- The Washington Times - Thursday, January 3, 2008

Revelers opening bottles of champagne to ring in the new year unwittingly got a glimpse of the beach. The bubbles that rise up and swirl around a champagne flute are similar in one way to the bubbles created by breaking waves.

A graduate research team at the University of Maryland in College Park is studying bubble phenomena in breaking waves. Although they are not considering champagne bubbles, the behavior of the bubbly imbues their experiment, which is in its sixth year, with a holiday interpretation.

“You need to have patience when you do such an experiment. It takes a long time to get results,” says Mostafa Shakeri, a postdoctoral student with a doctorate in mechanical engineering.

Bubbles are pressurized, thin-walled spheres of liquid containing air or gas that are ready to collapse. The bubbles in champagne and some of those from breaking waves contain carbon dioxide.

In the case of champagne, popping the cork lowers the pressure in the bottle, causing the carbon dioxide in the liquid to no longer be in equilibrium, says James H. Duncan, professor of mechanical engineering at the university.

“It can be at a certain pressure and temperature, but if it’s less, then the carbon dioxide has to come out. It’s called saturation. There’s a saturation quantity in liquid for each temperature and pressure,” says Mr. Duncan, who holds a doctorate in geophysical fluid dynamics.

The carbon dioxide is released from the liquid in the form of bubbles, Mr. Duncan says. Bubbles also form when champagne is poured into a flute where there are nucleation sites, such as imperfections in the glass and dirt, dust and fiber particles, he says.

Alternatively, bubbles occur in ocean waves when the surface of water is folded over, such as by a breaking wave, Mr. Duncan says. As the wave curls over, it traps air, which consists of nitrogen, oxygen and other gases, including carbon dioxide, he says.

The breaking wave forms a large cavity, or tube of air, that is crushed as the wave falls, says Dale Stokes, research oceanographer for the Scripps Institution of Oceanography, a center for marine-science research at the University of California in San Diego.

“This giant tube is crushed, or broken apart, into smaller tubes. It keeps getting crushed, or torn apart, by the turbulent water,” says Mr. Stokes, who holds a doctorate in oceanography. “It gets smaller until it forms the cloud of bubbles that you can see behind a breaking wave.

“When a wave overturns, there’s a lot of energy. Part of that energy is expended into turning the large air cavity into smaller and smaller bubbles,” he says.

As the number of bubbles increases, what is known as the air-water interface, or amount of surface area between the air and water, also increases, Mr. Duncan says.

“A wave comes along and engulfs pockets of air and breaks them into tiny bubbles, increasing the surface area between the air and water, therefore increasing the transfer rate of carbon dioxide and other things in the air into the ocean,” he says.

The action of the wave breaking and bubbles forming is over within a second, but the bubbles can linger for several seconds as they continue to break, Mr. Stokes says. Bubbles pushed into the water column rise back up to the sea surface through buoyant force and are tiny and fizzy, he says.

“Most of the bubbles escape at the surface. Only the very smallest ones are carried farther down into the water and dissolve,” Mr. Stokes says.

Mr. Shakeri and Mohammadreza Tavakolinejad, a University of Maryland graduate student in mechanical engineering, are measuring breaking waves produced by ships and the bubbles that result. Their research can be used to help explain ocean phenomena and for environmental studies, pollution models and biological research on water oxygenation, among other things, Mr. Duncan says.

“When a ship moves, it creates a wave,” Mr. Shakeri says. “When a wave breaks, it causes some energy to get lost. … That energy is supplied through the motor of a ship. Eventually, we hope to create an optimal ship hull so the energy loss will be minimized.”

Almost 40 percent of wave energy is lost at breaking, and up to 50 percent is expended from entrainment, or the entry of air into water, Mr. Tavakolinejad says.

“When a wave breaks, it traps a lot of air. That means a huge amount of energy of the wave is dissipated in the process,” he says. “If not, it would be damaging.”

Mr. Tavakolinejad and Mr. Shakeri are calculating the size distribution of bubbles made by the bow of a ship, using a wave tank on campus that produces wind and ship waves, Mr. Duncan says. They are studying the relationship between bubbles — their number, size and velocity — and the waves that produce them, he says.

The two are gathering their information using a laser beam to provide a bright enough light source to depict the bubbles and shadow graph images to mark the shadows of the bubbles, Mr. Duncan says. The shadows are recorded by two high-speed digital cameras, which can take 15 photographs per second, and compared to a target image for calculation purposes, he says.

Mr. Tavakolinejad and Mr. Shakeri are counting the number of bubbles and measuring the size distribution in each photograph, Mr. Tavakolinejad says.

A set of photographs includes about 180 photographs, or 90 image pairs, taken in about six seconds, Mr. Shakeri says.

“Because we take two images, one after the other, we can find out the velocity of the bubbles,” Mr. Tavakolinejad says. “The difference between the two images is one millisecond.”

One set of photographs takes a month or more to research, he says.

“But each setup takes less than an hour,” he says.

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