Magnetorheological fluids

Magnetorheological fluids have a viscosity which can be controlled by the application of a magnetic field. Usually they are dispersions of micronic magnetic particles in a carrier liquid. We shall present the basic phenomena related to the interplay between interparticle magnetic forces which are responsible for the gelation of the suspension and hydrodynamic forces which will break this gel and will allow the suspension to flow. We shall give some analytical predictions for the yield stress and emphasize how the combination of field and flow can give rise to a very rich rheology with hysteresis and shear-induced phase separation.