Magnetorheology of magnetic holes compared to magnetic particles

We compare the rheological behavior in a shear flow of two types of suspens ion of magnetic particles in the presence of a magnetic field. The first su spension is made of silica particles in a ferrofluid and the second one is made of carbonyl iron particles in silicone oil. The permeability curves of these two suspensions have been measured for different volume fractions as a function of the magnetic field in order to characterize the magnetic int eractions. We show for both cases the existence of two different yield stre sses: one associated with the solid friction of the particles on the plates of the rheometer and the second one with the rupture of the aggregates. Th is second yield stress presents a maximum with the volume fraction for the suspension of magnetic holes but grows faster than linearly with the volume fraction for the suspension of carbonyl iron. These features are explained by theoretical models based, respectively, on the deformation of aggregate s of macroscopic size and on ruptures between particles at the particle sca le. When plotted as a function of the Mason number, the curves for differen t fields well collapse on the same one but the slope of this master curve o n a log-log plot is quite different from the one of the Bingham law, especi ally for the suspension of magnetic holes. For both suspensions the exponen t depends on the volume fraction. (C) 2000 The Society of Rheology. [S0148- 6055(00)00501-0].