A new explanation for sonoluminescence

(June 1998)


Sonoluminescence is a fascinating effect which can be seen when ultrasonic waves break against the surface of a water bubble and heat the atoms inside until they glow. A new explanation of the atomic process behind Sonoluminescence was offered by an American physicist, Sanjay Khare, in a recent issue of the journal Physical Review Letters.

Up until now, even though scientists know a great deal about the motions of bubbles and ultrasonic waves, and even though Sonoluminescence was discovered in 1934, nobody has been sure exactly how Sonoluminescence works on an atomic level. Now scientists are using ultrasound to accelerate and enhance chemical reactions in a new branch of science called sonochemistry, mainly in the creation of new materials, so a good explanation is urgently needed.

The clue came from the observation that the ultrasound-stimulated bubbles emit light in very short pulses, as short as 10 parts in a trillionth of a second. Any single ''excited'' atom of the gas inside a bubble would take much longer to decay and emit light, but when many atoms decay together they sometimes decay faster.

In short, if the many atoms inside the bubble decayed at the same time, then the light waves would emerge in step with each other and at the same frequency. That would account for the short pulses. Khare and Mohanty, the two authors of the paper, suggest that it may be possible to vary the effect by varying the gas composition.

The temperature inside some bubbles can reach 10,000oc, twice the temperature at the Sun's surface, and potentially enough to fuse atoms and form new materials. Your reporter would never, however, stoop to commenting that research in this area seems to be hotting up.

The bubble fusion bubble

(March 2002)

A brawl broke out in physics in late February, when word got out of the decision to publish a controversial paper in Science, with the paper appearing in the March 8 issue. In research that recalls the heady days and steamy aftermath of the cold fusion fiasco, two researchers claimed to have created fusion in a beaker, bombarding a sample of acetone with neutrons to generate tiny bubbles. Next, they shocked the beaker with waves of sound, causing the bubbles to expand and then collapse, releasing flashes of light.

This is called sonoluminescence, and it is not all that surprising, but the authors, led by Rusi Taleyarkhan of Oak odge National Laboratory, had replaced the hydrogen in their acetone with deuterium, a heavier isotope of hydrogen. They argued that under the right conditions, the temperature might reach one million kelvin, at which point some of the deuterium might undergo fusion.

Their next step was to look for signs of this, in the form of high-energy neutrons and tritium (hydrogen-s, a radioactive isotope of hydrogen). The problem stems from the fact that even while the paper was undergoing peer review, two other Oak odge physicists tried to replicate the experiment, and failed to get the same result. Dan Shapira and Michael Saltmarsh used a more sensitive detector, and found that the results just did not add up.

The main problem seems to be that the neutrons used to seed the bubbles might easily be detected later and identified as 'products', while the source of the tritium is uncertain.

When word got out that the work was scheduled for publication in the March 8 issue of Science magazine, there was already a problem, as Robert Park from the American Physical Society pointed out in what Science snootily called ''an airy premature dismissal''.

If there was any airiness, it would appear to be where the riqor should have been in the Taleyarkhan et al. paper, and if there was anything premature in the matter, it was the publication in Science. Consider the evidence.

Robert Park, an eminent and courteous commentator, reminded his readers that the experiment had been repeated by two experienced nuclear physicists, Shapira and Saltmarsh, using the same apparatus, except for superior neutron detection equipment. They had found no evidence for 2.5 Mev neutron emission correlated with sonoluminescence. Any neutron emission was many orders of magnitude too small to account for the tritium production reported by the first group.

Park went on: ''Although distinguished physicists, fearing a repeat of the cold fusion fiasco 13 years ago, advised against publication, the editor has apparently chosen not only to publish the work, but to do so with unusual fanfare, involving even the cover of Science.

Perhaps Science magazine covets the vast readership of Infinite Energy Magazine.

Later, once the editorial comments in Science had come to light, Park replied:

An editorial by Don Kennedy in the March issue of' Science'... describes his courageous stand in publishing a controversial paper even though "it had become clear that a number of people didn't want us to publish this paper.

Park went on (and here, 'WN' refers to his refreshing newsletter on science, ''What's New'', which is distributed in the Internet each week, telling the truth as Park sees it, without fear or favor.):

Last week WN revealed that Science would carry an article by Taleyarkhan et at from Oak Ridge National Laboratory (WN I Mar02), claiming evidence of cold fusion correlated with sonoluminescence from collapsing bubbles in deuterated acetone. However, Shapira and Saltmarsh, also from oak Ridge, using purportedly superior detection and analysis equipment, found no evidence for fusion. Kennedy, it turns out, was merely urged to delay publishing the Taleyarkhan result until it could be accompanied by the Saltmarsh finding.

