An experimental investigation into the off-state viscosity of MR fluids

This study investigates the off-state rheological characteristics of magnetorheological (MR) fluids. Various carbonyl iron powder grades are tested, both micron- and nano-sized. The study considers both monodisperse and bidisperse MR fluid samples. The bidisperse MR fluids are both micron-sized only compositions and mixed micron- and nano-sized compositions. All fluid compositions employ a novel perfluorinated polyether (PFPE) oil as a base fluid. This base fluid is introduced and its qualities discussed. An off-state experimental investigation is conducted, for shear-rates from 0 to 200 s ^-1. All samples exhibit the well known shear-thinning property of MR fluids but with varying proportions. The off-state viscosity is of importance in a particular application, in an MR prosthetic knee. It determines how fast the knee joint can rotate in the absence of a magnetic field. A prominent MR fluid composition is selected for the proposed application which aims to maximize to rotational speed of the MR knee joint in the absence of a magnetic field.