Hexagons in a Bucket

Following on from my last post on the mysterious hexagon at Saturn's north pole, the Scientific American blog points out that the only real explanation offered so far, comes from:

Thomas Bohr (grandson of Niels), who last year produced geometric whirlpools by spinning the bottom of a water-filled bucket while the sides remain stationary.

From Nature:

Researchers at the Technical University of Denmark in Lyngby have created similar geometric shapes (holes in the form of stars, squares, pentagons and hexagons) in whirlpools of water in a cylindrical bucket1. The shapes appear easily enough once the bucket is spinning at a rate of one to seven revolutions per second, they say.

Tomas Bohr and colleagues made plexiglass buckets, 13 and 20 centimetres across, with metal bottoms that could be rotated at high speed by a motor. They filled the bucket with water and spun the bottom to whip up the liquid into a whirlpool that rose up the sides of the container.

This set-up is very similar to the rotating bucket that Isaac Newton used in the seventeenth century to investigate centrifugal forces.

The researchers found that once the plate was spinning so fast that the water span out to the sides, creating a hole of air in the middle, the dry patch wasn't circular as might be expected. Instead it evolved, as the bucket's spin sped up, from an ellipse to a three-sided star, to a square, a pentagon, and, at the highest speeds investigated, a hexagon.

And in the end, the hexagon in space turns out to not be that interesting at all. Go figure!