Triboluminescence
Triboluminescence is a phenomenon in which light is generated when a material is mechanically pulled apart, ripped, scratched, crushed, or rubbed. The phenomenon is not fully understood but appears in most cases to be caused by the separation and reunification of static electric charges, see also triboelectric effect. The term comes from the Greek τρίβειν and the Latin lumen. Triboluminescence can be observed when breaking sugar crystals and peeling adhesive tapes.
Triboluminescence is often a synonym for fractoluminescence. Triboluminescence differs from piezoluminescence in that a piezoluminescent material emits light when deformed, as opposed to broken. These are examples of mechanoluminescence, which is luminescence resulting from any mechanical action on a solid.
History
Early scientific reports
The first recorded observation is attributed to English scholar Francis Bacon when he recorded in his 1620 Novum Organum that "It is well known that all sugar, whether candied or plain, if it be hard, will sparkle when broken or scraped in the dark." The scientist Robert Boyle also reported on some of his work on triboluminescence in 1663. In 1675, astronomer Jean-Felix Picard observed that his barometer was glowing in the dark as he carried it. His barometer consisted of a glass tube that was partially filled with mercury. The empty space above the mercury would glow whenever the mercury slid down the glass tube.In the late 1790s, sugar production began to produce more refined sugar crystals. These crystals were formed into a large solid cone for transport and sale. This solid sugar cone had to be broken into usable chunks using a sugar nips device. People began to notice that tiny bursts of light were visible as sugar was "nipped" in low light, an established example of triboluminescence.
Mechanism of action
There remain a few ambiguities about the effect. The current theory of triboluminescence—based upon crystallographic, spectroscopic, and other experimental evidence—is that upon fracture of asymmetrical materials, charge is separated. When the charges recombine, the electrical discharge ionizes the surrounding air, causing a flash of light. Research further suggests that crystals that display triboluminescence often lack symmetry and are poor conductors. However, there are substances which break this rule, and which do not possess asymmetry, yet display triboluminescence, such as hexakisterbium iodide. It is thought that these materials contain impurities, which make the substance locally asymmetric. Further information on some of the possible processes involved can be found in the page on the triboelectric effect.The biological phenomenon of triboluminescence is thought to be controlled by recombination of free radicals during mechanical activation.
Examples
In common materials
Certain household materials and substances can be seen to exhibit the property:- Ordinary pressure-sensitive tape displays a glowing line where the end of the tape is being pulled away from the roll. Soviet scientists observed in 1953 that unpeeling a roll of tape in a vacuum produced X-rays. The mechanism of X-ray generation was studied further in 2008. Similar X-ray emissions have also been observed with metals.
- Opening an envelope sealed with polymer glue may generate light that can be viewed as blue flashes in darkness.
- When sugar crystals are crushed, tiny electrical fields are created, separating positive and negative charges that create sparks while trying to reunite. Wint-O-Green Life Savers work especially well for creating such sparks, because wintergreen oil is fluorescent and converts ultraviolet light into blue light.
Triboluminescence as a biological phenomenon is observed in mechanical deformation and contact electrification of epidermal surface of osseous and soft tissues, during chewing food, at friction in joints of vertebrae, during sexual intercourse, and during blood circulation.
Water jet abrasive cutting of ceramics creates a yellow/orange glow at the point of impact of very high-speed flow.
Chemicals notable for their triboluminescence
- Europium tetrakis triethylammonium emits particularly bright red flashes upon the destruction of its crystals.
- Triphenylphosphinebisthiocyanatocopper emits a reasonably strong blue light when crystals of it are fractured. This luminescence is not as extreme as the red luminescence; however, it is still very clearly visible to the naked eye in standard settings.
- N-acetylanthranilic acid emits a deep blue light when its crystals are fractured.
Fractoluminescence
Fractoluminescence is often used as a synonym for triboluminescence. It is the emission of light from the fracture of a crystal, but fracturing often occurs with rubbing. Depending upon the atomic and molecular composition of the crystal, when the crystal fractures, a charge separation can occur, making one side of the fractured crystal positively charged and the other side negatively charged. Like in triboluminescence, if the charge separation results in a large enough electric potential, a discharge across the gap and through the bath gas between the interfaces can occur. The potential at which this occurs depends upon the dielectric properties of the bath gas.EMR propagation during fracturing
The emission of electromagnetic radiation during plastic deformation and crack propagation in metals and rocks has been studied. The EMR emissions from metals and alloys have also been explored and confirmed. Molotskii presented a dislocation mechanism for this type of EMR emission. In 2005, Srilakshmi and Misra reported an additional phenomenon of secondary EMR during plastic deformation and crack propagation in uncoated and metal-coated metals and alloys.EMR during the micro-plastic deformation and crack propagation from several metals and alloys and transient magnetic field generation during necking in ferromagnetic metals were reported by Misra, which have been confirmed and explored by several researchers. Tudik and Valuev were able to measure the EMR frequency during tensile fracture of iron and aluminum in the region 100 THz by using photomultipliers. Srilakshmi and Misra also reported an additional phenomenon of secondary electromagnetic radiation in uncoated and metal-coated metals and alloys. If a solid material is subjected to stresses of large amplitudes, which can cause plastic deformation and fracture, emissions such as thermal, acoustic, ions, and exo-emissions occur.