The influence of particle size on the magnetorheological properties of an inverse ferrofluid

The dependence of magnetorheological properties on particle size is studied , using a model magnetorheological fluid consisting of nonmagnetic silica p articles suspended in a ferrofluid. For small particles a strong increase o f magnetorheological properties with particle size was found. At a certain particle size and field strength a crossover occurs to a regime where there is only a very limited size dependence. This can be understood in terms of the length of the chains in relation to the gap size of the geometry. Cros sover occurs when the average chain length becomes comparable to the gap si ze. The remaining size dependence may be explained by polydispersity, varia tions in the morphology, variations in the sterically stabilizing layer of the particles and surface roughness of the geometry. (C) 2000 American Inst itute of Physics. [S0021-9606(00)50429-2].