Science / Medicine : More Than Mere Froth : Scientists Are Finding New Substance in the Mechanics of Bubbles
Hard as it may be to conceive of bubbles as threatening, there are those who do not take the essence of effervescence lightly.
For example, scuba divers care, well, deeply about bubbles: Decompression sickness, generally caused by bubbles in the blood, can cause paralysis or death. To a lesser extent, astronauts and pilots face similar problems. And bubbles are a serious concern to the engineers who design systems that work with fluids: a cooling system for a nuclear reactor, for example, in which bubbles play a critical role.
New ways of avoiding these kinds of bubble trouble are being developed with the help of a 50-year-old theory about how bubbles form in liquids. In the last year or so, a team of researchers at Los Alamos National Laboratory in New Mexico has used the theories to make an important discovery about the nature of boiling, one that in the long term could affect the way nuclear power reactors are operated.
Another Los Alamos researcher, working in his spare time, has applied the theory to the physiology of scuba diving, with much more immediate results: His findings are now being incorporated into an electronic device to help scuba divers avoid injury, which could be on the market as soon as next year.
The Air Force and the National Aeronautics and Space Administration are doing similar work to help pilots and astronauts avoid the harmful effects of bubbles.
The theory being used by these researchers, known as bubble mechanics, explains what happens to gases in a liquid undergoing a sudden change in pressure. A common example is the gas bubbles that form in a bottle of carbonated beverage when it is opened, suddenly reducing the pressure inside.
According to bubble mechanics theory, the pressurized liquid had contained both dissolved gas and bubble “seeds,” which are extremely small amounts of undissolved gas trapped within thin spherical membranes of materials from the fluid. These seeds are of varying size but are typically far too small to be seen.
The surface tension of this membrane keeps the seeds in a state of equilibrium. Any change in pressure, however, ends this stability. A decrease in pressure--inside the bottle of soda being opened or inside the body of an ascending scuba diver, for example--causes dissolved gas to permeate the membranes of the seeds, turning them into bubbles.
In the body, these bubbles can block blood flow or bend nerve endings, causing decompression sickness, better known as the bends. In mild cases, the symptoms are joint pain, tingling sensations or numbness in patches of skin; the most severe cases can be paralyzing or lethal.
Divers try to avoid such problems by using standard tables that either limit the depth and duration of their dives or tell them how to decompress in stages after deep, long dives to avoid the bends. By far the most widely used of these tables are those of the Navy.
But the Navy tables are based on the behavior of dissolved gas alone and do not take bubble mechanics into account, said Andrew A. Pilmanis, a physiologist specializing in the effects of compression and decompression on the body. Major efforts to improve upon these tables are now under way at Navy facilities and elsewhere, and the opinions of researchers are sharply divided over the need to take bubble mechanics into account.
The first application of bubble mechanics, restricted to relatively simple forms of scuba diving, was led by David Yount at the University of Hawaii in the mid-1970s. More recently, Bruce R. Wienke, a physicist at Los Alamos, has expanded on Yount’s work to include repetitive diving over several days. “It’s not dissolved gas that causes decompression sickness, it’s bubbles,” Wienke said. “So it’s really bubbles you have to pay attention to. But since bubbles are coupled to dissolved gas, you have a loop, and it’s that loop we’re looking into.”
With the help of bubble mechanics, Wienke believes he has finally cleared up a puzzling phenomenon long observed in diving: In a series of dives to different depths, it is apparently safest to arrange the dives so that the first dive is deepest, the second not as deep as the first, and so on. Veteran divers have learned to do this by experience, even though diving tables do not explicitly require it.
Bubble mechanics, Wienke said, provide a physiological rationale for arranging dives in this way. Moreover, he thinks that the data can be used to improve on the Navy’s tables.
Wienke’s explanation begins with the fact that at depth, a diver’s body contains bubble seeds of various sizes. Depending on how deep the diver is, some of these seeds have been “stimulated”--gas has begun diffusing across their membranes, turning them into bubbles or increasing the probability that they will become bubbles.
As the diver goes deeper, smaller and smaller seeds are stimulated. This also means that the overall fraction of stimulated seeds is growing, since larger seeds were already stimulated at shallower depths.
