New Research Sheds Light on Bubble Science

Aug. 22, 2002 -- Those soothing sounds of roars and trickles as waves break against the shore have a surprisingly humble origin: Bubbles.

Not only are bubbles behind the dominant sounds above and beneath the sea, they also influence global climate, storm systems and the exchange of gasses and biological matter.

That's why researchers recently spent endless, painstaking hours in the lab counting millions of tiny bubbles in images snapped at the water's surface. By counting bubbles formed in ocean waves and noting their sizes, the team collected information that they believe can lead to a better understanding of the Earth's cycles.

"Understanding the formation of bubbles is key," says Grant Deane, a physicist at Scripps Institution of Oceanography at the University of California at San Diego who authored a study about bubbles with Scripps colleague M. Dale Stokes in this week's issue of Nature. "It's an important part of the puzzle to understanding global warming and underwater noise."

Bubble Magic

Bubbles influence the environment mostly as they form and break. As ocean waves crest and topple, air and sea water mix to form whitecaps. Inside each whitecap is a rich swirl of bubbles called the bubble plume.

As these bubbles form, they pocket gasses and transfer them between the air and sea. As they rise up through the water column, they scrub out and collect organic material and bacteria. Finally, at the surface, bubbles burst and release tiny droplets — and their contents — into the atmosphere.

These droplets contribute to clouds and weather systems and transfer matter and gasses long distances (one reason why cars rust more quickly in coastal towns is due to airborne salt released by bubbles). The same salty sea spray also cleanses pollution from the air as they form raindrops, which drag particles of pollution down into the water, a recent study found.

Sidney Perkowitz, a professor of physics at Emory University and author of the book, Universal Foam, adds that bubbles can even release tiny living things or pieces of living things when they burst above the water's surface.

"Bubbles could even have played a role in how life is distributed on Earth," he says.

Bubbles Big and Small

To better understand bubbles and their effects, Deane and Stokes captured video images of bubbles inside whitecaps cresting at the ocean's surface and from within waves created by a laboratory wave machine. To do this they built a so-called "BubbleCam" — a high-speed video camera with an intricate lens and focusing system that can snap finely sliced pictures as waves break.

A computer then counted most of the bubbles in the images, although some were left for human eyes to decipher. Once they charted the bubbles' sizes and distribution, Deane and Stokes determined there are two main kinds: Bubbles with a radius smaller than about 1 millimeter and those larger.

The larger bubbles, they learned, are formed by turbulence as a wave curls over on itself, creating the tube that surfers covet. The smaller ones are created by the splash of the wave's tip hitting its face. Smaller ones persist longer than larger ones thanks to a sturdier structure.

"These results are one more piece of information," said Deane. "Why do you get the number and sizes of bubbles you do in breaking waves? It's a very basic science question that we're trying to answer. It's like the Big Bangtheory of bubbles..."

Not only can the new information help scientists better understand bubbles' role in weather systems and climate change, it may also shed light on underwater acoustics.

Liquid Noisemakers

Peter Dahl, a physicist at the University of Washington, studies the noises bubbles make and how they make them. As bubbles form, Dahl explains, outside water pressure squeezes the bubble. This causes it to vibrate as it compresses and expands in a particular rhythm and the vibration creates a sound.

"There are bubbles of many sizes and each bubble radiates sound at a frequency that corresponds to its size," he says.

Bubbles also make noise as they become fragmented underwater and as they pop once they reach the ocean's surface. Together, these sounds, create the "roar" of an ocean wave.

Navy researchers have long studied the different sounds bubbles make since they use acoustical instruments when looking for submarines, wrecks or other items or when trying to communicate with people in underwater vessels. Sound is commonly used to scope the ocean since sound waves slip easily through seawater, while other forms of radiation do not.

Recognizing what sounds are bubbles can help engineers tune out the background noise of bubbles to get more accurate underwater readings. Climate researchers, meanwhile, can tune in to the noise of bubbles to estimate the flow of gas between air and sea.

It may seem easy to dismiss bubbles as insignificant air pockets, but, as Perkowitz argues, they're actually "a big deal."

"Thinking about how bubbles are made and their size is critical research," he says. "It's work of global importance."