Capillary flow of a suspension of non-magnetic particles in a ferrofluid under highly non-uniform magnetic field

When a ferromagnetic suspension flows through a capillary placed between two small strong permanent magnets, the magnetic force acts upon the non-magnetic (silica) particles dispersed in a ferrofluid and they tend to be extruded from the zone of high magnetic field. Particles get concentrated at the entrance section between magnets and form a plug. The increase of hydraulic resistance is due to the relative motion between particulate and ferrofluid phases in the presence of a field. If we keep the pressure difference constant, the flow rate will decrease when the field is applied and can eventually completely stop. In order to restart the flow a pressure difference, high enough to push the silica plug out of the capillary, is needed. The critical pressure of the flow blockage is nearly two times less than the pressure of the flow onset, both pressures being independent of the particle concentration in the suspension (except for near zero concentration). Such hysteresis of the flow onset/blockage has also been predicted in the frame of the proposed two-phase flow model, which has been used to calculate steady-state concentration profiles and discharge characteristics.