Avoiding clogs: the shape of arches and their stability against vibrations
Resumen: 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 in
the arch. The defects are quantitatively defined in terms of contact angles and we show that this is a key factor regarding the
endurance of arches.