  The present work reports a novel investigation into the collisional dynamics
of particles in the vicinity of the outlet of a 2D silo using molecular
dynamics simulations. Most studies on this granular system focus in the bulk of
the medium. In this region contacts are permanent or long-lived, so continuous
approximations are able to yield results for velocity distributions or mass
flow. Close to the exit, however, the density of the medium decreases and
contacts are instantaneous. Thus, the collisional nature of the dynamics
becomes significant, warranting a dedicated investigation as carried out in
this work. More interesting, the vicinity of the outlet is the region where the
arches that block the flow for small apertures are formed. It is found that the
transition from the clogging regime (at small apertures) to the continuous flow
regime is smooth in collisional variables. Furthermore, the dynamics of
particles as reflected by the distributions of the velocities is as well
unaffected. This result implies that there is no critical outlet size that
separates both regimes, as had been proposed in the literature. Instead, the
results achieved support the alternative picture in which a clog is possible
for any outlet size.
