Compaction Bubbles in Sand

We studied granular compaction by tapping a tilted tube filled with well-prepared, loosely-compacted, fine glass beads. Instead of uniformly compacting, the granular medium reduces its volume by forming a train of upward moving voids - that is, the bed bubbles after a tap. We found that the bubbling of the granular bed is robust in that the length and the velocity of bubbles are independent of the method of tapping. We investigated the properties of the bubbles as a function of the angle of the tube with respect to the horizontal and found a threshold angle below which the bubbling behavior disappears. The velocity of a bubble increases as it rises in the tube. By changing the ambient pressure of the system, we found that the interstitial gas plays an essential role in generating the bubbles.