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Splashdown. A drop of ethanol hits a smooth glass at atmospheric pressure (above) and 1/5 atmospheric pressure (below).

Sucking Away the Splatter

LOS ANGELES--Nature may abhor a vacuum, but a vacuum abhors a mess. In the absence of air, a droplet of liquid can crash into a smooth surface without splattering, physicists report. The odd phenomenon might be useful for controlling droplet formation in technological processes like inkjet printing.

It seems obvious and inevitable that a fast-moving droplet will splatter when it hits a hard surface. Researchers have studied the distribution of droplet sizes and energies in such splashes, and physicists Lei Xu, Sidney Nagel, and colleagues at the University of Chicago were searching for ways to control those sizes and energies when they discovered something unexpected: By pumping away some of the surrounding air they could eliminate the splatter entirely.

Within a tall vacuum chamber, the researchers released droplets of alcohol onto a dry glass plate from heights ranging from 20 centimeters to 3 meters. They recorded the resulting splashes with a high speed video camera as they varied the pressure in their apparatus, sucking it down as low as one hundredth of atmospheric pressure. The droplets struck the surface with speeds ranging from 2 to 7 meters per second, and for a given speed, the researchers found they could eliminate the splash by lowering the pressure beyond a specific threshold.

The team explains the results with a simple theory. As a drop strikes a surface, liquid begins to spread sideways at supersonic speed, creating a shockwave. The shockwave pushes back on the liquid, and if that force is greater than the internal forces holding the drop together, the shockwave will lift the liquid off the surface and create a splash. Reducing the pressure reduces the force exerted by the shock wave.

Ironically, the theory predicts that a thicker liquid should splash more than a thinner one. The researchers tested this curious prediction by studying the splash made by three types of alcohol with different viscosities. Indeed, the more viscous the alcohol, the lower the pressure needed to prevent splashing, the team reported here this week at a meeting of the American Physical Society.

"It's not uncommon to see a lovely phenomenon, but it is uncommon to get all the factors straight," says Walter Goldburg, an experimenter at the University of Pittsburgh in Pennsylvania. Bulbul Chakraborty, a theoretical physicist at Brandeis University in Waltham, Massachusetts, says the researchers' analysis opens the way to controlling splashing in, for example, spray coating surfaces with various substances.

Related sites
See the splash
The preprint of the paper