Vertical vibration is commonly used to agitate granular materials
in order to improve flowability. It is also used as an alternative
to fluid injection in order to fluidize a granular assembly. There are
cases when the behavior of a vibrated granular assembly can be
somehow counterintuitive, or in apparent defiance of gravity. One
example is the uphill motion of droplets of fine powders observed
by Caballero and Melo (2004), when they vibrated inclined surfaces
covered with grains. Another example is the reverse buoyancy
of objects immersed in vibrated granular columns, reported by
Shinbrot and Muzzio (1998). The term reverse buoyancy was used
∗ Corresponding author. Tel.: +58 212 5041534
E-mail addresses: ijsanche@ivic.gob.ve, ivanjo@gmail.com (I. Sánchez).
because objects with densities lower than the density of the granular
bulk sank, while objects denser than the granular bulk rose to
the surface, in contrast with normal buoyancy in fluids.