The induced field of magnetized wall on the static normal force of magnetorheological fluids

In this work, the effects of induced field generated by the magnetized wall on the static normal force of magnetorheological fluids have been studied both theoretically and experimentally. Firstly, the magnetic field distributions between two parallel ferromagnetic sheets in a uniform field have been investigated. The magnitude and direction of normal force created by the field gradient in different field regions are calculated based on the Biot-Savart theorem. In an attempt to obtain the correspondences between the magnetization of the magnetized sheets and the magnetic flux density of the field, we designed a uniform magnetic field generator for measuring the induced field of the magnetized sheets. The static normal forces of 30% volume fraction magnetorheological fluids with or without magnetized wall condition have been measured by a commercial plate-plate rheometer, and the field fluctuation at the edge of the magnetized sheet has been validated by controlling the radius of test area. The experimental results indicated that the magnetized sheets with a thickness of 0.2 mm can increase the static normal force by up to 20% when the thickness of suspensions is 0.5 mm. But the increment is limited by the saturation magnetization of material itself, in addition, the non-uniform magnetic field of the magnetized sheets will adversely affect the increase of the static normal force. The experimental results are basically consistent with the theoretical analysis when the radius of test area is 8 mm, but underestimate the field increment for the 10 mm-radius case which may caused by the uneven distribution of particles.