a spark in the dark

One wouldn't expect the 17th century English philosopher Francis Bacon to have much of a sweet tooth; he always struck me as a rather curmudgeonly sort, thoughts firmly fixed on Higher Matters, eschewing the paltry comforts of the flesh. But Jen-Luc Piquant suspects he might have had a secret fondness for hard candies, based on a passing remark Bacon made in his treatise, Novum Organum (published in 1620): "It is almost certain that all sugar, whether refined or raw, provided only it be somewhat hard, sparkles when broken or scraped with a knife in the dark."

Now, one could argue that Bacon was merely being an observant scientist following his natural curiosity, but stop and think for a moment under what conditions he might have discovered such an effect -- alone in a darkened room with a bag of hard candy and a knife to break up the pieces into smaller bits that were easier to consume. Sounds like a secret sweet tooth to me! This also establishes Bacon as the earliest to record the phenomenon, known as triboluminescence, a.k.a., "the Wint-O-Green Life Saver Effect."

I adored Wint-O-Green Life Savers as a child, mostly for the refreshing minty taste, but they also had loads of entertainment value. It's pretty well known that if you chew fresh, dry Wint-O-Green candies in a dark room -- or snap them in two using a pair of pliers -- you'll get a spark of greenish/bluish light. That's triboluminescence. In 1753, one Father Giambattista Beccaria wrote a treatise on "artificial electricity," in which he described how easy it was "to frighten simple people only by chewing lumps of sugar, and, in the meantime, keeping your mouth open, which will appear to them as if full of fire." One bets Beccaria was just a laugh-riot at parties. (Actually, considering the level of superstition at that time, he was fortunate not to be run out of town, or even burned at the stake.) 

By the late 1790s, sugar production began to produce more refined crystals of pure sugar in the shape of a large solid cone. Once transported and sold, the cone would be broken into smaller chunks using a "sugar nip" -- I'm guessing similar to, say, nail clippers or a similar type of pinching device. And of course, when the sugar cone was nipped in a darkened setting, there would be tiny bursts of visible light, and Beccaria's little party trick became less effective -- because everyone knew about it by then. It's not just sugar that shows the effect, either, When a diamond facet is being ground, or the gem is being sawed during the cutting process, the diamond may fluoresce blue or green. Open certain postal envelopes in the dark, or tear a Band-Aid wrapper very quickly, and you might see a brief blue-green glow.

There's actually rather a lot going on here, from a science standpoint. The simplest explanation is that when molecules are crushed or torn asunder, electrons are forced from their atomic fields and start colliding with nitrogen molecules in the air. The collisions cause a transfer of energy from the electrons to the nitrogen molecules, which begin to vibrate (an "excited state"). The nitrogen molecules want to get rid of the excess energy -- I guess the vibrations make them uncomfortable -- so they emit light, usually in the ultraviolet range, but there's usually a small amount of visible light as well.

Or, as Wikipedia succinctly puts it: "electrical fields are created, separating positive and negative charges than then create sparks while trying to reunite." It's rather like a lightning strike, in fact. The effect is usually observed with asymmetrical crystals; when those materials are scratched, crushed or rubbed, the chemical bonds are broken and a small flash of light is emitted. Hence the name triboluminescence from the Greek tribein ("to rub") and the Latin lumen ("light").

The effect is even more pronounced with Wint-O-Green Life Savers because of the flavoring used: wintergreen oil is technically known as methyl salicylate, a fluorescent chemical, which means it absorbs light of a shorter wavelength and then emits it as light of a longer wavelength. So when you crush a Wint-O-Green Life Saver between your teeth, negatively charged electrons break free; the atoms that once were their homes become positively charged, and the free electrons start dashing about in search of a new home. Meanwhile the sugar crystals disintegrate, causing nitrogen molecules from the surrounding air to attach themselves to the surface. The free electrons strike the nitrogen molecules, causing them to emit invisible ultraviolet radiation and a faintly visible glow. The molecules of methyl salicylate (wintergreen oil used for flavoring the candy) absorb the ultraviolet light (shorter wavelength), and then re-emit visible blue-green light (longer wavelength). And you have sparkage.

Most of us would be satisfied with that level of detail in an explanation, but scientists adhere to higher standards. They want to know precisely what is happening atom by individual atom, and preferably even at the smallest subatomic level. That way they can better control physical phenomena, and the more you can control them, the easier it is to exploit such effects in practical applications. That's why researchers at the University of Illinois at Urbana-Champaign began conducting experiments that exploited the effect to shed light -- literally -- on how materials fracture. They published their findings last year in the Journal of the American Chemical Society.

