  Using an event-driven molecular dynamics simulation, we show that simple
monodisperse granular beads confined in coupled columns may oscillate as a new
type of granular clock. To trigger this oscillation, the system needs to be
driven against gravity into a density-inverted state, with a high-density
clustering phase supported from below by a gas-like low-density phase
(Leidenfrost effect) in each column. Our analysis reveals that the
density-inverted structure and the relaxation dynamics between the phases can
amplify any small asymmetry between the columns, and lead to a giant
oscillation. The oscillation occurs only for an intermediate range of the
coupling strength, and the corresponding phase diagram can be universally
described with a characteristic height of the density-inverted structure. A
minimal two-phase model is proposed and linear stability analysis shows that
the triggering mechanism of the oscillation can be explained as a switchable
two-parameter Hopf bifurcation. Numerical solutions of the model also reproduce
similar oscillatory dynamics to the simulation results.
