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March 22, 2005 Press Contact: Steve Koppes
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Revealing a secret in plain sight: air makes liquids go splash

In the top frame, the drop splashes at atmospheric pressure. In the bottom frame, at a much lower pressure, there is no splashing and no apparent undulations in the rim. (Requires RealVideo; also available in AVI format.)
Still images
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.
Click to enlarge.
Photos courtesy of Lei Xu, University of Chicago

University of Chicago physicists have learned how to eliminate what scientists formerly regarded as the inevitable splashing that occurs after a liquid crashes onto a flat surface. It turns out that the removal of air eliminates the splash process.

It was a case of not being able to see something because it was hidden in plain sight, like “The Purloined Letter” of Edgar Allan Poe, said Wendy Zhang, Assistant Professor in Physics at the University of Chicago. “Things that are in front of you are hard to see because they’re around you all the time,” Zhang said.

Lei Xu (last name pronounced zhoo), a graduate student in Physics at the University of Chicago, will report his experimental findings on the physics of splashing March 22 at a meeting of the American Physical Society in Los Angeles. Xu’s co-authors are Zhang and Sidney Nagel, the Stein-Freiler Distinguished Service Professor in Physics at the University of Chicago. The discovery could lead to applications in technological situations where splashing occurs, including the combustion of liquid fuels, ink-jet printing and industrial washing.

“In an engine you break the gasoline into millions of pieces and then ignite them in a chamber, making a controlled explosion. You do that continuously in your car,” Xu said. A higher gas pressure might do a better job of breaking the fuel into smaller, more uniform pieces. But determining that would require further experiments more accurately simulating the splash process as it occurs under fuel-combustion conditions, he said.

Xu’s path to discovery began with an experiment designed to study energy distribution in the many pieces that fly away from a breaking droplet. The experiment showed that after a droplet hits a flat, dry surface, it forms a thin layer that breaks into thousands of pieces. Xu then repeated the experiment in a vacuum chamber and lowered the air pressure. He and Nagel figured that the drag of the surrounding air might play some role in the break-up process.

“Once we lowered the pressure and did the experiment, it didn’t have any splash,” Xu said. “So then we thought, well, this is much more interesting than the energy distribution. How about we investigate the role of air in this splash process?”

Researchers in the field previously had seen no reason for low atmospheric pressure to affect the results of their splash experiments. “There are many people working on this because it’s a very important technological area,” Zhang said. “Up until Lei’s experiment, the idea in the field of splashing is that of course the air doesn’t do anything, because it’s less dense and less viscous than water,” she said.

Instead, splash researchers generally focused on the interaction between the spreading droplet and the surface against which it collided. Their carefully controlled experiments produced reams of data but little understanding of the fundamental mechanism for splash formation.

“One of the things that is notorious in the splash-study community is that it’s very difficult to reproduce somebody else’s experiment. People have been working on this for more than a hundred years, and it is only recently that we understand enough about the different factors that affect the splash process to be able to reproduce results,” Zhang said.

The Chicago physicists isolated the effects of air pressure on splashing through a painstaking series of experiments involving four different gases and three different liquids. The gases selected—helium, air, krypton and sulfur hexafluoride—ranged from light to heavy. The fluids—methanol, ethanol and 2-propanol—have low surface tension, the force that makes a layer of liquid want to retract and form a droplet.

Xu tested water splash as well. Water exhibits the same behavior, but its higher surface tension narrows the range of splash-forming impact velocity and creates a much larger margin for experimental error.

“It’s much harder to splash than ethanol,” he said.

Xu found that he could precisely control the amount of splash by tuning the pressure. The next step, he said, might be to investigate splash outcomes under experimental scenarios involving very large or very small drops, or drops traveling at higher velocities.

Meanwhile, Nagel and his associates have completed another chapter in their ongoing research program that examines the surprising physics of everyday phenomena.

“If you look at anything in nature a little more carefully, it yields new insights and new phenomena. It’s inexhaustible. You can never say you’re done,” Nagel said.
Last modified at 11:32 AM CST on Thursday, April 07, 2005.

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