Amazing Bubble Column

This toy is simple, and very beautiful to watch. But there is also a bit of physics going on that leaves us completely open mouthed.

The toy consists of a clear plastic tube, containing a clear and very viscous liquid. At each end, there is a small circular compartment made of black plastic, with the same diameter as the main tube,which has a hole opening into the main compartment. This hole is smaller in diameter than the width of the tube, and we suspect that this is important. There is also a certain amount of air in the tube.

Normally, if you leave the tube standing upright,bubbles of air will rise up the tube, until they are all hidden inside the compartment at the top. You then turn the tube upside down, and the bubbles will again start rising slowly up the tube.

But what is really amazing is that if two bubbles are rising slowly up the tube and they are reasonably close to each other, the lower bubble appears to catch up the bubble above it, and becomes completely engulfed by it. It is as if the top bubble has swallowed the bottom one. Why should that happen? We are trying to find out...