Avoiding clogs: the shape of arches and their stability against vibrations

Abstract

A distinctive feature of discrete solids is their ability to form arches. These mechanically stable structures alter the isotropy of granular packings and can arrest the motion of grains when, for example, they flow through a bottleneck. Breaking arches can be achieved by means of an external vibration, which effectively eliminates clogging. Indeed, these phenomena and procedures are quite common in industrial applications. Nevertheless, there are not rigorous, well founded criteria to determine the most efficient way to break arches and restore the flow of grains. This happens in part because it is not known which are the relevant characteristics that boost the arch strength. In the experiment presented here, we have carried out a statistical analysis of the arches that block the exit orifice at the bottom of a two dimensional silo, and described their geometrical properties. We then submit the silo to an external vibration. We find that the larger the outlet size, the weaker the arches that clog it. This dependence is just the outcome of a more complicated process that involves geometrical defects inthe arch. The defects a are quantitatively defined in terms of contact angles and we show that this is a key factor regarding the endurance of arches.