Instead, 'Science' accompanied the Taleyarkhan paper with a glowing 'Perspectives' article, a 'News' report and an editorial Worse, 'Science' issued an embargoed press release. A press embargo is a device meant to suppress dissenting views the day a story breaks. We at WN are not press, however, nor did our in formation come from 'Science'.

After WN broke the story, 'Science' dropped its embargo. Both sides, Kennedy's editorial concludes, "would do well to wait for the scientific process to do its work." But in the end, it was 'Science' that refused to wait until it had a balanced report.

Working journalists, people whose daily work is writing and communicating about science are well aware of the brutal way in which Science beats into submission any journalist judged by them to have broken their embargo. Playing the part of judge, jury and executioner, junior staff at Science deny journalists access to stories that will be breaking in the journal, refuse requests to reconsider the matter, and ignore correspondence. So for the journal to put out an embargoed release on a matter already in the public domain shows a breath-taking arrogance matched only by their claim to have done the right thing in publishing when they did.

In short, the American Association for the Advancement of Science, the AAAS, has become a tool of a major business, the publication of Science, and the nominal members of the AAAS are treated with sublime contempt from on high. we will give Robert Park the last word, because he will never get it from Science:

The first warning sign that a scientific claim is voodoo is that it's pitched directly to the media. That didn't happen with the Taleyarkhan et at bubble-fusion paper (WN S Mar 02). The authors went through all the hoops, submitting their paper to a respected, peer-reviewed journaL It was Science that seemed determined to sensationalize the work. In the course ofa year, various drafts went to 13 or 14 reviewers, which does not inspire confidence. A number of reviewers reportedly advised against publication and some complain tnat 'science' did not tell tnem ot snapira and saltmaren's tailure to confirm fusion claims. The second warning sign of voodoo science is that any failure to confirm is blamed on an "establishment" conspiracy. A 'Business Week' story says one author of the Taleyarkhan paper "hinted" that Shapira and Saltmarsh were protecting ''the fusion establishment.

The main difference between this and the cold fusion case is that bubble fusion is at least a possibility, so it would be unwise to rule it out right away. On the other hand, this paper would not appear to offer any strong evidence that the possibility has been promoted to a reality, and that means there will be a few other attempts, around the world, to test the possibility in a more rigorous way.

ASK THE EXPERTS: PHYSICS

The bubbles produced by ultrasound in water (Sonoluminescence ) reach extremely high temperatures and pressures for brief periods. Could these conditions initiate or facilitate nuclear fusion, as suggested in the recent movie "Chain Reaction"?

A detailed discussion of the physics of can be found in the article Sonoluminescence: Sound into Light," by Seth J. Putterman (Scientific American, February 1995). In it, the author outlines a different interpretation of the phenomenon from the one given below, though he agrees that the likelihood of getting fusion to occur in sonoluminescence bubbles is insignificant. Andrea Prosperetti in the department of mechanical engineering at the Johns Hopkins University has studied this question in detail. He responds:

"It must first of all be stressed that the 'extremely high temperatures' referred to are, at least for now, speculation. While many researchers would concede temperatures of up to, say, 10,000 kelvins (which is way too low for nuclear fusion), a much smaller number would feel comfortable with temperatures in the millions of degrees range. The computations that indicate such extreme conditions inside a pulsating bubble are based on rather extreme idealizations. "The most fundamental one is the fact that the bubble remains absolutely spherical during its radial oscillations. On theoretical grounds, there are many reasons to doubt this premise: a collapsing sphere is highly unstable (which is the reason why attempts at producing fusion by causing the implosion of gas-filled micro-balloons with powerful pulses of laser light have so far failed), and liquid jets may develop that span the bubble.

"Furthermore, experiment suggests that the light emitted by a bubble has a weak directional asymmetry, which would be incompatible with perfect sphericity. Hence, while it is not absolutely possible to rule out the occurrence of nuclear reactions inside a pulsating bubble on the basis of the present knowledge, the actual occurrence of such reactions is, to say the least, doubtful."

The Johns Hopkins University has also provided this official statement on Dr. Prosperetti's work:

Sonoluminescence, the puzzling glow emitted by a bubble in a field of high-pitched sound waves, may be caused by a tiny jet of liquid that shoots across the interior of the bubble at supersonic speed and slams into the opposite side, a Johns Hopkins researcher has proposed. At the point where this powerful jet strikes the bubble wall, it "fractures" the liquid, releasing energy in the form of light, says Andrea Prosperetti, an internationally respected expert on the mechanical properties of bubbles.

