The "Cheerios" Effect

The tendency for certain floating things to clump under the action of surface tension---things such as Cheerios cereal bits in your breakfast bowl, bubbles in a glass of beer, pepper flakes on water, even strands of hair up against a washbasin---has important potential engineering implications, such as for the design of self-assembling circuits and devices.

Study of the clumping phenomenon has a long history. For example, an excellent summary was prepared by no less than James Clerk Maxwell for the Encyclopedia Britannica as long ago as 1875. Now a Harvard professor, Lakshminarayanan Mahadevan, and an undergraduate student, Dominic Vella (now a graduate student at Cambridge University), have taken up the subject and written a pedagogical review, hoping to rescue the subject from the obscuring algebraic complexity that has settled around it (as Mahadevan argues) and concentrate on the pertinent relatively simple physics principles. They emphasize that contrary to general belief, chemical interactions are oftentimes not paramount in determining whether clumping occurs; instead a simple equilibrium of forces and torques---including things such as buoyancy and surface tension---are the deciding factors.

Even objects denser than water can float if the geometry is right: See this picture of a floating thumbtack. Even more interestingly, one can control the strength and sign of this interaction; indeed, there are indications that insects that live on the air-water interface might even use this effect to great advantage.

Vella and Mahadevan, American Journal of Physics, September 2005
Contact: lm@deas.harvard.edu
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