Credit: NASA; (inset) Chi M. Phan 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....