A constitutive equation for magnetorheological fluid characterization

A microstructural model of the motion of particle pairs in MR fluids is proposed that accounts for both hydrodynamic and magnetic field forces. A fluid constitutive equation is derived, from the model that allows the prediction of velocity and particle structure fields. The analysis is similar to that of bead-spring models of polymeric liquids with replacement of the elastic connector force by a magnetic force. Results for simple shear flow are presented for the case when the two particles remain in close contact so they are hydrodynamically equivalent to an ellipsoid with an aspect ratio of two and only the component of the magnetic force normal to the connecting vector between the centers of the two particles affects motion. The model predicts oscillatory motion of the particle pairs at low magnetic fields. The fluid reaches a steady state at high magnetic fields. The time required to reach the steady state for a given shear rate reduces significantly as the field increases.