7 - Stick-Slip Instabilities in Magnetorheological Fluids

Abstract

In this chapter, we review our recent experimental and theoretical results on the flow instabilities observed in magnetorheological (MR) fluids subjected to applied magnetic fields. We study three types of MR fluids, consisting of suspensions of, respectively, silica-coated iron particles, carbonyl iron particles and cobalt-nickel (CoNi) fibers dispersed in a liquid carrier, and subjected to two types of flow, shear flow between parallel plates and pressure-driven flow through a capillary. In all the cases, we found that the flow curves showed a decreasing branch corresponding to a region of negative differential viscosity, for which a stable steady-state flow was impossible. We found different physical reasons for the observed instabilities. For shear flow between parallel plates of highly concentrated suspensions of spherical iron particles, fracture of the magnetic field-induced particle structures in combination with shear localization was at the basis of the observed instabilities. Periodic formation and fracture of the particle structures due to the magnetic field gradient and the hydrodynamic drag forces, respectively, was responsible for instabilities in the case of pressure-driven flow through a capillary of suspensions of spherical iron particles. In the case of suspensions of CoNi fibers, interparticle friction forces were at the origin of the obtained negative differential viscosity, as a consequence of hindering fiber reorientation inside the aggregates as they were deviated by shear forces from the direction of the applied field. In addition to the significance of this work from a fundamental point of view, its practical relevance stands on the importance of detailed knowledge of the conditions that give rise to instabilities in order to rule them out and avoid malfunction of technological devices based on the MR effect.