We just installed a huge plastic swimming pool, and besides playing with
all sorts of cool wave propagation and focussing effects, I finally got a
chance to mess with Falaco solitons:
These are ring-vortices, but underwater, and only the lower half of the
ring is there. They are easily disturbed, so one needs to remain outside
the swimming pool while creating and observing them. The author of the
above website notes that, if they can be created in perfect half-ring
shape, their lifetime is incredibly long.
I found that an old Frisbee works great for generating them. I use it as
a scoop, dipping it half-way in, pushing it along, and slowly withdrawing
it upwards so the underwater "smoke ring" stays laminar. The termination
at the surface of the water cause lens effects, and in sunshine the shadow
of the ring is visible on the pool bottom.
One can launch a fast ring at a slow one. If aimed right, the fast one
will pass through the hole, but one shrinks and the other expands. If
they hit each other, they disrupt (although one black shadow-spot dies
first, and it seems to take awhile for the wave of disruption to reach the
other black spot.) Two people, one at either end of the pool, can have
long, slow, smoke-ring fights. The rings of course cannot be seen, and
all we have is pairs of circular black shadow-splotches which drift very
slowly across the pool bottom.
I occasionally could succeed in producing one with my hands, but with the
Frisbee I reliably made one every time.