(518j) Monte Carlo Simulations on Phase Change of Internal Structures of Aggregates of Magnetic Rod-like Particles in an External Magnetic Field

Magnetic particle suspensions have a great potential as an application in engineering fields and therefore a variety of studies on these functional fluids have been conducted in various fields, including the traditional fluid engineering field and the recent bio-engineering field such as an application to the drug delivery system. Magneto-rheological properties significantly depend on the formation of aggregates of magnetic particles. This aggregate formation is governed by a variety of factors such as the magnetic field strength, magnetic particle-particle interactions, translational and rotational Brownian motion, and flow situations. In the present study, we focus on a ferromagnetic rod-like particle suspension to discuss the phase change of aggregate structures of magnetic rod-like particles. The characteristics of the phase change are investigated by Monte Carlo simulations for thermodynamic equilibrium. From Monte Carlo simulations, we attempt to clarify the dependence of the phase change of aggregate structures on the field strength, the magnetic particle-particle interaction strength, the volumetric fraction, etc. From the present simulations, it is seen that the cluster formation of magnetic particles significantly appears for a strong magnetic interaction larger than a certain value, which is much larger than that for a magnetic spherical system. This is because the rotational Brownian motion has a significant effect on the aggregate formation for a magnetic rod-like particle suspension. If the magnetic interaction strength is sufficiently large for clusters being formed, thick linear chain-like clusters are formed and the magnetic moments of the constituent particles of the cluster of interest incline almost in the same direction. These clusters align in the opposite directions to the neighboring clusters in a weak applied magnetic field situation. As the magnetic field is increased, thick chain-like clusters grow and become thicker by adsorbing the neighboring clusters that align in the opposite direction to the magnetic field direction. This phase change of aggregates is quantitatively verified in the characteristics of the order parameter and the radial distribution function. The increase in the volumetric fraction of particles induces the phase change from thick chain-like clusters into wall-like clusters. Moreover, the increase in the particle aspect ratio induces instability of thick chain-like clusters to give rise to more complex cluster formation.