Two years ago, the explosion of the Deepwater Horizon oil rig covered hundreds of square miles of the Gulf of Mexico with oil (main image). The oil floated because it is less dense, and therefore lighter, than water. But now scientists say that water can sometimes float on oil—and their findings, which were published last month in Langmuir, could help to mop up oil slicks like the one created by the 2010 disaster. Using a theoretical model, the scientists calculated the forces acting on water when it is dripped onto an oil surface. Taking into account surface tension, the property that allows some insects to walk on water, they showed that a water droplet can "hang" from the oil's surface. The oil surface droops like a rubber membrane, allowing the above air—which is much lighter than oil and water—to extend beneath the surface's average level. With help from the surface tension, this air pocket balances the weight of the water droplet, preventing it from sinking. The scientists confirmed the model's predictions in tests and found that water droplets up to 170 microliters in volume could float on oil (inset image and video). Such water droplets would be large enough to harbor natural microbes that consume oil. If they were sprayed over an oil slick, the scientists say, the cleanup process could be much faster.
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