Avalanching behavior of magnetic granular mixtures

We conducted avalanching experiments with an external magnetic field and granular samples of different grain sizes (3.18 mm, 6.35 mm, and 8.73 mm) and different materials (low-carbon steel, alloy steel, stainless steel, and brass). The magnetic field was varied to control the magnetic Bond number (the ratio between the magnetic and the gravitational forces in the system). For each test, we compared the angle of repose and the surface roughness of the material in its postavalanche state. The samples containing only steel beads transitioned through three flow regimes as the magnetic field increased. Initially, the grains flowed freely. Above a threshold magnetic field, the material began to move in clumps, and above a second threshold, it solidified completely. The steel-brass mixtures with low magnetic susceptibilities only transitioned through the first two states. We find that the angle of repose and surface roughness increase linearly with magnetic cohesion in the first regime, but that the trends in the second regime depend on the composition and magnetic susceptibility of the mixture. When the angle of repose and surface roughness are expressed in terms of the magnetic Bond number, the homogeneous samples that vary in grain size and magnetic susceptibility collapse onto a single curve, but the mixtures (i.e., the samples that contain more than one type of material) do not.