Dependence of the Regime of Aggregate Structures of Magnetic Rod-Like Particles on the Magnetic Model

In the present study, we attempt to discuss the dependence of the regime of the aggregate structures of magnetic rod-like particles on the magnetic model. Moreover, we briefly discuss the characteristic magneto-rheological properties of each magnetic model. Three representative magnetic models are here addressed for a magnetic rod-like particle, that is, (a) a spherocylinder particle with a dipole moment at the particle center (dipole model), (b) a spherocylinder particle with a plus and a minus charge at the center of each hemi-sphere (charge model) and (c) a spherocylinder with a dipole moment in a direction normal to the particle axis direction at the particle center (hematite model). For each magnetic model, molecular simulations based on the Monte Carlo method have been performed in order to elucidate the influences of magnetic particle-particle and particle-field interactions on the aggregate structures of magnetic spherocylinder particles in thermodynamic equilibrium. For the case of the dipole model, long stable raft-like clusters are gradually formed with increasing magnetic particle-particle interaction strength, and these raft-like clusters dissociate into the formation of chain-like clusters with increasing magnetic field strength. For the case of the charge model, long and thick chain-like clusters are more significantly formed with increasing magnetic interactions, and the thick chain-like clusters in the field direction become further thicker with increasing magnetic field strength. For the hematite model, long raft-like clusters are formed and these clusters still remain and incline in the field direction in a strong magnetic field situation. From these results, it is evident that the different magnetic model gives rise to the significantly different regime of the aggregate structures. Moreover, Brownian dynamics simulations have been conduced in order to clarify the dependence of the magneto-rheological characteristics on the regime of the above-mentioned particle aggregates. Among these magnetic models, the charge model yields the largest magneto-rheological effect, whereas the hematite model provides the negative viscosity due to the magnetic properties of particles.