In concrete, we have attempted to clarify the influences of the magnetic field strength, magnetic interactions between particles, and volumetric fraction of particles, on particle aggregation phenomena. In order to discuss quantitatively the aggregate structures of particles, we have focused on the radial distribution and orientational pair correlation function. For no applied magnetic field cases, long column-like clusters are formed as magnetic particle-particle interactions increase. Characteristics of these clusters are that particles incline in a certain direction with their magnetic moments alternating in direction between the neighboring particles. For applied magnetic field cases, the magnetic moments of the particles incline in the magnetic field direction, so that the columnar clusters are not formed. The brick wall-like aggregates are formed as the influences of the magnetic field and magnetic particle-particle interactions become significantly dominant.Copyright

Quasi-2D Monte Carlo Simulations of Phase Transitions and Internal Aggregate Structures in a Highly Dense Suspension Composed of Magnetic Plate-Like Particles

We have investigated phase transitions and internal aggregate structures of a highly dense suspension composed of magnetic plate-like particles with a magnetic moment normal to the particle axis, by means of the Monte Carlo method. In the present study, we have considered a quasi-2D system in order to clarify the influences of volumetric fraction of particles and magnetic field strength on particle aggregations and phase transitions. Internal structures of particle aggregates have been discussed quantitatively in terms of pair correlation and orientational pair correlation functions. The main results obtained here are summarised as follows. When the influence of magnetic interactions between particles is of the same order of the magnetic field strength, the particles form column-like clusters, and the internal structure of a suspension shows solid-like structures. For the case of strong applied magnetic field, the internal structure is transformed from solid-like structures into isotropic ones. However, as the volumetric fraction increases, the particles form brick wall-like structures under circumstances of a strong applied magnetic field, and the internal structure exhibits solid-like ones.Copyright

Brownian Dynamics Simulations of Colloidal Dispersion Composed of Ferromagnetic Spherocylinder Particles in a Simple Shear Flow (For the Case of an Applied Magnetic Field Normal to the Shear Plane)

We have investigated aggregate structures and rheological properties of a colloidal dispersion composed of ferromagnetic spherocylinder particles with a magnetic moment along the particle axis direction, by means of Brownian dynamics simulations. In concrete, we have attempted to clarify the influences of the flow field, magnetic field strength, magnetic interactions between particles and volumetric fraction of particles. In order to discuss quantitatively the internal structures of clusters, we have concentrated our attention on the radial distribution and orientational distribution functions. The present results are compared with those of the theoretical analysis for dilute dispersions and also non-dilute dispersions; the results for the latter were obtained by means of the mean-field approximation, which magnetic particle-particle interactions can be taken into account. Some important results are summarized as follows. For the case of the magnetic field strength and magnetic interactions between particles are more dominant than the viscous forces due to a simple shear flow, chain-like like clusters are formed along the magnetic field direction, although they are slightly tilted to the flow direction. When magnetic particle-particle interactions become over a certain value, such cluster formation leads to a significant increase in the viscosity of the dispersion.© 2009 ASME

Quasi-2D Monte Carlo Simulations of a Colloidal Dispersion Composed of Magnetic Plate-Like Particles With Magnetic Moment Normal to the Particle Axis

We have investigated aggregation phenomena of a colloidal dispersion composed of magnetic plate-like particles by means of Monte Carlo simulations. Such plate-like particles have been modeled as disk-like particles which have a magnetic moment normal to the particle axis at the particle center, with the section shape of a spherocylinder. The main objective of the present study is to clarify the influences of magnetic field strength and magnetic interactions between particles on particle aggregation phenomena. We have concentrated our attention on a quasi-2D system from an application point of view such as development of surface changing technology using such magnetic plate-like particles. A magnetic field was applied along a direction perpendicular to the plane of the monolayer. Internal structures of particle aggregates have been discussed quantitatively in terms of radial distribution and orientational pair correlation functions. The main results obtained here are summarized as follows. For the case of strong magnetic interactions between particles, the particles form long column-like clusters with their magnetic moments alternating in direction between the neighboring particles. These tendencies appear under circumstances of a weak applied magnetic field. However, as the magnetic field strength increases, the particles incline toward the magnetic field direction, so that the particles do not form such clusters.Copyright

Negative Viscosity Due to Magnetic Properties of a Semi-Dense Colloidal Dispersion Composed of Ferromagnetic Rod-Like Particles With Magnetic Moment Normal to the Particle Axis

We have investigated the negative viscosity of a colloidal dispersion composed of ferromagnetic rod-like particles, which have a magnetic moment normal to the particle axis. A simple shear flow problem has been treated to clarify the particle orientational distribution and rheological properties of such a semi-dense dispersion, under circumstances of an external magnetic field applied in the direction normal to the shear plane of a simple shear flow. The results obtained here are summarized as follows. For the cases of a very strong magnetic field and magnetic interactions between particles, the magnetic moment of the rodlike particles is significantly restricted in the magnetic field direction, so that the particle approximately aligns in the shear flow direction. Also, the particle can easily rotate around the axis of the cluster almost freely even in a simple shear flow. Characteristic orientational properties of the particle cause negative viscosity, as in the previous study for a dilute dispersion. However, magnetic particle-particle interactions have a function to make such negative viscosity decrease.© 2007 ASME