Structure formation and instability in a tube of sand.

A new instability in the combined flow of fine grains and gas is investigated by means of experiments, simulations, and analytic techniques. When a bubble of air rises through a granular packing in a tube, a sequence of smaller bubbles spontaneously forms in front of it. The existence of this instability is shown from the experiments, simulations, and theoretical considerations. Moreover, the simulations and experiments agree on the quantitative level. In particular, when the tube is tilted away from the vertical the experiments and the simulations show the same increase in the speed of the rising bubble.