09/01/97
By Jeffrey Chuang / The Dallas Morning News
When you bite down on a Wint-O-Green Life Saver, sparks fly. Light actually sparkles from the sugar and flavoring in the candies when teeth crunch them.
For years no one understood why. But now, scientists are deciphering the molecular shapes and symmetries that control the phenomenon, in which brilliant flashes of light appear as certain kinds of crystals are crushed.
With their insights, researchers are building ornate crystals, demolishing them, and then watching all the colors of the rainbow appear.
Some scientists say the sweet little flashes could help explain eerie lights at the bottom of the ocean and the enigmatic process of sonoluminescence, in which bubbles in water uncannily light up as sound waves muscle through them. Others say that the flashing crystals could have applications such as the detection of cracks in materials.
The flashing of light when a material is fractured or deformed is known as triboluminescence, from tribo, meaning "friction," and luminesce, meaning "to emit light." When a triboluminescent material is crunched, crushed, ripped or pulverized, it transforms that jarring mechanical energy into cold, sparking flashes.
"The phenomenon of triboluminescence is of great curiosity," says Arnold Rheingold, a chemist at the University of Delaware.
Triboluminescence is everywhere in the world around us, says Dr. Rheingold. Among the materials that flash when fractured are table sugar, adhesive tape and window glass.
"In a weak way, almost everything does," says Dr. Rheingold. He and Linda Sweeting of Towson University in Towson, Md., have identified the internal structure of rice grain-sized triboluminescent crystals.
Different materials can spark red, blue, green or virtually any other color, says Jeffrey Zink, a professor of chemistry at the University of California in Los Angeles.
"The first time you see it, you say, 'Holy cow!' " says Dr. Rheingold. "You kind of jump back thinking the thing's going to blow up."
In the case of crystals in wintergreen candies, the energy comes from gnashing teeth, but in other materials, the source of mechanical energy is different.
"If you rip off an adhesive bandage in the dark, you can see light emitted," says Dr. Rheingold. "It's excess energy being emitted in the form of light."
The English scholar Francis Bacon observed triboluminescence as he chopped blocks of cane sugar at night in 1605.
But scientists did not have many clues to the puzzle of why certain materials spark under pressure until more than three centuries later. In the 1920s, they noted that the flashes seemed related to the shapes of the molecules in triboluminescent materials.
"One would believe it would have to have something to do with how the atoms are arranged" in crystals, says Dr. Rheingold.
Research was slow in the field, but Dr. Zink and other scientists characterized the shapes of crystals that triboluminesce and the ones that don't in the 1970s.
But those studies gave mixed results, writes Dr. Sweeting, in a paper published in the May issue of the journal Chemistry of Materials.
Namely, some of Dr. Zink's crystals that were not expected to spark actually did spark when mashed. That puzzled scientists.
So Dr. Sweeting synthesized organic molecules known as esters and crystallized them. Then she and several students sequestered themselves in a pitch-black room, mashed the crystals in a transparent container, and watched for sparks.
Meanwhile, Dr. Rheingold took samples of the crystals Dr. Sweeting was using and bounced X-rays off them. By watching the pattern of where the X-rays bounced, he was able to deduce the internal structure of the tiny crystals.
"It's a little like one of those mirrored balls in a dance room," says Dr. Rheingold. If you know where the spots on the wall are and where the spotlight is, you can work backward to tell where all the mirrors are, he says.
"We can't see our mirrored ball because it's too finely structured for us to see ... but we can see the pattern it produces," he says.
Dr. Sweeting compared her findings about which crystals had sparked with the structures that Dr. Rheingold had found.
She saw that almost all of the sparking crystals had a peculiar asymmetry in their structure.
In those crystals, the individual molecules lined up with each other to point like one collective arrow. That was possible because the local arrangement of tiny molecules in the crystal was imbalanced, she said. There was a single pointy side, like the letters A, V or Y.
In the nonsparking crystals, on the other hand, the molecules didn't line up to form an arrow. Those patterns were balanced, says Dr. Rheingold, like the letters O, H or X.
"You can take one end of the letter X and stretch it out to the other side and it's still an X," says Dr. Rheingold.
Dr. Sweeting's work seemed to confirm what many chemists had long suspected - that these molecular crystals had to point in some direction in order to be triboluminescent.
However, as in Dr. Zink's work, there were a few irregularities in Dr. Sweeting's results.
Several of the symmetric crystals flashed when they were crushed, contrary to what she had expected. So Dr. Sweeting did the experiment again after purifying her crystals. This time, the symmetric crystals did not flash.
The work shows that molecular crystals spark only if they are asymmetric or if some impurity is distorting the internal structure, writes Dr. Sweeting. Impurities make a symmetric crystal spark the way an asymmetric crystal would, she writes.
That asymmetry is necessary to create light, says Charles Strouse, a professor of chemistry at UCLA. It allows positive and negative charges in the crystal to separate from each other as the crystal cracks, he says.
But when opposite charges separate, they pull back on each other, like a rubber band. When the rubber band snaps back, light shoots out of the crystal, says Dr. Strouse.
"The charges will recombine like a bolt of lightning," says Dr. Rheingold. The light that comes out is an electrical spark, he says.
Recently, the phenomenon has crackled its way into potentially important roles in two scientific mysteries.
In the late 1980s, marine biologists discovered a strange light coming from sea vents on the floor of the Atlantic Ocean. The researchers say that the light at these hot vents in Earth's crust may have been the evolutionary force behind photosynthesis - the process by which organisms use light to make food - as life got started deep in the ocean.
The light is not the incandescence flatly emitted by hot rock, says Alan Chave, a marine physicist on the research team, from the Woods Hole Oceanographic Institution. One of the possibilities is that the light comes from triboluminescence, he says.
This fall, researchers will descend to the ocean depths to study whether that light is indeed caused by triboluminescence.
Some of the hot compounds that swell up out of these vents, such as zinc sulfide, are known to be strongly triboluminescent, says Dr. Zink, who is advising the Woods Hole team.
"When it hits the cold water, the thermal shock might cause the cracking to cause triboluminescence," says Dr. Zink.
Another mystery that triboluminescence may help unravel is the strange flashing of bubbles in water - sonoluminescence - that physicists first reported in the 1920s.
At special pressures and impurity concentrations, sound waves can make bubbles in water spontaneously gleam with light, says Andrea Prosperetti, a professor of mechanical engineering at Johns Hopkins University.
In a paper published in April in the Journal of the Acoustical Society of America, Dr. Prosperetti proposes that triboluminescence may be the reason bubbles flash.
Under special conditions, even liquid water can crack like a crystal, says Dr. Prosperetti.
"Silly Putty, if you pull it slowly, it flows like honey," says Dr. Prosperetti. But pull it fast enough and you can snap it, he says.
Water is pulled at very high speeds during sonoluminescence, says Dr. Prosperetti, and it snaps just like the putty. When the water snaps, sparks flash - the same way sparks flash when triboluminescent crystals are crushed, he theorizes.
Technological applications for triboluminescence remain scarce. Researchers have used the effect to detect cracks in airplane rotors, says Dr. Zink. And there have been reports of light coming from the ground during earthquakes, says Dr. Zink, so a few people have considered using triboluminescence to measure the strength of seismic disturbances. But for the most part, there have been few spinoffs, he says.
Until there are, candy crunchers and scientists alike can keep watching for the strange light in dark places all over the world.