Oil and Water Don't Mix
Alan Turing, who is arguably the world's first computer scientist , was interested not only in artificial automata, but in autonomous natural processes also. He proposed what is called the Turing hypothesis of pattern formation in nature to explain such things as a zebra's stripes and a leopard's spots. The hypothesis is built on the physical principles of reaction and diffusion, and it explains many natural patterns.
One such pattern occurs in a surface reaction of a mixture of oil and detergent on the surface of water that evolves through time in a peculiar way. "Spreading oil on the waters" as a way to calm the surface was studied by Benjamin Franklin as early as 1774. His account, published in the Transaction of the American Philosophical Society, recalls how he had observed spear fishermen putting oil on the water's surface to dampen small surface waves to make it easier to see the fish. An interesting experiment involving oil on water can be performed by mixing a little detergent with mineral oil, and then placing a drop of the mixture onto water. The oil layer will expand and contract in a regular fashion, and it will eventually reach a static state. What is curious about this reaction, and a key to the mechanism, is that the expansion and contraction will stop if the water vessel is capped.
A group of scientists from the Massachusetts Institute of Technology have studied this reaction and their results will appear in a future issue of the Journal of Fluid Mechanics . The actor in this effect is changes in surface tension mediated by evaporation. The detergent, a surfactant, reduces surface tension, and it would rather be at the interface between the water and oil, rather than in solution with the oil. As the detergent diffuses to the interface, the decreased surface tension causes the oil droplet to expand. Since the center of the oil droplet is thicker than the edges, there's a differential surface tension (a shear) that causes the generation of waves from the center to the edge of the droplet. This causes the detergent to be expelled from the droplet, so the surface tension of the surrounding water is reduced, and the oil droplet contracts. Evaporation of the surfactant from the water is needed to reset the reaction, so preventing evaporation by capping the vessel will stop the cycle.
1. OK, there's Babbage and Ada Lovelace, but that's why I say, "arguably."
2. Denise Brehm, "Case closed: MIT gumshoes solve "throbbing" oil mystery" (MIT Press Release, July 17, 2007).