Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

MIT gumshoes solve "throbbing" oil mystery

18.07.2007
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.

Written by Denise Brehm
MIT Department of Civil and Environmental Engineering

Elizabeth A. Thomson | MIT News Office
Further information:
http://www.mit.edu

More articles from Ecology, The Environment and Conservation:

nachricht Preservation of floodplains is flood protection
27.09.2017 | Technische Universität München

nachricht Conservationists are sounding the alarm: parrots much more threatened than assumed
15.09.2017 | Justus-Liebig-Universität Gießen

All articles from Ecology, The Environment and Conservation >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Neutron star merger directly observed for the first time

University of Maryland researchers contribute to historic detection of gravitational waves and light created by event

On August 17, 2017, at 12:41:04 UTC, scientists made the first direct observation of a merger between two neutron stars--the dense, collapsed cores that remain...

Im Focus: Breaking: the first light from two neutron stars merging

Seven new papers describe the first-ever detection of light from a gravitational wave source. The event, caused by two neutron stars colliding and merging together, was dubbed GW170817 because it sent ripples through space-time that reached Earth on 2017 August 17. Around the world, hundreds of excited astronomers mobilized quickly and were able to observe the event using numerous telescopes, providing a wealth of new data.

Previous detections of gravitational waves have all involved the merger of two black holes, a feat that won the 2017 Nobel Prize in Physics earlier this month....

Im Focus: Smart sensors for efficient processes

Material defects in end products can quickly result in failures in many areas of industry, and have a massive impact on the safe use of their products. This is why, in the field of quality assurance, intelligent, nondestructive sensor systems play a key role. They allow testing components and parts in a rapid and cost-efficient manner without destroying the actual product or changing its surface. Experts from the Fraunhofer IZFP in Saarbrücken will be presenting two exhibits at the Blechexpo in Stuttgart from 7–10 November 2017 that allow fast, reliable, and automated characterization of materials and detection of defects (Hall 5, Booth 5306).

When quality testing uses time-consuming destructive test methods, it can result in enormous costs due to damaging or destroying the products. And given that...

Im Focus: Cold molecules on collision course

Using a new cooling technique MPQ scientists succeed at observing collisions in a dense beam of cold and slow dipolar molecules.

How do chemical reactions proceed at extremely low temperatures? The answer requires the investigation of molecular samples that are cold, dense, and slow at...

Im Focus: Shrinking the proton again!

Scientists from the Max Planck Institute of Quantum Optics, using high precision laser spectroscopy of atomic hydrogen, confirm the surprisingly small value of the proton radius determined from muonic hydrogen.

It was one of the breakthroughs of the year 2010: Laser spectroscopy of muonic hydrogen resulted in a value for the proton charge radius that was significantly...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ASEAN Member States discuss the future role of renewable energy

17.10.2017 | Event News

World Health Summit 2017: International experts set the course for the future of Global Health

10.10.2017 | Event News

Climate Engineering Conference 2017 Opens in Berlin

10.10.2017 | Event News

 
Latest News

Terahertz spectroscopy goes nano

20.10.2017 | Information Technology

Strange but true: Turning a material upside down can sometimes make it softer

20.10.2017 | Materials Sciences

NRL clarifies valley polarization for electronic and optoelectronic technologies

20.10.2017 | Interdisciplinary Research

VideoLinks
B2B-VideoLinks
More VideoLinks >>>