Prosperetti's theory appears in the April 1997 issue of the Journal of the Acoustical Society of America. His paper offers an alternative to the widely held view that the bubble glows because of shock waves that concentrate energy in its center as it shrinks. His theory also deflates the hope among some researchers that sonoluminescence generates enough pressure and heat to produce nuclear fusion, a potential source of cheap, clean energy. Some scientists have speculated that bubble temperatures during sonoluminescence exceed 2 million degrees Fahrenheit, near the levels needed for fusion. This idea became a key plot point in the motion picture "Chain Reaction," starring Keanu Reeves. But if Prosperetti's theory holds true, the heat inside the bubbles would peak at about 10,000 degrees F, the level found at the sun's surface. "It's enough to explain the chemical activity, but it's far below the amount needed to produce nuclear fusion," says Prosperetti, who is the Charles A. Miller, Jr. Distinguished Professor of Mechanical Engineering at Hopkins.

Sonoluminescence was discovered in 1934 by two German physicists who immersed powerful ultrasound generators in a vessel of water, creating a cloud of tiny bubbles that gave off a glow. Scientists were intrigued but found it was too difficult to study in detail the unwieldy mass of short-lived bubbles. In 1989, however, Lawrence Crum, then a professor at the University of Mississippi, and his graduate student, Felipe Gaitan, were able to induce sonoluminescence in a single bubble trapped within a sound field inside a cylinder of water.

Since then, scientists have been able to study the phenomenon more closely. Much to their surprise, they realized that this "single-bubble" luminescence was different from the massive "multiple bubble" phenomenon first observed 60 years earlier and -- as it turns out -- far more mysterious. For example, the flash of light lasts an incredibly short time, a few tens of trillionths of a second. Also, the phenomenon is extremely sensitive to the nature, purity and temperature of the liquid and to the presence of dissolved gases in it.

Sound waves passing through the liquid cause the bubble to compress and expand repeatedly. At its largest point, the bubble's diameter is about that of a human hair. Scientists believe the sound energy is concentrated during the bubble's compression phase, then is released as light near the point where its size is smallest. But the exact mechanism has remained a mystery.

In his new paper, Prosperetti says it is unlikely that shock waves within the shrinking bubble trigger sonoluminescence because the bubble would need to maintain a near-perfect spherical shape. "I think it is absolutely impossible for the bubble to remain spherical," he says. "In a sound field, there is a very well-defined mechanism that will prevent this from happening. The fluid wants to push a jet, a finger of liquid, through the bubble, hitting the other side. What you see in sonoluminescence is the initial result of this 'hammer of water.'" This jet, moving at perhaps 4,000 miles per hour, or more than five times the speed of sound in air, strikes so quickly that water molecules do not have time to flow away from the point of impact. Instead, the fluid fractures. "This is what happens with Silly Putty, for instance," Prosperetti says. "If you pull slowly, it just stretches or flows. But if you pull it really hard, it snaps, and you get a brittle fracture." Ice and even Wint-O-Green Lifesavers candy sometimes give off light when they crack, and water molecules could produce the same effect, the Hopkins researcher suggests. His theory holds the promise of explaining many facets of the phenomenon. For example, bright light emission requires tiny amounts of a noble gas such as xenon, argon or helium dissolved in the liquid because, Prosperetti believes, these inert atoms create flaws or weaknesses in water's crystal-like structure that provide a foothold where the fracture begins. In his paper, Prosperetti urges other researchers to test his theory. He suggests several lab experiments for this purpose, including the firing of a hyperfast bullet or fluid jet at water in a controlled setting to see if it produces luminescence.

Lawrence A. Crum of the Applied Physics Laboratory at the University of Washington expands on the above response: "If one is to consider the possibility of nuclear reactions produced by sonoluminescence, it is helpful first to consider some simple physics, particularly the energy levels associated with these various systems.

"When a sound wave propagates through a fluid, the amount of energy density in the wave is quite small. The reason we think the sound of a jet aircraft is really loud has more to do with the sensitivity of our remarkable auditory system than with the energy in the sound wave itself. Our ear is so sensitive that as newborn babies, we can hear molecular displacements on the order of angstroms--about the diameter of an atom. Even though we lose this sensitivity with age, our adult ears can still detect molecular displacements on the order of nanometers. Thus, if one considers the energy density in a sound field capable of producing sonoluminescence, one finds it to be quite small--on the order of 10-11 electron volt per molecule. The electron volt may seem a crazy unit, but we shall see later why it is a convenient one.

"When a sound field propagates through a liquid such as water, the molecules of the liquid are held together by molecular bonds that are relatively strong. Thus, it is very difficult for the negative pressures existing in a propagating sound field to tear apart the water--and it practically never happens. What does happen is that the sound field interacts with any small gas bubble that may exist in the water and causes the bubble to grow dramatically during the passage of the negative pressure portion of the sound field--the water essentially 'boils'--because the pressure is below the vapor pressure. During the negative pressure cycle, the bubble can grow to many times its original size--say a factor of 1,000 in volume.

