As waters part, polygons appear.

Imagine a hurricane with an eye in the shape of a propeller amid the swirling clouds. Physicists have observed something almost as strange in whirlpools that they made by swirling liquids in a novel way. Within the whirlpools, they've seen three-blade-propeller shapes as well as regular polygons, including squares and hexagons.

The behavior of liquids in rotating containers has long fascinated physicists. For instance, in a famous late-1600s study, Isaac Newton pondered why the surface of water in a rotating bucket becomes concave.

In the new experiments, Tomas Bohr and his colleagues at the Technical University of Denmark in Lyngby observed liquids in a cylindrical, Plexiglas container that doesn't actually turn. Instead, a plate attached to a motor-driven shaft spins at up to 7 revolutions per second inside the container, while the vessel itself remains still.

As expected, in experiments with water or with viscous ethylene glycol, the spinning platter swirled the liquid above it to create whirlpools. But the throats of those whirlpools tapered to surprising shapes at the platter's surface, the team reports in the May 5 Physical Review Letters.

In the water experiments, those shapes transformed as speed increased, changing from circular to elliptical to propeller-shaped to square to pentagonal and finally to hexagonal. Ethylene glycol whirlpools formed shapes with no more than three sides.

Curiously, the polygons themselves rotated, although more slowly than their parent whirlpools.

Rotating fluids play important roles in systems ranging from industrial equipment, such as pumps, to atmospheric disturbances, such as tornadoes and hurricanes. Although the newfound shapes remain unexplained, Bohr says that their discovery may eventually lead scientists to a deeper understanding of fluids' rotational behaviors.

--P.W.