One of nature’s loveliest creations is the splash ring, or crown, that forms milliseconds after a drop of fluid strikes a surface. University of Chicago physics graduate student Lei Xu recently figured out what creates this form and splashes in general: air.
Physicists have been investigating the mechanics of splashing for more than a century, but until Xu came along no one had thought to look at what impact air might have. To find out, Xu and his adviser Sidney Nagel along with colleague Wendy Zhang used high-speed photography to study the splash of ethanol drops as they landed on a glass plate inside a vacuum chamber. The pressure of air inside the chamber was steadily lowered. At one-fifth to one-sixth atmospheric pressure, the drops still plopped but without a hint of a splash. They also tried the experiment using atmospheres of different gases, including krypton and helium. The results were consistent.
Xu and his colleagues have worked out why. Normally, as a drop hits the surface, it expands rapidly outward and pushes on the surrounding air. The air pushes back with a force of its own. As it does, it breaks the surface tension that binds together molecules in the drop, shattering it. Take away the air, Xu says, and you also take away that force, resulting in a splashless impact. Removing splashes, he says, could improve the quality of ink-jet printing.