On the Yield Stress of Magnetorheological Fluids

Magnetorheological fluids (MRFs) are a category of functional materials that exhibit magneto-mechanical coupling. These materials exhibit a reversible and instantaneous change from a free-flowing Newtonian fluid to a semi-solid state upon application of a magnetic field. In contrast to ordinary fluids, MRFs can tolerate shear stresses up to a yield value in the presence of a magnetic field. The yield stress strongly depends on intensity of the applied magnetic field and volume fraction of magnetic particles. As the yield stress is the most important parameter of an MRF and must be considered in the design of MR devices, in this work, effects of magnetic field and volume fraction of particles are investigated both experimentally and theoretically. MRF samples with the same carrier fluid but different particle concentrations are analyzed, and an empirical model is proposed for the yield stress of MRFs that covers a wide field strength range and also captures magnetic saturation of the MR fluids. Though the model is mathematically simple, it also includes the effect of particle concentration such that once calibrated, it can be utilized for different particle concentrations as well. Moreover, a modified form of the magnetic dipole model is proposed to model the yield stress of MRFs where an exponential distribution function is utilized to describe the arrangement of particle chains in the presence of a magnetic field. It is shown that, though the model has a simple mathematical formulation, it leads to a reasonable distribution of chains compared to previous similar models. (C) 2022 The Author(s). Published by Elsevier Ltd.