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Bursting bubbles blow smoke rings

Physicists have long used high-speed video to watch bubbles burst: A small hole appears in the bubble, it grows as the film of liquid smoothly contracts, and finally, the thickening film disintegrates in a spray of droplets. But, something equally spectacular—and ordinarily invisible—occurs in the gas rushing out of the burst bubble, two mechanical engineers report.

The gas does not simply rush out of the bubble in all directions, as Ali al Dasouqi and David Murphy of the University of South Florida in Tampa show in videos like the one above of water bubbles filled with smoke. Instead, the gas rises up in delicate vortex rings—the same sorts of swirls that are commonly known a smoke rings, the team will report Tuesday at the meeting of the American Physical Society’s Division of Fluid Dynamics in Seattle, Washington.

If it’s big enough, a bubble can release a whole stack of vortex rings resembling a multitiered Japanese pagoda, the videos show. Perhaps surprisingly, the pattern in the escaping gas is even prettier and more delicate than the bubbles themselves.