  The hourglass is one of the apparatuses familiar to everyone, but reveals
intriguing behaviors peculiar to granular materials, and many issues are
remained to be explored. In this study, we examined the dynamics of falling
sand in a special form of hourglass, i.e., a wedge-shaped hopper, when a
suspended granular layer is stabilized to a certain degree. As a result, we
found remarkably different dynamic regimes of bubbling and cavity. In the
bubbling regime, bubbles of nearly equal size are created in the sand at a
regular time interval. In the cavity regime, a cavity grows as sand beads fall
before a sudden collapse of the cavity. Bubbling found here is quite visible to
a level never discussed in the physics literature and the cavity regime is a
novel phase, which is neither continuous, intermittent nor completely blocked
phase. We elucidate the physical conditions necessary for the bubbling and
cavity regimes and develop simple theories for the regimes to successfully
explain the observed phenomena by considering the stability of a suspended
granular layer and clogging of granular flow at the outlet of the hopper. The
bubbling and cavity regimes could be useful for mixing a fluid with granular
materials.
