  A jet of water entering into a pool of the same liquid can generate air
entrainment and form bubbles that rapidly raise to the surface and
disintegrate. Here we report the equivalent phenomenon produced by a plunging
dry granular jet, so far unexplored. For grains smaller than a critical size,
the granular jet entering into the pool produces air entrainment that leads to
bubbles formation. The bubbles formed are covered by a shell of grains attached
to the bubble air-water interface due to capillary-induced cohesion. In
contrast to classical air bubbles, these "granular bubbles" are stable over
time because the granular shell prevents coalescence and keeps the air
encapsulated either if the bubbles rise to the surface or sink to the bottom of
the pool, which is determined by the competition of the buoyant force and the
weight of the assembly. Experimentally, we show how the bubble size and volume
of entrained air depend on the grain size, liquid properties and jet impact
velocity.
