Particle Dynamics Study of Microstructural Evolution and Mechanical Response in Magnetorheological Fluids

Magnetorheological fluids (MRFs) exhibit field-dependent mechanical properties that are governed by the evolution of their internal particle structures. In this work, a particle-dynamics-based numerical method is used to investigate the microstructure formation and shear response of monodisperse and bidisperse MRFs at volume fractions of 22% and 33% under the same magnetic field and carrier-fluid viscosity. A mechanical model based on interparticle interactions is employed to describe particle motion and magnetic-field-induced chain assembly. To quantify the structural evolution during deformation, the average coordination number is introduced as a microstructural descriptor. The results show that bidisperse MRFs exhibit distinct chain formation mechanisms from monodisperse systems, particularly with decreasing particle size ratio and increasing concentration. Under steady shear, both systems display a fluctuating increase in stress, accompanied by structural rupture, migration and reorganization. However, concentration markedly affects the dominant evolution mode, while the incorporation of small particles in bidisperse systems weakens chain integrity, leading to stronger stress fluctuations and generally lower peak stress. These results provide insight into the coupling between microstructure evolution and mechanical response in MRFs.