  Recently, the existence of so-called granular bubbles and droplets has been
demonstrated experimentally. Granular bubbles and droplets are clusters of
particles that respectively rise and sink if submerged in an aerated and
vibrated bed of another granular material of different size and/or density.
However, currently there is no model that explains the coherent motion of these
clusters and predicts the transition between a rising and sinking motion. Here,
we propose an analytical model predicting accurately the neutral buoyancy limit
of a granular bubble/droplet. This model allows the compilation of a regime map
identifying five distinct regimes of granular bubble/droplet motion.
