Dynamics of a 2D silo discharge: A competition between structural domains before clogging

A two-dimensional silo filled with identical disks of radii d is experimentally studied. At the critical value of the aspect ratio of the silo, D/d (length-opening), a stationary flow of disks is established (a gravity-driven silo discharge). Throughout the silo discharge and before clogging, disks conform into dynamic dense structural arrays (domains), and a size-competition between two principal domains is observed to occur. Interestingly, the size-competition remains continuous as clogging occurs, indicating the absence of a sudden increase or decrease in the local density due to the clogging. A well-defined shape factor is implemented at the scale of single disks, and hence we can identify the spatial structure of the domains during the discharge process. We observe that the size of the domains shows some sort of correlated competition, as one of them decreases another one increases. On the other hand, the statistical fluctuations of the linear and angular velocities of the domains, commonly referred to as the granular temperature, show a continuous transition when the flow is suddenly blocked by the formation of an arch. Notwithstanding that the dynamics of the domains, characterized by a shape factor, and the complete granular temperature does not provide tools to predict clogging, in this work we introduce a novel dynamical approach to describe the temporal local distribution of the elements of a flow of granular matter.