Triboluminescence is a type of luminescence where a material produces light from friction, crushing, or other tearing. The word comes from Greek and Latin words, essentially meaning “light from rubbing”. Simple examples of triboluminescence include blue light from crushing a wintergreen candy or yellow light from rubbing quartz crystals together.
Take a closer look at how triboluminescence works and explore easy ways you can see it.
How Triboluminescence Works
There are a few different ways a material produces light when it’s torn or broken. First, the mechanical stress gives electrons energy. When excited electrons return to a more stable state, they release light. Another mechanism is that the action separates electrical charges, which release light when they are reunited. A third possibility is that the stress generates an electrical current that ionizes molecules (such as trapped gas), making them glow.
While friction, tearing, and crushing release some heat, the light does not come from incandescence. So, another name for triboluminescence is cold light.
The flashes of light come in a variety of colors. The most common colors are blue, white, yellow, orange, and red. Sometimes the emitted light extends beyond the visible spectrum. For example, triboluminescence sometimes releases ultraviolet light or x-rays. Higher energy light sometimes activates phosphorescent or fluorescent compounds, producing visible colored light.
Triboluminescence vs Piezoluminescence
A related phenomenon is called piezoluminescence. In piezoluminescence, light results from deformation rather than fracturing. For this reason, another name for triboluminescence is fractoluminescence. Both phenomena are types of mechanoluminescence, which is light resulting from mechanical action. Many materials are both triboluminescent and piezoluminescent. Usually, the molecules or crystals contain impurities, have an asymmetric shape, or display other irregularities that allow for charge separation and collection.
Examples of Triboluminescence
If you want to see triboluminescence for yourself, you have a lot of options. Explore these examples of triboluminescence in a dark or dimly lit room:
- Sugar and Wint-O-Green Lifesavers: Crushing a sugar cube or hard candy releases blue light. In this case, the result is miniature lightning. Breaking sucrose crystals separates positive and negative charges. When enough charge accumulates, a tiny static electric charge results, ionizing nitrogen in air. The nitrogen then emits ultraviolet and blue light. Wint-O-Green Lifesaver candies glow especially well because the methyl salicylate (wintergreen flavor) is fluorescent. So, the ultraviolet light from triboluminescence excites electrons in methyl salicylate, making even more blue light.
- Ice: Cracking an ice cube tray fresh out of the freezer releases light. Much of this light is ultraviolet, but there is also a blue glow.
- Tape: Rapidly pulling away a piece of duct tape or regular sticky tape (e.g., Scotch tape) produces a blue glow. Other adhesives make light, too. Sealed envelopes, Band-Aid™ wrappers, and friction tape make a glowing line as they pull away from a surface. Tape also release x-rays, but air absorbs most of them.
- Quartz Crystals: Rubbing quartz crystals together produces yellow light. The Uncompahgre Ute put quartz crystals in rawhide rattles to flash lights when shaken in the dark. You can use any large chunks of quartz (including rose quartz or amethyst). Walking on very dry sand at night similarly releases light.
- Tiles: Water jet cutting of ceramic tile releases a yellow or orange light.
- Diamonds: Although not a sight you’ll probably experience, faceting some diamonds produces red or blue light.
Biological tissues also display triboluminescence, even though you can’t see it. Chewing food, moving vertebrae, and blood circulation all release a bit of triboluminescent light.
Triboluminescence chemical compounds include europium tetrakis (dibenzoylmethide)triethylammonium (bright red), N-acetylanthranilic acid (deep blue), and triphenylphosphinebis(pyridine)thiocyanatocopper(I) (bright blue).
Minerals That Display Triboluminescence
Quartz is not the only mineral that displays triboluminescence. Geologists estimate around 50% of crystalline minerals are triboluminescent. For example, here is a small selection of the many minerals that produce light:
- Opal (sometimes)
In most cases, these minerals produce orange, yellow, or white light.
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