running hot and cold

A few years ago, in 2004, a Ben and Jerry's ice cream store in New York City celebrated Earth Day with an unveiling of a prototype "thermoacoustic chiller": basically, a freezer that kept the pints of Cherry Garcia and Chunky Monkey nicely chilled on a warm sunny day by using sound waves, instead of vapor compression of harmful chemicals like hydrofluorocarbons (HFCs) — the mechanism behind the modern refrigerator. The prototype "chiller" was developed by Matt Poese and Steve Garrett, both physicists at Penn State University.

The underlying effect  has been known for over 100 years, after glass blowers in the 19th century observed that tones were being generated by hot glass bulbs attached to a cool tube. Anything that combines thermodynamics with acoustics is A-OK in my book. It's essentially the same basic concept as a standard heat engine, which derives energy from differences in temperature (ref. Sadi Carnot and Maxwell's Demon). I like to think of it as being the temperature equivalent of dropping a ball from a given height. The ball gains more potential energy the higher it is raised, which converts into kinetic energy when the ball is dropped. The higher the ball, the more potential energy is stored, and the more kinetic energy you get when you drop it. Applied to the heat engine, this means that the greater the difference in temperature, the more potential energy there is to convert into kinetic energy. Maybe it's not a perfect analogy, but it works for me.

Of course, unless you can harness that energy to do something useful, it's largely wasted effort. The Penn State scientists figured out how to do that. The concept derives from the fact that sound waves travel by compressing and expanding the gas (air) in which they are generated. This mechanical energy can be used to cool and heat stacks metal plates in the path of the sound wave. Some get hotter, some get colder, and the result is that critical temperature difference that gives rise to usable energy. Put a couple of heat exchangers on that sucker, and you've got a nifty little cooling chamber. Time magazine declared it one of "The Most Amazing Innovations of 2004."

Even better: the gas used is helium, much safer than HFCs. We don't see a lot of thermoacoustic refrigerators on the market just yet because their energy efficiency isn't competitive with the conventional technology. But give it time: scientists are ingenious sorts, and they're making improvements all the time. Thermoacoustic refrigeration is already being used to cool biological samples on board the Space Shuttle. 

Poese and Garrett aren't alone. There are lots of research groups working on various fundamental and applied approaches to exploiting this unusual effect — groups like Orest Symko's at the University of Utah. Symko has a long-standing interest in building tiny versions of thermoacoustic refrigerators for cooling electronics. (Considering how hot my MacBook Pro tends to run, such a breakthrough would be very welcome in the industry.) A couple of years ago, he expanded that program to include all kinds of thermodynamics devices that convert heat into sound, and sound into electricity, for a broad range of possible applications. He and five of his graduate students were on hand at the ASA meeting in Salt lake City to present their latest achievements.

Heat is basically wasted energy, but Symko's devices harness heat that would normally be wasted — like that emitted by the dual core microprocessor in my MacBook Pro — and convert it into usable electricity. It's the same basic structure as the thermoacoustic chiller: a small cylinder (the "resonator") that fits in the palm of your hand, containing a stack of metal plates, placed between a cold heat exchanger, and a hot heat exchanger. Take a blowtorch to one end, and air begins to move down the tube, creating sound waves, similar to how a flute produces tones. Also inside the tube is a piezoelectric crystal, a "smart material" that responds to an increase in pressure by producing an electric spark. (Those old cigarette lighters in cars — since replaced by auxiliary plug-ins — used piezoelectric crystals.) The tube's dimensions determine the frequency of the sound, in the present case, in the audible range; very small ones could produce ultrasound waves.

Voila! Heat turns into sound turns into electricity. It's not a lot of energy, mind you: Symko estimates that only about 10-25% of the heat is converted into sound, and a little more is lost in the conversion of the sound into electricity, although that's a much more efficient conversion: generally, 80-90% of the sound is converted. Still, you won't see these things being used to power Microsoft's corporate headquarters any time soon. But as an alternative to solar cells in small niche applications, Symko's thermoacoustic devices could be ideal.

