Rotating Fluid Polygons

by Tomas Bohr

Most of us know what happens, when a bucket of water is rotated around its central axis: if the rotation is even, the water will slowly come to rest in the rotating frame, and the only effect of the rotation is that the surface will be curved into the shape of a paraboloid. Newton took this as an argument for the existence of “absolute space” relative to which his “inertial frames” should be defined. The rotating system is not an inertial frame, since the “fictitious” centrifugal force acts there.

Now, what happens if only the bottom rotates? Recently, a team at the Physics Department at the Technical University of Denmark performed such an experiment. A cylindrical container, where the bottom plate can turn, is filled with water up to a certain height, and a motor sets the bottom plate rotating. Right away, the situation looks like what we are used to: the surface starts to bend since the water is forced out against the sides. Surprisingly enough, however, the surface can spontaneously deform, break the axial symmetry and create shapes that are most aptly described as “polygons”. They are stationary in a rotating frame - not, however, one that rotates with the speed of the bottom, but considerably slower.

The team consisted of three bachelor-students from the University of Copenhagen - Thomas Jansson, Kåre Hartvig Jensen and Martin Haspang - a visitor from Paris - Pascal Hersen - and Tomas Bohr from the Physics Department. A paper describing the experiment has recently appeared in Physical Review Letters (96, 174502 (2006), erratum 98, 049901 (2007) and has given rise to many comments and descriptions on e.g. Nature's homepage, Science News, Scientific American and the Danish Ingeniøren.

Currently there is no theory for this experiment, so it is a challenge for all those who believe that Fluid Dynamics is a closed, well-understood part of physics.  

For more pictures look here 

Links, comments and decriptions:
http://physorg.com/news66924222.html
http://www.physicsweb.org/articles/news/10/5/8/1
http://www.zpenergy.com/modules.php?name=News&file=article&sid=1893