Ink released into water shows the swirling motion inside a vortex ring as well as its deformation and breakup upon stagnation against a wall. Although humans are known to make such vortex rings with smoke or bubbles, they are common in nature as well. Buoyant plumes often feature vortex rings at their head; dolphins and whales play with bubble rings; volcanoes blow smoke rings; and mosses use them to distribute spores.
Two vortex rings collide head-on in this video. If their vorticities and velocities are matched in magnitude and opposite in direction, their collision results in a stagnation plane—essentially a wall across which the fluid does not pass. In reality, there are slight variations that result in non-zero velocities where the vortices meet, so some mixing occurs, but the overall symmetry remains striking. The collision breaks up the vortex ring into filaments, some of which cross-link with the other vortex’s filaments, resulting in the little halo-like eddies around the perimeter. Videos of the same experiment at different Reynolds numbers can be found here. (Submitted by Charlie H; Video credit: T. Lim and T. Nickels)