For several decades people have been playing in the dark with triboluminescence using wintergreen-flavored Lifesavers candy. The idea is to break the hard, donut-shaped candy in the dark. Usually, a person looks in a mirror or peers into a partner's mouth while crunching the candy to see the resulting blue sparks.
How to Make Candy Spark in the Dark
- wintergreen hard candies (e.g., Wint-o-Green Lifesavers
- teeth, hammer, or pliers
You can use any of a number of hard candies to see triboluminescence, but the effect works best with wintergreen-flavored candy because wintergreen oil fluorescence enhances the light. Select a hard, white candy, as most clear hard candies do not work well.
To see the effect:
- Dry your mouth with a paper towel and crunch the candy with your teeth. Use a mirror to see light from your own mouth or else watch someone else chew candy in the dark.
- Place the candy on a hard surface and smash it with a hammer. You can also crush it beneath a clear plate of plastic.
- Crush the candy in the jaws of a pair of pliers
You can capture the light using a cell phone that works well in low light or a camera on a tripod using a high ISO number. The video is probably easier than capturing a still shot.
How Triboluminescence Works
Triboluminescence is light produced while striking or rubbing two pieces of a special material together. It is basically light from friction, as the term comes from the Greek tribein, meaning "to rub," and the Latin prefix lumin, meaning "light". In general, luminescence occurs when energy is input into atoms from heat, friction, electricity, or other sources. The electrons in the atom absorb this energy. When the electrons return to their usual state, the energy is released in the form of light.
The spectrum of the light produced from the triboluminescence of sugar (sucrose) is the same as the spectrum of lightning. Lightning originates from a flow of electrons passing through the air, exciting the electrons of nitrogen molecules (the primary component of air), which emit blue light as they release their energy. Triboluminescence of sugar can be thought of as lightning on a very small scale. When a sugar crystal is stressed, the positive and negative charges in the crystal are separated, generating an electric potential. When enough charge has accumulated, the electrons jump across a fracture in the crystal, colliding with exciting electrons in the nitrogen molecules. Most of the light emitted by the nitrogen in the air is ultraviolet, but a small fraction is in the visible region. To most people, the emission appears bluish-white, although some people discern a blue-green color (human color vision in the dark is not very good).
The emission from wintergreen candy is much brighter than that of sucrose alone because wintergreen flavor (methyl salicylate) is fluorescent. Methyl salicylate absorbs ultraviolet light in the same spectral region as the lightning emissions generated by the sugar. The methyl salicylate electrons become excited and emit blue light. Much more of the wintergreen emission than the original sugar emission is in the visible region of the spectrum, so wintergreen light seems brighter than sucrose light.
Triboluminescence is related to piezoelectricity. Piezoelectric materials generate an electrical voltage from the separation of positive and negative charges when they are squeezed or stretched. Piezoelectric materials generally have an asymmetric (irregular) shape. Sucrose molecules and crystals are asymmetric. An asymmetric molecule changes its ability to hold electrons when squeezed or stretched, thus altering its electric charge distribution. Asymmetric, piezoelectric materials are more likely to be triboluminescent than symmetric substances. However, about a third of known triboluminescent materials are not piezoelectric and some piezoelectric materials are not triboluminescent. Therefore, an additional characteristic must determine triboluminescence. Impurities, disorder, and defects are also common in triboluminescent materials. These irregularities, or localized asymmetries, also allow for an electrical charge to collect. The exact reasons why particular materials show triboluminescence can be different for different materials, but it is probable that crystal structure and impurities are primary determinants of whether or not a material is triboluminescent.
Wint-O-Green Lifesavers aren't the only candies that exhibit triboluminescence. Regular sugar cubes will work, as will just about any opaque candy made with sugar (sucrose). Transparent candy or candy made using artificial sweeteners will not work. Most adhesive tapes also emit light when they have ripped away. Amblygonite, calcite, feldspar, fluorite, lepidolite, mica, pectolite, quartz, and sphalerite are all minerals known to exhibit triboluminescence when struck, rubbed, or scratched. Triboluminescence varies widely from one mineral sample to another, such that it might be unobservable. Sphalerite and quartz specimens that are translucent rather than transparent, with small fractures throughout the rock, are the most reliable.
Ways to See Triboluminescence
There are several ways to observe triboluminescence at home. As I have mentioned, if you have wintergreen-flavored Lifesavers handy, get in a very dark room and crush the candy with pliers or a mortar and pestle. Chewing the candy while watching yourself in a mirror will work, but the moisture from saliva will lessen or eliminate the effect. Rubbing two sugar cubes or pieces of quartz or rose quartz in the dark will also work. Scratching quartz with a steel pin may also demonstrate the effect. Also, sticking/unsticking most adhesive tapes will display triboluminescence.
Uses of Triboluminescence
For the most part, triboluminescence is an interesting effect with few practical applications. However, understanding its mechanisms may help explain other types of luminescence, including bioluminescence in bacteria and earthquake lights. Triboluminescent coatings could be used in remote sensing applications to signal mechanical failure. One reference states that research is underway to apply triboluminescent flashes to sense automobile crashes and inflate airbags.