Bubble racing in a Hele-shaw cell*

We study theoretically and experimentally the propagation of inviscid bubbles in a Hele-Shaw cell under a uniform background flow, in the regime where each bubble remains approximately circular. New experimental results for the velocity of an isolated bubble are found to agree well with theoretical predictions. For a train of three collinear bubbles, we observe that the middle bubble closes in on the leader, again in agreement with theory. Finally for a system of two non-identical bubbles, we find that the larger bubble overtakes the smaller one, and they avoid contact by rotating around each other while passing.