Microstructural Evolution and Transient Magnetization in Bidisperse Magnetic Colloidal Suspensions

Magnetic colloidal suspensions have attracted increasing interest due to their tuneable macroscopic responses under external magnetic fields. These responses arise from the formation of microscopic structures by magnetic particles suspended in a carrier Newtonian fluid. Previous studies revealed that particle size bidispersity—suspensions with two particle sizes—significantly affects the particle dynamics. However, these studies focused on weak magnetic interactions. The present work examines the impact of strong magnetic interactions in the formation of microstructure. Brownian dynamics simulations were employed to study bidisperse magnetic colloidal suspensions under a uniform magnetic field. The microstructural evolution was tracked over time, along with the time-dependent behavior of the magnetization, a macroscopic property that characterizes the collective response of the suspension. The simulations revealed a variety of structures that were not observed in suspensions of identical particles. At weak magnetic fields, large particles drive the aggregation of small ones around them, resulting in shell-like structures. This leads to a decay in the time-dependent increase of magnetization, as more small particles cluster around the large ones. In contrast, at strong magnetic fields, large particles promote bulk chain-like structures, enhancing magnetization. These findings may support more accurate magnetization models and new applications based on these suspensions.