Diverse microstructures of magnetorheological fluid induced by shear flows (A direct numerical investigation)

Abstract

This study presents numerical simulations of magnetorheological (MR) fluids under a wide range of shear rates using the Euler-Lagrange hybrid scheme. In a confined space between plane-parallel walls with gravity neglected, under a constant magnetic field and particle volume fraction, the formation of magnetic particle microstructures and the shear-thinning behavior of the apparent viscosity of the MR fluid are investigated. As the shear rate increases, the microstructures formed by the magnetic particles transition from chain-like to sheet-like formations around a Mason number (Mn) of 1. Beyond a certain shear threshold, these structures begin to aggregate toward the stationary wall, while particles move away from the moving wall. For Mn = 30, hydrodynamic two-phase separation is observed with respect to the vertical direction of the walls: a particle-free region appears on the moving wall side, while an aggregate structure without interparticle contact forms on the stationary wall side. With further increase in shear rate to Mn = 100, the upper-layer particles of the aggregates detach and disperse. The two-phase separation observed at Mn = 30 is induced by magnetic and hydrodynamic interactions, and the aggregation toward the stationary wall is attributed to the constraint of particle rotation by applied magnetic field. These findings indicate that the presence or absence of particle rotation influences the hydrodynamic lift forces acting on the particles. The ratio of the magnetic torque induced by the applied magnetic field to the hydrodynamic torque is as a key factor governing the microstructure of magnetic particles in MR fluids.