There's always a net loss any time you convert one type of energy into another — that's the basis of the second law of thermodynamics, namely, entropy, a.k.a., The Ultimate Killjoy. It might be a losing battle, but that doesn't mean it's not worth fighting. Someone who wasn't, to my knowledge, in Salt lake City, but perhaps should have been, was Australian scientist Luke Zoontjens, who made news in 2005 with his work on using the sound waves produced from heat derived from car exhaust gases to run car air conditioners. Then a PhD student at the University of Adelaide, Zoontjens sought to exploit the same kinds of thermoacoustic devices, urning heat into sound, and sound into cold air, just like Penn State's thermoacoustic chiller. 

You wanna talk about inefficiency? The average gas engine in a car only gets a 30% return in usable energy on the gas it burns; 70% is released as wasted energy, mostly heat. It says something about the extent of the energy problem, and our society, that this is considered an acceptable loss. Zoontjens' scheme converts the heat from a car's exhaust pipe into sound waves, which are amplified inside the tube to as much as 180 decibels. That energy can then be harnessed to cool the car's interior.  We're looking at a mere 20% efficiency once everything's been converted, but considering it all comes from what would otherwise be wasted energy, technically, it's a tiny net gain.

Most of us never really stop to think about how powerful sound really is. For instance, we take ultrasound imaging for granted, and because it's so safe, many people might not realize that ultrasound at higher frequencies can burn — which is why it can be used to cauterize bleeding, particularly in vital organs that have hundreds of tiny blood vessels. It "cooks" the proteins in the blood just like the whites of eggs. I wrote about therapeutic uses of ultrasound several years ago for the (sadly) now defunct magazine, The Industrial Physicist. One of my sources, Larry Crum of the University of Washington, pointed out that the late Princess Diana (back in the news yet again, thanks to a new tell-all biography from publishing doyenne Tina Brown) died from uncontrolled bleeding from all those tiny vessels in the vital organs; had handheld therapeutic ultrasound devices been available at the time of her fatal car crash, the princess might have survived.

Sound is also capable of producing extremely high temperatures through the phenomenon of sonoluminescence, possibly even on a par with nuclear fusion. That was the premise of the 1996 film Chain Reaction,  in which Keanu Reeves was implausibly cast as a PhD physicist. (Jen-Luc Piquant notes that donning a white coat and glasses really can't overcome the actor's trademark stoned surfer dude demeanor, any more than it could turn a smokin' hot chica like Elisabeth Shue into a mousy wallflower scientist in The Saint.)

Keanu and his co-star, Rachel Weisz, play physicists who have discovered  how to exploit sonoluminescence to achieve "bubble fusion," except instead of a Nobel Prize nomination, Keanu gets framed for the murder of his boss. Oh, and his experiment has been rigged up like an atomic bomb, so he's not only got to clear his name, he has to save the world, too.  Just a typical week in the life of the average research physicist, Future Spouse assures me. (Jen-Luc concurs. When she's not trying to diffuse a bubble fusion bomb and foil an international conspiracy, she's struggling to stabilize extra dimensions of spacetime. C'est la vie!)

It might sound like a load of Hollywood hooey, but sonoluminescence is a very real phenomenon — "the emission of short bursts of light from imploding bubbles in a liquid when excited by sound," per Wikipedia. It was first observed in the 1930s by scientists working on sonar. (There's some debate over who and when, but I'll go with the more detailed story, because it makes for livelier copy.) H. Frenzel and H. Schultes — neither of whom bore any resemblance to Keanu Reeves (or Rachel Weisz) — were trying to speed up the photographic development process by placing an ultrasound transducer into a tank of developing fluid. Instead, it caused tiny dots to develop on the film. The fluid had bubbles, you see, and those bubbles were emitting tiny flashes of light whenever the ultrasound was turned on. Since film is photosensitive — designed to react to light — the dots appeared on the developed film.

This came as quite a surprise to scientists, but there was one creature who rolled its eyes in disdain over how thick-headed these humans could be sometimes: the lowly pistol shrimp, a.k.a., the snapping shrimp. This species of shrimp has a set of asymmetrical claws, and the larger one produces a loud snapping sound — loud enough that the pistol shrimp vies with the sperm whale and beluga whale for the title of "loudest animal in the sea." That snapping sound gives rise in turn to a shock wave powerful enough to stun or kill the shrimp's prey (small fish). Such a shock wave also creates bubbles that collapse and produce a flash of light. Granted, it's of a very low intensity, and usually not visible to the naked eye, but still — the pistol shrimp species would like us to tell you that "shrimpoluminescence" really should have been noticed much sooner than October 2001. If anyone deserves a Nobel Prize for the discovery of sonoluminescence, it's the shrimp.

