A striking example of levitation is encountered in the “kugel fountain” where a granite sphere, sometimes weighing over a ton, is kept aloft by a thin film of flowing water. In this paper, we explain the working principle behind this levitation. We show that the fountain can be viewed as a giant ball bearing and thus forms a prime example of lubrication theory. It is demonstrated how the viscosity and flow rate of the fluid determine (i) the remarkably small thickness of the film supporting the sphere and (ii) the surprisingly long time it takes for rotations to damp out. The theoretical results compare well with measurements on a fountain holding a granite sphere of one meter in diameter. We close by discussing several related cases of levitation by lubrication.

Topics
Friction, Thermodynamic states and processes, Minerals, Chemical elements, Educational assessment, Fluid dynamics, Lubrication flows, Buoyancy, Awards and Prizes, Physicists
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See the information on various websites, for instance, that of Clute's Kugel in the State Botanical Garden of Georgia in Athens, Georgia at <http://clutebarrow.org/kugel.html>.
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From the brochure by Kusser Aicha Granitwerke on Floating sphere and floating object fountains, see <http://www.kusser.com/>.
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See <http://midwarks.info/kenglobe/> for a highly readable report on the granite sphere fountain (also known as the “groovy ball project”) in Kenilworth, UK.
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See the entry Kugel ball at <http://en.wikipedia.org/>, including the frequently-asked-questions section.
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Due to the extremely small thickness of the fluid layer, the water has hardly any freedom to explore the normal direction once it has left the nozzle region and is immediately forced into the parallel direction. As noted in Figs. 4 and 7, the size of the nozzle region covers only about 10% of the total range of θ. For the spherical fountain this constitutes an area of ∼(0.1)2∼0.01 of the total immersed area, making the contribution of the region that is not treated by our analysis of the order of 1%.
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© 2014 American Association of Physics Teachers.
2014
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