The Surprising New Science of Splashes
Photographs of a drop of alcohol hitting a smooth, dry, glass surface. Each row shows the drop at four times. The first frame shows the drop just as it is about to hit the surface. The next three frames in each row show the evolution of the drop at .276 milliseconds, .552 milliseconds and 2.484 milliseconds after impact. In the top row, the drop splashes at atmospheric pressure (100 kilopascals). In the second row, under a lower air pressure, the drop emits only a few droplets. In the third row, at an even lower pressure, no droplets are emitted and no splashing occurs, although the thickness of the rim undulates. In the fourth row, at the lowest pressure, there is no splashing and no apparent undulations in the rim. These photographs were taken with a digital camera that can snap 47,000 images per second.
Credit: Lei Xu, University of Chicago.

One raindrop splattering on the sidewalk might seem like any other. But a new study reveals the amount of splash depends on surrounding air pressure.

Researchers have studied splashing for over a hundred years without noticing this link.

Lei Xu, a graduate student at the University of Chicago, made the surprising discovery while taking high-speed pictures of drops falling in a vacuum chamber. With less atmospheric pressure, the drops squish down without fragmenting into tinier droplets.

"What we discovered is that you can decrease the splashing by sucking out some of the air from the chamber," said Xu's adviser Sidney Nagel.

"Typically, what we believed was that air is so low in density and viscosity, how can it matter?" Nagel told LiveScience. "Not only does it matter, it plays a dominant role."

A liquid crown

When a drop hits a dry, flat surface, it flattens out into a sheet, or film, of liquid. Under normal pressures and at sufficient impact velocity, tiny droplets form on the rim of the sheet, like the gems of a crown.

By lowering the atmospheric pressure, the droplets can be made to disappear, while the sheet expands along the surface more smoothly. The simplest explanation, according to Nagel, is that the air resists the expansion, causing the film to destabilize and break apart into tiny pieces.

"But if there's no air, there's nothing to keep [the film] from spreading," Nagel said.

Nagel, Xu and co-author Wendy Zhang found they could tune the amount of splashing by varying the pressure, as well as the types of liquids and gases.

Besides normal air, they experimented with lighter gas like helium and heavier gases such as krypton and sulfur hexafluoride. For liquids, they employed the alcohols methanol, ethanol and 2-propanol, all of which have low surface tension - the force that resists changes to a liquid's shape.

Water has such a high surface tension that its drops need to be traveling faster than alcohol drops to induce a splash. This made water impractical for the experimental setup.

In the future, the researchers plan to explore drop speed and size, as well as surface roughness and liquid viscosity.

"We'll follow where the science leads us," Nagel said.

Don't spill

Since liquids are all around us, the findings are relevant to a host of different applications. Imagine eliminating the splash of an inkjet printer, for example.

"Whenever you deal with sloshing, splashing, or dripping, you want to control that process," Nagel said.

It may not be necessary to use vacuums to make the liquids behave. The researchers found that a lighter gas, like helium, offers less resistance, and therefore produces less splashing.

Xu presented these findings at last month's American Physical Society meeting. Movies depicting the splashes can be found here.