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See Inside August 2006

Platonic Liquids

Isaac Newton spun a bucket of water like a top and observed a round vortex. Now Danish researchers have squared that circle by rotating just the bottom of a water-filled bucket. As they dialed up the speed to a few spins per second, the vortex adopted a trefoil shape, then became square, pentagonal and hexagonal. The sluggish outer layer of water may be amplifying small variations in the radius of the vortex, creating corners, says bucket spinner Tomas Bohr of the Technical University of Denmark (and grandson of Niels), whose group reports in the May 5 Physical Review Letters. Storms are similar in principle to a semistationary bucket, which may explain structured hurricane eyes, and the underlying principle might apply to bathtub drains, too, Bohr says. “I've never seen a triangular bathtub vortex,” he adds, “but who knows?”

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