Bubble racing in a Hele-Shaw cell*

We study theoretically and experimentally the propagation of 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. We examine a system of two non-identical bubbles on different streamlines of the background flow, with the larger one initially behind. We find that the larger bubble overtakes the smaller one, and they avoid contact by rotating around each other while passing.