Dense bubble flow in a silo: an unusual flow of a dispersed medium
  The dense flow of air bubbles in a two-dimensional silo (through an aperture D) filled with a liquid is studied experimentally. A particle tracking technique has been used to bring out the main properties of the flow: displacements of the bubbles, transverse and axial velocities. The behavior of the air bubbles is observed to present similarities with non-deformable solid grains in a granular flow. Nevertheless, a correlation between the bubble velocities and their deformations has been evidenced. Moreover, a new discharge law (Beverloo-like) must be considered for such a system, where the flow rate is observed to vary as D^{1/2} and depends on the deformability of the particles. 
Yann Bertho, Christophe Becco and Nicolas Vandewalle