Cohesion induced by a rotating magnetic field in a granular material

We report experiments on a magnetic cohesive granular material made of steel spheres in the millimeter range. A magnetic field magnetizes the spheres, so that an interaction force between grains appears. A rotating magnetic field is applied parallel to plane of the quasi-two-dimensional cell containing the spheres so that only the time averaged force between two particles will be considered. Both maximum angle of stability and angles of repose are measured. The maximum angle of stability is found to depend linearly on the interaction force. Another noticeable feature is the lack of dependence of the maximum angle of stability on the initial height of the heap. We show that the angle of repose is less sensitive to the magnetic interaction force than the maximum of stability. At last, we discuss the importance of using a rotating field rather than a constant one. In particular, we report some measurements of both the maximum angle of stability and the angle of repose in constant field, which show a strong dependence of the angles of avalanche on the direction of the field.