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.
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 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 and 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 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 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.