Air rings blown by dolphins swimming underwater and rings of smoke emitted by jet engines are just two examples of vortex rings. These doughnut-shaped structures and their mesmerizing movement have been studied for decades given their role in propulsion and—in the case of jellyfish and other invertebrates—biological locomotion.

A team of researchers at New York University and NYU Shanghai has uncovered a remarkable property of vortex rings that has been overlooked for more than a century—one that illuminates how these rings respond when they move through water and reach air (i.e., at the water-air interface).

When a vortex ring traveling sideways and up through water reaches the surface and meets air, it can rebound while largely maintaining its shape—much like a tennis ball bouncing off a wall. After the reflection, the ring loses only a small fraction of its energy. However, if the vortex ring moves more directly upward, it breaks apart instead of bouncing.