Dial-a-Splash: Thin air quells liquid splatter

In a classic image of high-speed photography, a drop of milk landing on a surface explodes into an ornate crown with beads of fluid leaping from its rim. Now, a study of other splashes finds that the air in which such bursts unfold is a previously overlooked actor in that performance.

AFTER THE FALL. In air at normal pressure (left), an ethanol drop spatters into a delicate and elaborate crown upon striking a glass slide. At 17 percent of ordinary pressure (right), the drop makes no splash. Xu

“I don’t think anyone ever thought poor little old air could do anything to the splash,” says physicist Sidney R. Nagel, who led the investigation. Yet he, Lei Xu, and Wendy W. Zhang, all of the University of Chicago, have discovered that even modestly reducing the air pressure completely quells the rococo exuberance of crashing drops.

“Flabbergasting,” comments Detlef Lohse of the University of Twente in Enschede, the Netherlands.

Nagel, Xu, and Zhang made their unexpected observations while releasing alcohol drops onto glass slides in a sealed chamber with adjustable air pressure. To monitor the impacts, the team filmed them at 47,000 video frames per second.

The experimenters also found that replacing air with lighter gases, such as helium, diminished splashing and that heavier gases, such as krypton, enhanced it.

To explain their results, the Chicago researchers posit that the leading edge of a fallen, squashed drop rushes outward, compressing a thin layer of gas next to the glass surface. At normal pressure, as the gas resists this compression, it forces up the film’s edge, which then breaks up. The result: a splash.

But at reduced pressures—or if the gas is light—the compressed gas resists less vigorously, generating a weaker splash or none at all. The splash is “very tunable,” Xu says.

The team details its findings in a report on the Internet site called ArXiv, where physicists often post new results (http://www.arxiv.org/abs/physics/0501149).

Splashing occurs, for better or worse, in many industrial processes, such as fuel combustion, ink-jet printing, and the coating and washing of various products. The Chicago findings could have practical importance, comments David Quéré of Collège de France in Paris, because they have revealed “a new and very efficient way to prevent the splash.”

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