Ryo Hayasaka

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.

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

Brownian Dynamics Simulations of Sedimentation Phenomena of Ferromagnetic Spherical Particles in a Colloidal Dispersion

We have investigated sedimentation phenomena of a colloidal dispersion composed of ferromagnetic spherical particles in the gravity field, by means of Brownian dynamics 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 sedimentation phenomena of such magnetic particles. In order to discuss quantitatively the sedimentation process and the internal structures of particle aggregates after the sedimentation, we have concentrated our attention on the local radial distribution function of each layer. The main results obtained here are summarized as follows. For the case of a weak magnetic field and a weak magnetic force between particles, layered structures are formed. As the magnetic field increases, clusters are formed in upright formation along the gravity or the magnetic field direction. As magnetic particle-particle interactions increase, particles combine with each other to form aggregate in other directions, and new types of clusters are formed in the bottom area. In this situation, therefore, the upright-standing clusters come to disappear. For a dilute case, relatively small clusters are formed apart from each other in almost equal space. As the volumetric fraction increases from such a situation, clusters with voids in the center area of the clusters come to be observed, but such formation disappears and layered structures are formed with further increasing the volumetric fraction.Copyright

Brownian Dynamics Simulations of Sedimentation Phenomena of Ferromagnetic Spherical Particles in a Colloidal Dispersion under Gravity Field

We have investigated sedimentation phenomena of a colloidal dispersion composed of ferromagnetic spherical particles in the gravity field, by means of Brownian dynamics 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 sedimentation phenomena of such magnetic particles. In order to discuss quantitatively the sedimentation process and the internal structures of particle aggregates after the sedimentation, we have concentrated our attention on the time change in the local number density of particles and local radial distribution function of each layer. The main results obtained here are summarized as follows. For the case of a weak magnetic field and a weak magnetic force between particles, layered structures are formed. As the magnetic field increases, clusters are formed in upright formation along the gravity or the magnetic field direction. As magnetic particle-particle interactions increase, particles combine with each other to form aggregates in other directions, and new types of clusters are formed in the bottom area. In this situation, therefore, the upright-standing clusters come to disappear. For a dilute case, relatively small clusters are formed apart from each other in almost equal space. As the volumetric fraction increases from such a situation, clusters with voids in the center area of the clusters come to be observed, but such formation disappears and layered structures are formed with further increasing the volumetric fraction.

Transport Coefficients and Orientational Distributions of a Dilute Colloidal Dispersion Composed of Hematite Particles: Analysis for an Applied Magnetic Field Parallel to the Angular Velocity Vector of Simple Shear Flow

We have studied the influences of the magnetic field, shear rate, and random forces on transport coefficients such as viscosity and diffusion coefficient, and also on the orientational distributions of hematite particles composed of a dilute colloidal dispersion. Hematite particles are modeled as spheroids with a magnetic moment normal to the particle axis. In the present analysis, these particles are assumed to conduct the rotational Brownian motion in a simple shear flow as well as an external magnetic field. The basic equation of the orientational distribution function has been derived from the balance of the torques and solved by the numerical analysis method. The results obtained here are summarized as follows. With increasing the magnetic field, since the magnetic moment is strongly restricted to the magnetic field direction, the motion of the particle is forced to rotate in directions normal to the shear flow direction. In the case of a strong magnetic field and a smaller shear rate, the rodlike particles can freely rotate in the xy-plane with the magnetic moment remaining pointing to the magnetic field direction. On the other hand, for a strong shear flow, the particle has a tendency to incline in the flow direction with the magnetic moment pointing to the magnetic field direction. Additionaly, the diffusion coefficient gives rise to smaller values than expected, since the rodlike particle sediments with the particle inclining toward directions normal to the moment direction.Copyright