At least one company seems sold on Wienke’s work. Scubapro, a Rancho Dominguez, Calif.-based maker of equipment for recreational divers, is in the “early design stage” of a project to build an electronic meter, using bubble mechanics principles, to help divers avoid decompression problems, according to Doug Toth, a project engineer at the company. The meter, worn on the arm, guides the diver through an underwater excursion interpreting such information as the diver’s depth and time underwater according to bubble mechanics principles.
Those who discount bubble mechanics--including the Navy--question its relevance, not its validity. “There are two issues here,” said Capt. Edward D. Thalmann, a doctor and head of the diving medicine department of the Naval Medical Research Institute in Bethesda, Md. “The first is, what is the underlying biophysics of decompression sickness? The second is, do you need to take that into account to compute safe decompression tables? The answer to No. 2 is probably not.”
The Navy prefers a statistical approach, using a database of detailed information on about 5,000 dives, mostly military ones, done since the 1950s. It plans to start testing updated tables, constructed using this database, early next year, in hopes of releasing them by the end of 1991. Richard Vann, director of applied research at the Hypo- and Hyperbaric Center at Duke University Medical Center, called the Navy work “one of the most important developments” in this century.
Vann represents yet another camp: He argues that bubbles are caused primarily by bones, tendons and muscles moving against one another in and around joints. He bases this conclusion on his own research and on similar work at Scripps Institution of Oceanography in San Diego.
Vann and Pilmanis have recently turned their attention to altitude decompression sickness, which affects astronauts and pilots. When they don space suits to leave the space shuttle, astronauts experience a decrease in pressure from the shuttle’s one atmosphere to about 0.3 of an atmosphere in their suits.
A staged-decompression procedure developed by NASA has prevented any serious injuries. But no such procedures exist for military pilots, who are also exposed to significant pressure drops when they fly to high altitudes or are tested in altitude chambers, which is required every three years.
“The Air Force has 100 reported cases from the chambers alone, and a smaller number of cases from flights,” said Pilmanis, chief of high altitude protection in the School of Aerospace Medicine at Brooks Air Force Base in Texas. “We all know there’s a much higher number out there, but we don’t know what it is.” Pilots often do not report mild cases of decompression sickness, he said, because “they tend to be grounded, and no pilot wants to be grounded.”
Last year, Pilmanis began what he believes will be a four-year effort to develop a single set of altitude decompression tables useful to both pilots and astronauts. In formulating the tables, he is evaluating “a number of different approaches, including bubble mechanics,” he said. Three of the researchers he is collaborating with--Hugh D. Van Liew at the State University of New York at Buffalo, Christian Lambertsen of the University of Pennsylvania Medical Center in Philadelphia and Michael Gernhardt of Ocean Systems Engineering in Houston--have done work related to bubble mechanics, he added.
Bubble mechanics is also being applied by researchers interested in the physics of boiling liquids, particularly in nuclear reactors. Although it seems to be a simple phenomenon, boiling is actually poorly understood on a fundamental level, according to Ralph A. Nelson, a staff researcher at Los Alamos.
“The engineering community has looked at the boiling process in detail for 50 or 60 years, and generated about 2,000 to 5,000 research papers, and we still cannot characterize the process,” he said.
Partly because of this uncertainty, many nuclear power reactors are operated very conservatively--at levels three to five times below those they are believed able to safely sustain. “That’s a lot of power; it’s millions of dollars a day,” Nelson said. Through the use of detailed computer simulations of reactors, based on reactor design and thermodynamics principles, he believes it may be possible to safely reduce these margins someday.
His research team at Los Alamos’ Nuclear Technology Division recently took a small but significant step toward that long-term goal. Using supercomputers running programs based on bubble mechanics, the team found that the amount of heat absorbed by a boiling system is dependent on where the first bubble originates in the system. Indeed, the differences in heat value recorded when simulated bubbles began in different places were great enough to account for the 100% scattering observed.
“We think bubble mechanics theory is really starting to bear fruit,” Nelson said. “We need to do some real good experimental work to back up the analytical work, but it’s a start.”
