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No. 119: Sep-Oct 1998

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Curious Effects Department

The spinning-egg water-sprinkler. It is generally known that you can separate fresh eggs from hard-boiled eggs by spinning them like a top. If they are hard-boiled they will spin on one of the ends. The fluid contents of fresh eggs, however, will slosh around and prevent top action. O.K.! this is not very curious, so we'll place a hard-boiled egg in a flat pan containing a thin layer of water and give it a spin. Not only does the egg spin on end but a layer of water creeps up the side of the egg. When the water is about half way up the side of the egg it breaks up into droplets and sprays out horizontally like a rotating lawn sprinkler.

No mysterious forces are involved, nor are there spooky quantum mechanics effects. The major forces operating are gravity, centrifugal force, and adhesion between the egg surface and the water. As the film of water creeps up the egg, the centrifugal force increases and overcomes the force of adhesion. Then, water droplets spray outward.

(Gutierrez, Gustavo, et al; "Fluid Flow up the Wall of a Spinning Egg," American Journal of Physics, 66:442, 1998.)

Creating fluid corners in kitchen sinks. When a smooth column of water from your kitchen faucet hits the sink, it flows out radially. At a calculable radius, its height suddenly rises. This smooth, circular ridge is called a "hydraulic jump." Here, some of the kinetic energy of the falling water is converted into the potential energy of the deeper layer of water. Nothing particularly mysterious here.

But, if a liquid more viscous than water is used, the circular ridge is transformed into a neat polygon with surprisingly sharp corners. Different flow rates create different polygons. Polygons with as many as 14 corners have been observed. Interestingly, identical flow rates can result in different stable polygons. See the referenced article for all the math.

(Ellegaard, Clive, et al; "Creating Corners in Kitchen Sinks," Nature, 392:767, 1998.)

Spinning hard-boiled egg water sprinkler A spinning hard-boiled egg creates a water sprinkler.

From Science Frontiers #119, SEP-OCT 1998. 1998-2000 William R. Corliss