The "sticky" sands of time : effects of cohesion on silo discharge*

The flow rate of cohesionless granular materials exiting a silo is commonly described by the ``Beverloo's law" which captures the role of the aperture size and grain diameter. Yet, the role of interparticle cohesive force in the emptying of a silo remains unclear. Indeed, the presence of cohesion can lead to clogging of the aperture. Even when the cohesive grains freely flow through the aperture, the flow rate must account for the cohesion. Here, we experimentally rely on model cohesive grains to extend the Beverloo's law to cohesive particles. A flat-bottomed cylindrical silo with a circular aperture is filled with cohesion-controlled particles. First, more inter-particle cohesion requires larger openings for any flow. For large enough apertures, flow is indeed observed for cohesive particles as well, but the discharge is systematically less for greater cohesion. These observations are quantitatively connected to the magnitude of cohesion between the particles.