  We report on the nature of flow events for the gravity-driven discharge of
glass beads through a hole that is small enough that the hopper is susceptible
to clogging. In particular, we measure the average and standard deviation of
the distribution of discharged masses as a function of both hole and grain
sizes. We do so in air, which is usual, but also with the system entirely
submerged under water. This damps the grain dynamics and could be expected to
dramatically affect the distribution of the flow events, which are described in
prior work as avalanche-like. Though the flow is slower and the events last
longer, we find that the average discharge mass is only slightly reduced for
submerged grains. Furthermore, we find that the shape of the distribution
remains exponential, implying that clogging is still a Poisson process even for
immersed grains. Per Thomas and Durian [Phys. Rev. Lett. 114, 178001 (2015]),
this allows interpretation of the average discharge mass in terms of the
fraction of flow microstates that precede, i.e. that effectively cause, a
stable clog to form. Since this fraction is barely altered by water, we
conclude that the crucial microscopic variables are the grain positions; grain
momenta play only a secondary role in destabilizing weak incipient arches.
These insights should aid on-going efforts to understand the susceptibility of
granular hoppers to clogging.
