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September 10, 1997, Wednesday

SECTION: LIFESTYLE; Ed. 1,2,3,6,7,8; Pg. E-2

LENGTH: 766 words

HEADLINE: Solving riddle of flashing crystals

SERIES: QUEST

BYLINE: Jeffrey Chuang; THE DALLAS MORNING NEWS

BODY:
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. When a triboluminescent material is crunched, crushed, ripped or pulverized, it transforms that mechanical energy into cold, sparking flashes.

"The phenomenon of triboluminescence is of great curiosity," says Arnold Rheingold, a chemist at the University of Delaware in Newark, Del.

Among the materials that flash when fractured are table sugar, adhesive tape and window glass.

"In a weak way, almost everything does," says 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. "The first time you see it, you say, Holy cow!' " says 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 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 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 Rheingold.

Research was slow in the field, but scientists characterized the shapes of crystals that triboluminesce and the ones that don't in the 1970s.

But those studies gave mixed results, Sweeting wrote in a paper published in the journal Chemistry of Materials.

So 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, Rheingold took samples of the crystals 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 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.

Sweeting compared her findings about which crystals had sparked with the structures that Rheingold had found.

She saw that almost all of the sparking crystals had a peculiar asymmetry in their structure. 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.

The work shows that molecular crystals spark only if they are asymmetric or if some impurity is distorting the internal structure, says Sweeting. It allows positive and negative charges in the crystal to separate from each other as the crystal cracks.

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.

"The charges will recombine like a bolt of lightning," says Rheingold. The light that comes out is an electrical spark.

LANGUAGE: ENGLISH

LOAD-DATE: September 11, 1997