(558bf) The Aggregate Structures of Magnetic Cubic Particles in Thermodynamic Equilibrium By Means of Brownian Dynamics Simulations

A suspension composed of magnetic particles has a great potential for application in a variety of engineering fields. Typical application areas of magnetic particle suspensions are the fields of fluid engineering, biomedical engineering and environmental engineering. In the field of fluid engineering, a magnetorheological effect may be controlled by an external magnetic field and therefore magnetic particle dispersion is expected to be applicable for developing mechanical actuators and dampers. Magnetorheological properties significantly depend on the regime of aggregate structures. Recently, in the field of biomedical engineering, magnetic hyperthermia treatment has been actively investigated as a hopeful application of magnetic particle suspensions. This treatment utilizes the heating effect arising from the relaxation of magnetic moments in an external alternating magnetic field. Several simulation studies for the case of spherical particle suspensions in an alternating magnetic field have already been conducted by means of Brownian dynamics method. On the other hand, non-spherical particles such as rod-like, disk-like and cube-like particles are quite difficult from a simulation point of view in a time-dependent magnetic field. In the case of magnetic cube-like particles, a suspension of cubic particles is expected to exhibit a sufficiently strong magnetorheological effect since cubic particles tend to aggregate to form closely-packed clusters where constituent cubic particles are located in a face-to-face contact configuration. Brownian dynamics simulation methods may be desirable for investigating the dynamic characteristics of a magnetic cubic particle suspension. However, in contrast to axisymmetric particles such as cylindrical and spheroidal particles, the diffusion coefficients or friction coefficients of cubic particles are not well known as analytical expressions or numerical results. From this background, we have already evaluated the translational and rotational diffusion coefficients of a cube in a wide rage of the Reynolds number. In the present study, we attempt to verify the validity of the Brownian dynamics simulation method with these translational and rotational diffusion coefficients by addressing the aggregation phenomena in a cube-like magnetic particle suspension in thermodynamic equilibrium. In concrete, we have investigated the dependence of the regime change in aggregate structures on the various factors such as the magnetic particle-particle interaction strength and the magnetic particle-field interaction strength. The results obtained by Brownian dynamics simulations have been compared with those of Monte Carlo simulations that may be regarded as theoretical solutions. Snapshots of particle aggregates are addressed for qualitative discussion, and order parameters and radial distribution functions are focused on for quantitative discussion regarding the phase change and the internal structure of the aggregates of cubic particles. In no applied magnetic field, if the magnetic interaction strength is sufficiently large, closely-packed aggregate structures are formed in the system. A regime change in the internal structure of particle aggregates appears within a narrow range of the magnetic interaction strength. As the magnetic field is increased, the closely-packed aggregate structures are transformed into wall-like clusters where the magnetic moments of constituent particles strongly incline in the magnetic field. From comparison with Monte Carlo results, it is seen that the characteristics of the aggregate structures and the phase change are in significantly good agreement with those of Monte Carlo method. From these results, we may conclude that the present Brownian dynamics simulation method is a useful particle-based simulation tool for a suspension composed of magnetic cube-like particles. In the next stage, this simulation method may be applied to the investigation of the dynamic characteristics of a magnetic cube-like particle suspension such as magnetorheological characteristics in a non-equilibrium situation.