[Editors] MIT gumshoes solve "throbbing" oil mystery

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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Case closed: MIT gumshoes solve "throbbing" oil mystery
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For Immediate Release
TUESDAY, JULY 17, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO, VIDEO AVAILABLE

STORY ONLINE AT: http://web.mit.edu/newsoffice/2007/cool-science-0717.html

CAMBRIDGE, Mass.--Hey kids! Try this at home. Pour clean water onto a 
small plate. Wait for all the ripples to stop. Then mix a small 
amount of mineral oil with an even smaller amount of detergent. 
Squeeze a tiny drop of that mixture onto the water and watch in 
amazement as the oil appears to pump like a beating heart.

It's a simple experiment, but explaining what makes the drop of oil 
throb-and then stop when deprived of fresh air-has long mystified the 
scientific community. Now, in work that could have applications in 
fields from biology to environmental engineering, an MIT team has 
cracked the case.

In the July 25 issue of the Journal of Fluid Mechanics, MIT 
Professors Roman Stocker of civil and environmental engineering and 
John Bush of mathematics explain what happens when an oil drop 
containing a water-insoluble surfactant (or material that reduces the 
surface tension of a liquid, allowing easier spreading) is placed on 
a water surface.

"It's an easy experiment to make. But getting the theory for it was 
not straightforward," Bush said. "Roman turned a microscope loose on 
the problem-which was key to finally understanding it."

The question of the physical phenomenon of oil spreading on a surface 
has been around for some time. Benjamin Franklin wrote about it in 
1774 in the Transaction of the American Philosophical Society, after 
he saw Bermuda spear fishermen use oil to damp waves so they could 
more easily see fish under the ocean surface.

The question Stocker and Bush examined had another dimension: why oil 
with an added surfactant doesn't come to rest, but instead contracts 
and repeats the process in a periodic fashion.

The mechanism, they now know, is surface tension, or more precisely, 
evaporation-induced variations in surface tension. These changes in 
surface tension cause the drop to expand, then contract, and repeat 
the process every couple of seconds until it runs out of gas, which 
in this case, is surfactant. Covering the experiment stops the 
process because it prevents evaporation of the surfactant.

"We're dealing with three interfaces: between the oil drop, the water 
in the Petri dish, and the air above it," Stocker said, explaining 
surface tension. "A detergent is a surfactant, which reduces the 
surface tension of a liquid. The detergent molecules we added to the 
oil drop prefer to stay at the interface of the oil and water, rather 
than inside the oil drop."

Think of the oil detergent drop as a small lens with a rounded 
bottom. The surfactant in the drop moves to the bottom surface of the 
lens, where it interacts with the water to decrease the surface 
tension where oil meets water. This change in tension increases the 
forces pulling on the outer edges of the drop, causing the drop to 
expand.

The center of the drop is deeper than the edges, so more surfactant 
settles there, reducing the surface tension correspondingly. This 
causes the oil and surfactant near the outer edges of the drop to 
circulate. This circulation creates a shear (think of it as two 
velocities going in opposite directions), which generates very tiny 
waves rolling outward toward the edge. When these waves reach the 
edge, they cause small droplets to erupt and escape onto the water 
surface outside the drop. Videomicroscopy - essentially, attaching a 
video camera to a microscope - was critical in observing this step in 
the process. Those droplets of oil and surfactant disperse on the 
water and decrease the surface tension of the water surface, so the 
drop contracts.

As the surfactant evaporates, the surface tension of the water 
increases again, and the system is reset. Forces pull at the outer 
edges of the lens, and the cyclical process begins again.

But the beating ceases instantly when Stocker and Bush put a lid over 
it. If the surfactant can't evaporate, the oil drop remains stable. 
In the end, it was being able to stop the beating process that made 
it clear to the researchers that evaporation played a central role in 
the mechanism.

"This is a bizarre and subtle mechanism. Everybody was flummoxed," 
said Bush, whose recent research includes understanding how some 
insects walk on water.

He first heard about the oil drop phenomenon from Professor Emeritus 
Harvey Greenspan of mathematics, who had pondered it for some time. 
Bush in turn talked to Stocker, who was then an instructor in the 
Department of Mathematics. It took about three years of sporadic work 
(without funding), and the help of two undergraduate students who 
carried out the lab repetitions-Margaret Avener and Wesley Koo-but 
Stocker and Bush finally solved it.

To what end, the researchers don't yet know. "One rationalizes the 
physical world by understanding the mechanisms," said Bush, 
explaining the importance of basic scientific research. "One can 
never predict which mechanisms will be important."

"Oil contamination of water resources is a prominent problem in 
environmental engineering," said Stocker. "Awareness of the 
fundamental mechanisms governing the interaction between the two 
phases is critical to devise sound engineering solutions for 
remediation."

Spontaneous oscillations are observed in many natural systems, 
including nerve cells, muscle tissue, and the biological clocks 
responsible for circadian rhythms, the professors said. And previous 
work published on the oil drop problem had been carried out by 
scientists interested in seeing if the mechanism could explain 
biological oscillations.

--MIT--

Written by  Denise Brehm
MIT Department of Civil and Environmental Engineering