Dimorphic magnetorheological fluids: exploiting partial substitution of microspheres bynanowires

Abstract

Magnetorheological (MR) fluids typically are suspensions of spherical micron-sizedferromagnetic particles suspended in a fluid medium. They are usually thought of asBingham-plastic fluids characterized by an apparent yield stress and viscosity.Partial substitution of the micron-sized iron particles with rod-shaped nanowiresconstitutes a dimorphic MR fluid. In this study, we investigate the influence thatnanowires have on the magnetorheological and sedimentation properties of MRfluids. A variety of conventional and dimorphic MR fluid samples were consideredfor this study with iron loading ranging from 50 to 80 wt%. The nanowires usedin this study have mean diameters of 230 nm and a length distribution of7.6 ± 5.1 µm, while the spherical particles have a mean diameter of8 ± 2 µm. Flow curves were measured using a parallel disk rheometer and a sedimentationmeasuring instrument was constructed for quantifying sedimentation velocity. TheBingham yield strength and sedimentation velocity of the dimorphic MR fluids are thencompared to those of conventional MR fluids incorporating spherical particles.