Spark in the Dark

Lock yourself in a closet, wait a few minutes until your eyes adjust to the dark, then look in a mirror and bite down hard on a Wint-O-Green Life Saver with your mouth open. For a split second a tiny blue-white flash crackles between your teeth, like lightning in a jar. It isn’t a trick of your eyes or a bit of candy-aisle magic. You’ve just set off a small electrical storm, and the light it gives off has a name almost nobody can pronounce on the first try: triboluminescence.

The word comes from the Greek tribein, “to rub,” plus the Latin lumen, “light”: light made by friction, crushing, or tearing. Once you know it’s there, it turns up in some surprising places, from 17th-century sugar bowls to Ute ceremonial rattles to a roll of office tape that, in the right conditions, can produce enough X-rays to image a human finger.

A spark noticed for centuries

People were noticing these sparks long before anyone could explain them. In 1605 the English philosopher Francis Bacon wrote that hard sugar “will sparkle” when it is broken or scraped in the dark, one of the earliest written descriptions of the effect, as the history of triboluminescence records. In 1663 Robert Boyle, better known for his gas law, reported something similar.

Others got there earlier still. The Uncompahgre Ute people of what is now Colorado made ceremonial rattles from buffalo rawhide and filled them with clear quartz crystals collected in the mountains. When the rattle was shaken at night, the crystals ground against one another and the translucent hide glowed from inside with faint flashes. It may be the oldest known human use of light made by crushing crystals.

What’s actually happening when you crunch

Triboluminescence still isn’t fully understood. It’s much easier to watch than to model, and the mechanism varies from one material to the next. For sugar, though, the basic story is well established, largely thanks to chemist Linda M. Sweeting of Towson University, who spent years studying the effect and wrote a clear explanation of the Wint-O-Green flash.

Sugar is a crystal. When a sugar crystal cracks, the two new faces don’t split their electric charge evenly: one side ends up with extra positive charge and the other with extra negative charge. Across a gap that narrow, the voltage is big enough to tear electrons out of the air in the crack. The electrons slam into nitrogen molecules, which absorb the energy and give it back as light. It’s the same thing that happens in a lightning bolt, just shrunk down to the width of a crack in a candy.

There’s a catch. Most of the light that excited nitrogen gives off is ultraviolet, which our eyes can’t see. Crush a plain sugar cube or a peppermint in the dark and you’ll get only a faint, bluish glimmer, made by the small part of the nitrogen glow that falls in the visible range.

Why Wint-O-Green is the star

The wintergreen flavor is what makes Wint-O-Green special. It comes from methyl salicylate, the same compound found in oil of wintergreen, and methyl salicylate is fluorescent. It soaks up ultraviolet light and gives it back as visible blue light. So when the sugar cracks and the nitrogen gives off mostly invisible UV, the flavoring around it turns much of that UV into a blue you can see. The result is a flash many times brighter than plain sugar can make. Scientific American’s “Candy Lightning” activity walks through the same two steps: a discharge, then fluorescence.

A few practical tips if you want to try it. Let your eyes adjust to total darkness for several minutes first. Crunch hard and fast, because the crack is what makes the light, so slowly dissolving the candy won’t work. Have a friend chew in front of a mirror, or crush the candy with pliers instead of your teeth. And make sure the candy is a classic one made with real sugar. Sugar-free mints are made from sugar alcohols that don’t form the same kind of crystals, and they tend to disappoint. Humidity matters too. Damp candy discharges less, so a dry winter night tends to give the best show.

Sticky tape and a burst of X-rays

If crunching candy makes visible light, what about peeling something apart? Pull a strip of ordinary clear tape off its roll in a dark room and you can sometimes see a faint blue glow along the line where the tape leaves the roll. The physics is related. As the adhesive separates, charge builds up on the two surfaces, and when it discharges, light comes out.

In 2008 a team at UCLA took this much further. Physicists Carlos Camara, Juan Escobar, Jonathan Hird and Seth Putterman built a machine that unrolled ordinary Scotch tape at about 3 centimeters per second inside a vacuum chamber. With the air pumped out, the discharges no longer fizzled in a thick gas. Electrons were accelerated across the gap with enough energy to give off X-rays when they hit the tape. The bursts lasted around a billionth of a second each, and they were strong enough that the team put a finger in the beam and made an X-ray image of the bone. The work was published in Nature, and Nature‘s news team covered the result under the delightfully blunt headline “Sticky tape generates X-rays.”

The finding wasn’t entirely new. Soviet scientists had reported X-rays from peeling adhesive tape in a vacuum back in 1953, a result that drew little attention in the West. The UCLA experiment confirmed it with modern detectors and linked the X-ray bursts to the tape’s jerky “stick-slip” motion as it peels. Don’t worry about your desk drawer, though. In normal air the effect stays tame: the gas around the tape absorbs the energy long before anything like an X-ray can form.

The discovery wasn’t just a curiosity. The team pointed out that charge separated by peeling could someday power cheap, portable X-ray sources that need no high-voltage supply, and Putterman went on to co-found a company to develop the idea.

Where else the dark sparkles

Once you start looking, triboluminescence turns up in all kinds of everyday places:

• Quartz. Rub or strike two quartz pebbles together in the dark and they light up with a warm orange glow, the same effect the Ute rattles used. Rock hounds sometimes show it off on camping trips.
• Envelopes and bandages. The flash of an old lick-to-seal envelope being ripped open, or a bandage wrapper being peeled apart, is caused by charge separating as an adhesive pulls away.
• Sugar cubes and some medicine crystals. Many crystalline solids, including some pharmaceutical compounds, flash when crushed. Researchers have found that the effect often depends on tiny defects and asymmetries in how the crystal is packed. A recent study highlighted on EurekAlert! found that structural irregularities let many more materials flash under pressure than once thought.

Engineers have also started putting the effect to work. Because some triboluminescent materials flash when they crack, researchers have proposed building them into coatings and composites as built-in damage sensors. In theory, a bridge girder or an aircraft panel could literally light up where it’s starting to fail.

A small mystery you can hold in your hand

What makes triboluminescence charming is that it’s at once trivial and deep. A child can set it off with a candy and a mirror. Yet scientists still argue over the details of how charge builds up when solids crack, why some crystals flash and others don’t, and how much energy those tiny discharges can really pack. Museums have caught on: the Oregon Museum of Science and Industry even publishes a do-it-yourself activity sheet built around this same experiment.

So the next time a power cut leaves you sitting in the dark, don’t just wait for the lights to come back. Find a roll of tape, a couple of quartz pebbles, or a bag of the green-wrapped mints. Let your eyes adjust and crunch down. The spark you see is physics older than candy, older than tape, and as old as cracking crystals themselves.

Featured image: a quartz pebble in normal light (left) and glowing by triboluminescence in the dark (right). Photo: Yaroslav Ulyushin, Wikimedia Commons, CC BY 4.0.

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