Frenzel and Schultes discovered what is now known as multi-bubble sonoluminescence (MBSL). They weren't able to do much detailed analysis, because there were far too many bubbles, and those bubbles weren't around long enough (only a few hundred picoseconds) to make detailed measurements or observations. It took over 50 years before scientists were able to produce single bubble sonoluminescence (SBSL) — coincidentally, that honor belongs to the aforementioned Dr. Crum, and his collaborator, Felipe Gaitan.

Perhaps the shrimp have a valid point about us lagging way behind on the initial discovery, but scientists have made some impressive advances on the sonoluminescent front since Crum and Gaitan's pioneering work on SBSL in 1989. They can make a single bubble expand and collapse over and over again periodically, emitting that telltale flash of light each time it collapses.

With SBSL, it was easier to analyze this complicated process by focusing on a single bubble, which is how scientists learned that the temperature inside the bubble was hot enough to melt steel. Theorists predicted that it could get even hotter, perhaps above 1 million Kelvins. This was exciting because it meant that it might be possible to use sonoluminescence to achieve thermonuclear fusion. (Take that, pistol shrimp, thinkin' you're all that and a bag of chips, just because you have big, loud snappy claws!)

And this is where the controversial topic of bubble fusion — a.k.a. sonofusion — comes in. It's possible in principle, per the work of UCLA's Seth Putterman, although he has yet to successfully demonstrate sonofusion in the lab. Someone who claims to have done so is Rusi Taleyarkhan of Oak Ridge National Lab — a claim that has sparked a heated debate and even raised allegations of scientific misconduct. The so-called "string wars" might dominate the mainstream media coverage, but there's been just as much finger-pointing, name-calling, and snarky put-downs in sonofusion — and I'd argue it's an equally sexy topic. (Nota bene: this is not — repeat, not — cold fusion, even though the apparatus operates at room temperature. The nuclear reactions — assuming that's what they are — occur at the very high temperatures inside the core of the imploding bubbles, which shock wave simulations indicate could be as high as 10 megakelvins,)

The chronological chain of events is a bit confusing, given the amount of back and forth that's gone on, and the fact that all the Wikipedia entries on the subject are marked as being "disputed." So I hope people will feel free to post corrections and clarifications in the comments section. But from what I've been able to gather, Taleyarkhan et al. published a paper in Science in 2002 claiming that the results from their experiments on acoustic cavitation were consistent with fusion (most notably, the amount of neutrons released, and tritium produced from the "reactions").

The trouble began when ORNL colleagues repeated the experiments and announced that their neutron and tritium production was more in line with random coincidence. Taleyarkhan's team published a rebuttal, and followed up with published papers with new claims of bubble fusion in 2004 and in 2005, the latter appearing in the peer-reviewed journal Physical Review Letters. But the number of skeptics grew. Among the most vocal was UCLA's Brian Naranjo, who openly questioned the validity of the Purdue results in a 2006 article in Nature.

Part of the problem is that even Taleyarkhan admits that the reaction doesn't always work correctly, and they are still investigating what the critical experimental parameters might be for achieving sonofusion. His claims were extraordinary, and therefore elicited more scientific doubt than usual. Things got really nasty when allegations of misconduct emerged — namely, that Taleyarkhan had attempted to actively thwart the efforts of several university colleagues to test his claims — and a special review committee at Purdue was appointed to investigate the matter.

The story ends fairly positively for Taleyarkhan and his collaborators. Earlier this year, Purdue rejected the allegations of research misconduct, stating that "the evidence does not support the allegations" and concluding that "vigorous, open debate of the scientific merits of this new technology is the most appropriate focus going forward." In other words, fight it out in the pages of peer-reviewed journals, people, and leave university administrators out of it.

So the jury is still out on whether these sonofusion research results are valid, i.e., reproducible. Putterman, for one, has not been able to duplicate Taleyarkhan's experiments. Apparently, the BBC documentary series Horizons commissioned Putterman's reproduced experiment — how cool is that? I dream of a day when CNN, for instance, sponsors such an experiment to resolve a scientific dispute. Ultimately it all comes back down to energy sources, and given global warming, the price of gasoline, and the myriad of other problems associated with how we power our daily lives, it definitely qualifies as being of broad public interest. At least it should.