Abstracts:

The path of a circular air bubble rising in a viscous fluid of a Hele-Shaw cell is found to change from a straight path to a zig-zag path when the Reynolds number of the bubble (proportional to the bubble terminal velocity) exceeds a threshold. By visualizing the wake structures of the rising bubbles, and measuring the frequency of the zig-zag paths, we demonstrate that the path instability is a consequence of the vortex shedding behind the bubble.