Microstructures in Strongly Interacting Dipolar Fluids

The formation of ring- and chain-like microstructures in ferrofluids consisting of interacting magnetic particles coated by a surfactant layer is studied by Monte Carlo simulations. For thin coating layers, it is found that ring- and chain-like structures coexist. The ring-like structures are suppressed by thicker coating layers. These observations are in agreement with recent experiments. Also, the ring-like structures are formed by dynamical aggregation of particles, instead of bending of linear chains. The latter process is forbidden by a substantial energy barrier. More generally, it is found that the ring structures exist when the parameter α=μ² /[k_BT (d+2δ)³] is higher than a critical value α_c, where μ is the magnitude of magnetic moment, d is the diameter, and δ is the coating thickness of a particle. We find that when α<α_c, the ring structures cannot be formed spontaneously, while the chain-like structures still exist. Furthermore, the critical value α_c is almost independent of magnetic particle volume density in the low density range.