Viscometric and Sedimentation Characterization of Bidisperse Magnetorheological Fluids

Magnetorheological fluids (MRFs) are typically thought of as Bingham-plastic (BP) fluids characterized by a yield stress. Partial substitution of micrometer-scale Fe particles with nanometer-scale Fe particles leads to bidisperse MRFs. This partial substitution mitigates particle sedimentation, but can reduce yield stress for high nanoparticle concentrations. We examine tradeoffs between increasing suspension stability versus decreasing in yield stress as nanoparticles are substituted for microparticles. Four groups of fluids with total Fe concentration of 50, 60, 70, and 80 wt% were considered. A sedimentation measuring device quantified sedimentation velocity of MRFs in a gravitational field. This sensor relates the rate of change of inductance relative to settling rate as Fe wt% decreases because of sedimentation. MRF flow curves were measured using a parallel disk rheometer and yield stress was identified using the BP flow model.