If you've ever managed to try Cheerios, you may have noticed how those last few Cheerios in the cereal bowl seem to cluster together in the centre and along the edges. It's called the "Cheerios effect." Now an international team of physicists has discovered a reverse Cheerios effect. They described their results in a new paper in the Proceedings of the National Academy of Sciences.
The Cheerios effect may not be an especially exotic phenomenon — we also see it in pollen floating atop a pond, and the foamy heads of beer — but the actual physical mechanisms at work weren't clearly outlined until a 2005 paper in the American Journal of Physics. The culprits: buoyancy, surface tension, and something called the "meniscus effect."
Buoyancy is what determines whether something will sink or float, while surface tension is a property arising from water molecules pulling on one another in a dance of mutual attraction. The liquids essentially cling to each other so tightly that they form a kind of skin over the top of the liquid.
The meniscus effect is what happens when you place a single Cheerio in a bowl of milk. Its mass will form a dent in the milk's surface. Place a second Cheerio in the bowl, and it will do the same. If the two Cheerios are close enough together, they will drift toward each other. It's a microcosm of general relativity, whereby the mass of the Sun warps the fabric of spacetime, pulling the planets into orbit around it. Place yet another Cheerio near the edge of the bowl, and it will follow the curve of that meniscus, looking for all the world like it's clinging to the edge of the bowl.
And now there's a way to get an inverted Cheerios effect, by swapping the roles of Cheerio and liquid. This latest work explores what happens when you have liquid drops resting on a soft solid surface. Even better, the physicists discovered they could actually control how those liquid drops clustered together across that surface.
"The droplets deform the surface on which they live, and due to this deformation, they interact — somewhat reminiscent of general relativity, from which we know that galaxies or black holes interact by deforming space around them," co-author Stefan Karpitschka, now at Stanford University, said in a statement. "What is remarkable about our case though is the fact that the direction of the interaction can be tuned by the medium, without modifying the particles themselves."
The original Cheerios effect led to advanced materials and insight into how galaxies collapse via gravity. Its inverse also has lots of potential applications, according to co-author Lorenzo Botto of Queen Mary University of London. "[T]he physical phenomena we have highlighted in this paper suggest ways to design surfaces that prevent fogging or control heat transfer," he said. This would make it possible to "create car windows that are always transparent despite high humidity or surfaces that improve heat management in conditioners or boilers."