Reverse Janssen effect with non-spherical grains.

Granular matter exhibits unusual mechanical properties. For instance, when filling a cylindrical container with grains, the weight measured at the bottom of the granular column does not scale linearly with the added mass, but asymptotically saturates towards a constant apparent weight. This observation is the well-known Janssen effect, ultimately reflecting that the "lost" weight is supported by frictional forces between the grains and the wall of the container confining the grains. When filling narrower containers with spherical grains, however, an intermediate regime consisting of an overshoot in measured weight appears; in this case, before the usual Janssen saturation is observed, frictional forces contribute to increase the apparent weight. Here, we experimentally study the role of particle shape in this reverse Janssen effect and find that it disappears faster with increasing cylinder diameter for non-spherical grains compared to spherical grains. We then discuss the experimental results, analyzing some of the possible aspects that could potentially explain the influence of grain shape.