Placing an obstacle in front of a bottleneck has been proposed as a sound alternative to improve the flow of discrete materials in a wide variety of scenarios. Nevertheless, the physical reasons behind this behavior are not fully understood and the suitability of this practice has been recently challenged for pedestrian evacuations. In this work, we experimentally demonstrate that for the case of inert grains discharging from a silo, an obstacle above the exit leads to a reduction of clog formation via two different mechanisms: i) an alteration of the kinematic properties in the outlet proximities that prevents the stabilization of arches; and ii) an introduction of a clear anisotropy in the contact fabric tensor that becomes relevant when working at a quasi-static regime. Then, both mechanisms are encompassed using a single formulation that could be inspiring for other, more complex, systems. The mechanisms underlying clogging of granular materials exiting a container have been widely studied, but findings have been sometimes contradictory for other systems or active matter in general. The authors experimentally analyze the effect of an obstacle to prevent silo clogging, finding that the obstacle has a dual role altering both the kinematic properties of the system and the distribution of contact orientations

The existence of a transition from a clogged to an unclogged state has been recently proposed for the flow of macroscopic particles through bottlenecks in systems as diverse as colloidal suspensions, granular matter, or live beings. Here, we experimentally demonstrate that, for vibrated granular media, such a transition genuinely exists, and we characterize it as a function of the outlet size and vibration intensity. We confirm the suitability of the "flowing parameter" as the order parameter, and we find out that the resealed maximum acceleration of the system should be replaced as the control parameter by a dimensionless velocity that can be seen as the square root of the ratio between kinetic and potential energy. In all the investigated scenarios, we observe that, for a critical value of this control parameter S-c, there seems to be a continuous transition to an unclogged state. The data can be resealed with this critical value, which, as expected, decreases with the orifice size D. This leads to a phase diagram in the S-D plane in which clogging appears as a concave surface.

Flowing grains can clog an orifice by developing arches, an undesirable event in many cases. Several strategies have been put forward to avoid this. One of them is to vibrate the system in order to undo the clogging. Nevertheless, the time taken to break an arch under a constant vibration has a distribution displaying a heavy tail. This can lead to a situation where the average breaking time is not well defined. Moreover, it has been observed in some experiments that these tails tend to flatten for very long times, exacerbating the problem. Here we will review two conceptual frameworks that have been proposed to understand the phenomenon and discuss their physical implications.