"When you break a pencil, you actually have to have broken chemical bonds," Illinois professor Kenneth Suslick told the New York Times last year. "Yet our understanding of the process is surprisingly poor. I fact, when you look at the quantum mechanics of that, it isn't exactly clear how the breakage occurs." What is clear is that when one breaks a material, invariably this drives chemical reactions. And a reaction like triboluminescence "gives us a spectroscopic probe to see what's going on right at the fracture point," said Suslick.

But triboluminescence, as it naturally occurs, is a very small effect. The Illinois researchers had to figure out some way to amplify the effect in order to glean useful information from the fracture point (a spectral fingerprint, if you will). Suslick and his collaborators filled a test tube with a slurry of small sugar crystals and liquid paraffin, then immersed a vibrating titanium rod into it. This generated ultrasound waves, creating lots of tiny bubbles constantly growing and collapsing in the paraffin (acoustic cavitation). The shock waves caused the sugar crystals to collide, nitrogen and oxygen bubbled through the slurry, and the result was bursts of light 100 to 1000 times brighter than the usual triboluminescence. So far they've found the presence of carbon monoxide, CO2 ions, and other products of combustion, and are now working on determining the chemical reactions taking place during triboluminescence.

Research in this area continues to yield surprising results. Last month, physicists at UCLA announced that unspooling a simple roll of Scotch tape produces sufficient X-rays to make clear images of their fingers. Seth Putterman and several students constructed a device to unspool the 3M brand Scotch adhesive tape at a steady rte of about 1.3 inches per second, and placed it in a vacuum. They measured emitted light and X-rays.

Surprisingly, the tape did not emit X-rays continuously, but in short bursts -- enough energy to produce an X-ray image of a finger in a second. (A dental X-ray takes about one-third of a second.) The next step is to build a device that brings two pieces of tape together, and then rapidly separates them -- at a rate of around 1000 times per second -- using a piezoelectric device. The idea is control the effect, and hopefully miniaturize it for potential applications.

Something similar happens with cloth tape, which displays a glowing line where the end of the tape is being pulled away from the rest of the roll. Putterman's team has found most brands of clear adhesive tape also give off x-rays, albeit with a different spectrum of energies. Duct tape does not, and they haven't gotten around yet to testing masking tape. Apparently you also get triboluminescence when you tear off the piece of tape at the end of a roll of photographic film.

Given Putterman's prior work on things like sonoluminescence, it's not surprising that when asked about potential applications, his thoughts naturally turn to nuclear fusion. The idea is that energy from the breaking adhesive could somehow -- aye, there's the sticking point! -- be directed away from the electrons to heavy hydrogen ions that would be cunningly implanted into the tape. Those ions would accelerate and (hopefully) collide with sufficient force that they could fuse, emitting energy in the process. But fear not, OSHA! Ripping off a piece of Scotch tape is not exposing us to dangerous radiation on a daily basis in the workplace -- unless you happen to work in a vacuum. Air molecules otherwise intercept the X-rays, rendering them harmless pretty quickly.

A more realistic possibility is finding some way to exploit the phenomenon in simple medical devices to destroy tumors with bursts of x-rays. And like the Illinois researchers, Putterman's UCLA group is eying a potential application to detect x-ray emissions from composite materials as they start to fatigue (i.e., become more prone to cracking); current methods, which work very well with metals, don't always reveal weak areas composites in time to prevent a crisis. Since composite materials are increasingly used to build airplanes. I, for one (in a burst of self-interest), would support any new technology capable of telling ground crews whether or not the plane I'm boarding is likely to break into pieces mid-flight, 37,000 feet in the air. Just sayin'...

So, wintergreen-flavored candies and Scotch tape could be potential energy sources in the future -- bet the manufacturers never saw that coming. Science is weird. And in this case, truly stranger than fiction.

Comments

[Gordon]

Fascinating!

[Pastafarian]

Oh boy! (grabs bowl of popcorn, waits for fireworks)

[Jonathan260]

How about a column on Arthur Clarke's Second Law and famous quote. "Any sufficiently advanced technology is indistinguishable from magick."

[Adrian Morgan]

I've just cast my vote for the 3QD Prize in Science. My vote went to Blake Stacey's essay on "the necessity of mathematics", but I want to say that this article would have been my second preference, if only the voting were based on a preferential system. It's a readable and engaging bit of writing that's full of genuinely fascinating information, but I had to deduct points for generalising too much in the introduction ("one wouldn't expect" - why not?; "it's pretty well known" - among whom?). It might not have got my vote, but let this comment be a consolation prize - it came very close.