The odd weight of granular packings: Effect of grain geometry

Abstract

Granular materials are collections of solid grains that exhibit unusual mechanical properties. In 1895, the engineer Janssen found that when filling a silo with corn grains, the pressure at the bottom of the column rather than increasing linearly with the added mass showed a saturation [5]. Recently, this experiment has been repeated using spherical grains on narrow containers, where the pressure at the bottom exhibits an overshoot region, meaning that the apparent mass is larger than the added mass before saturating [14]. In this work we revisit those experiments now using two different grain geometries, being oblate and prolate grains, with the aim of studying its behaviour on narrow containers and if the reversed Janssen effect is still observed using these grain geometries. It is found that for tubes with large diameters both, oblate and prolate particles, exhibit the saturation of mass for large added mass, the qualitative response described by Janssen. As the diameter of the tube decreases, the overshoot regions gain more importance at intermediate added masses. However, the model introduced in [14] for spheres does not describe properly the results obtained. Now, the magnitude of the overshoot decays faster with the increasing size of the container. Furthermore, it is observed that the average packing fraction of the granular columns does increase until saturating, which may be related to the saturation of the pressure on the granular column.