An Experimental Investigation into the Off-State Viscosity of MR Fluids

Abstract

This study investigates the off-state rheological characteristics of magnetorheological fluids. Various carbonyl iron powder grades are tested, both micron- and nano-sized. The study considers both unimodal and bimodal MR fluid samples. The bimodal MR fluids are both micron-sized only compositions and mixed micron- and nano-sized compositions. All fluid compositions employ a novel 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.