"When the sound field eventually turns positive, the pressure is now above the vapor pressure; the vapor rapidly condenses, and all the energy that was given to the bubble during its growth process is available to be concentrated into a small region as the bubble is driven to an implosive collapse. This process is called acoustic cavitation. Because this implosive collapse is dominated by the inertia of the liquid surrounding the bubble, and there is little stiffness supplied by the condensing vapor (only the small amount of residual gas contained within the bubble), the energy density can become much larger than that originally present in the sound field itself. The energy concentration is now so high that the residual gas contained within the bubble is heated to incandescence temperatures and emits light. This process is called sonoluminescence. Because these electromagnetic emissions are on the order of an electron volt, and they probably come from a single molecule, or atom, or electron, we can now say that the energy concentration is now on the order of one electron volt per molecule--an increase of a factor of 1011 or so.

"Energies on the order of an electron volt are typical on an atomic basis and correspond to an effective temperature on the order of 10,000 kelvins. This is a pretty high temperature, of course, and can influence chemical reactions. Thus, sonoluminescence is often associated with 'sound chemistry'--or 'sonochemistry.' The fact that a rather benign mechanical mechanism such as a propagating sound field can produce atomic reactions is a quite remarkable and has attracted considerable scientific attention (see "The Chemical Effects of Ultrasound," by Kenneth S. Suslick in Scientific American, Vol. 260, No. 2, pages 8086 [or 62-68 for non-U.S. readers]; February 1989).

"Although energies on the order of an electron volt per molecule are relatively large for our macroscopic world, they are the typical energies of reaction in the atomic world. When we consider thermonuclear fusion, on the other hand, we need to move from the atomic to the nuclear scale. Because a proton or a neutron is on the order of a million times smaller than an atom, nuclear fission and fusion typically require energies on the order of millions of electron volts (MeV). The substance of the question posed by the reader is essentially: Can this benign mechanical sound field now interact at the nuclear level? Of course, our immediate response is that we are still six orders of magnitude too small in energy, and there is no possibility for nuclear fusion from sonoluminescence.

"Given that controlled fusion is such an attraction because of our nearly inexhaustible source of hydrogen as fusion fuel and that existing devices designed to harness this energy are of enormous dimensions and costs, it would seem desirable to see if there is some mechanism to boost the energy density by another six orders of magnitude. There has been a glimmer of hope in this direction when it was determined that there are strong indications that the collapsing bubble can generate an imploding shock wave within the gas contained within the interior of the collapsing bubble. This imploding shock wave can compress the interior of the bubble's contents even more; indeed, William C. Moss and his colleagues at Lawrence Livermore National Laboratory have obtained theoretical estimates of the temperatures achievable with an imploding shock wave, and these values approach those required for nuclear fusion. "Is an imploding shock wave possible? Seth Putterman and his colleagues at the University of Califnornia at Los Angeles have measured the velocity of the bubble interface and have determined that it can reach values on the order of four to five times that of the velocity of sound in the undisturbed gas. These data seem very promising. Andrea Prosperetti--see his recent comments here in 'Ask the Experts'--has suggested, however, that the bubble must remain spherical for the shock wave to develop much strength--which he believes is not very likely. Tom Matula and his colleagues at the University of Washington have observed a shock wave in the liquid after bubble collapse, which might be a consequence of a shock wave in the gas. Values of the amplitude of this waterborne shock wave correspond to predicted values, assuming it arose in the interior of the gas, so there is additional evidence of the effect.

"The state of the art of sonoluminescence research at the moment is that investigators are trying to understand the bubble collapse process and look for any evidence of the shock wave within the bubble itself.

"It is difficult to understand just what the writers of the movie Chain Reaction intended in their screenplay. Certainly the science was so awful it turned off any serious scientist. Their hypothesis that sonoluminescence generated hydrogen (and no oxygen) was kind of silly. In the movie, a 'chain reaction' occurred, but it was difficult to determine if this was a nuclear chain reaction or just a big hydrogen explosion. And the fact that Keanu Reeves was able to outrun the shock wave from the explosion on a motorcycle suggests that it was actually pretty mild--even though it did demolish several city blocks.

"As one who is involved in sonoluminescence research, I was particularly disappointed that the writers really messed up the science. I think that they sold the public short and that a little more authentic science would have attracted an explosion of interest among young kids on the Internet and would have greatly improved the attendance at the theaters. It frustrates me that movies and TV shows that depict doctors and lawyers are made to look quite authentic, but when it comes to physics or chemistry, Hollywood seems not to have made it past the third grade!"

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