Triboluminescence

Triboluminescence is an optical phenomenon in which light is generated through the breaking of chemical bonds in a material when it is pulled apart, ripped, scratched, crushed, or rubbed (see tribology). The phenomenon is not fully understood, but appears to be caused by the separation and reunification of electrical charges. The term comes from the Greek τρίβειν ("to rub"; see tribology) and the Latin lumen (light). Triboluminescence can be observed when breaking sugar crystals and peeling adhesive tapes.

Triboluminescence is often used as a synonym for fractoluminescence (a term sometimes used when referring only to light emitted from fractured crystals). Triboluminescence differs from piezoluminescence in that a piezoluminescent material emits light when it is deformed, as opposed to broken. These are examples of mechanoluminescence, which is luminescence resulting from any mechanical action on a solid.

History

File:UteQuartzRattle.jpg

An Uncompahgre Ute Buffalo rawhide ceremonial rattle filled with quartz crystals. Flashes of light are visible when the quartz crystals are subjected to mechanical stress in darkness.

Uncompahgre Ute Indians

The Uncompahgre Ute Indians from Central Colorado are one of the first documented groups of people in the world credited with the application of mechanoluminescence involving the use of quartz crystals to generate light. The Ute constructed special ceremonial rattles made from buffalo rawhide which they filled with clear quartz crystals collected from the mountains of Colorado and Utah. When the rattles were shaken at night during ceremonies, the friction and mechanical stress of the quartz crystals impacting together produced flashes of light visible through the translucent buffalo hide.

Later descriptions

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 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 cone of sugar had to be broken into usable chunks using a device known as sugar nips. People began to notice that as sugar was "nipped" in low light, tiny bursts of light were visible.

A historically important instance of triboluminescence occurred in Paris 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. Whenever the mercury slid down the glass tube, the empty space above the mercury would glow. While investigating this phenomenon, researchers discovered that static electricity could cause low-pressure air to glow. This discovery revealed the possibility of electric lighting.

Mechanism of action

Materials scientists have not yet arrived at a full understanding of the effect, but 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 electric discharge ionizes the surrounding air, causing a flash of light. Research further suggests that crystals which display triboluminescence must lack symmetry (thus being anisotropic in order to permit charge separation) and be poor conductors. However, there are substances which break this rule, and which do not possess asymmetry, yet display triboluminescence anyway, such as hexakis(antipyrine)terbium iodide. It is thought that these materials contain impurities, which confer properties of asymmetry to the substance.

The biological phenomenon of triboluminescence is conditioned by recombination of free radicals during mechanical activation.

Examples

A diamond may begin to glow while being rubbed. This occasionally happens to diamonds while a facet is being ground or the diamond is being sawn during the cutting process. Diamonds may fluoresce blue or red. Some other minerals, such as quartz, are triboluminescent, emitting light when rubbed together.

Ordinary Pressure-sensitive tape ("Scotch tape") displays a glowing line where the end of the tape is being pulled away from the roll. In 1953, Soviet scientists first observed that unpeeling a roll of tape in a vacuum produced X-rays. The mechanism of X-ray generation was studied further in 2008.

Also, when sugar crystals are crushed, tiny electrical fields are created, separating positive and negative charges that then create sparks while trying to reunite. Wint-O-Green Life Savers work especially well for creating such sparks, because wintergreen oil (methyl salicylate) is fluorescent and converts ultraviolet light into blue light.

Triboluminescence is biological phenomen observed in mechanical deformation and contact electrization of epidermal surface of osseous and soft tissues, at chewing food, at friction in joints of vertebrae, during sexual intercourse, and during blood circulation.

Fractoluminescence

Fractoluminescence is often used as a synonym for triboluminescence. It is the emission of light from the fracture (rather than rubbing) 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 (EMR) during plastic deformation and crack propagation in metals and rocks have 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 emissions. Recently, Srilakshmi and Misra reported an additional phenomenon of secondary EMR during plastic deformation and crack propagation in uncoated and metal-coated metals and alloys.

Theory

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 (1973–75), which have been confirmed and explored by several researchers. Tudik and Valuev (1980) were able to measure the EMR frequency during tensile fracture of iron and aluminum in the region 1014¬¬ Hz by using photomultipliers. Srilakshmi and Misra (2005a) 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, exo-emissions occur. With the discovery of new materials and advancement in instrumentation to measure effects of EMR, crack formation and fracture; the EMR emissions effect becomes important.

Deformation induced EMR

The study of deformation is essential for the development of new materials. Deformation in metals depends on temperature, type of stress applied, strain rate, oxidation and corrosion. Deformation induced EMR can be divided into three categories: effects in ionic crystal materials; effects in rocks and granites; and, effects in metals and alloys. EMR emission depends on the orientation of the grains in individual crystals since material properties are different in differing directions. Amplitude of EMR pulse increases as long as the crack continues to grow as new atomic bonds are broken and it leads to EMR. Pulse starts to decay when crack halts. Observations from experiments showed that emitted EMR signals contain mixed frequency components.

Test methods to measure EMR

Most widely tensile test method is used to characterize the mechanical properties of materials. From any complete tensile test record, one can obtain important informations about the material’s elastic properties, the character and extent of plastic deformation, yield and tensile strengths and toughness. These informations which can be obtained from one test justifies the extensive use of tensile test in engineering materials research. Therefore, investigations of EMR emissions are mainly based on the tensile test of the specimens. From experiments, it can be shown that tensile crack formation excites more intensive EMR than shear cracking, increasing the elasticity, strength and loading rate during uniaxial loading increases amplitude. Poissions ratio is a key parameter for EMR characterization during triaxial compression. If the poissions ratio is lower, it is harder for the material to strain transversally and hence higher is the probability of new fractures. Mechanism of plastic deformation is very important for safe operation of any component under dynamic conditions.

Uses and applications

This EMR can be utilized in developing sensors/smart materials. This technique can be implemented in powder metallurgy technique also. EMR is one of these emissions which accompany large deformation. If an element can be identified which gives maximum EMR response with minimum mechanical stimulus then it can be incorporated into main material and thus set new trends in the development of smart material. The deformation induced EMR can serve as a strong tool for failure detection and prevention.

Orel V.E. invented the device to measure EMR whole blood and lymphocytes in laboratory diagnostics.

See also

• Piezoelectricity
• Earthquake light
• List of light sources
• Sonoluminescence

References

• Martín Gil Jesús and Martín Gil Francisco J. "Triboluminescence of new uranyl salts", Journal of Chemical Education, 1978, 55(5):340.
• Page on "Wintergreen Candy and Other Triboluminescent Materials"; from Towson University, Department of Chemistry; by Dr. Sweeting.
• 1997; article from science news explaining Dr. Sweeting's work.

External links

• 2006 article Sound Science Behind Glowing Sugar; on PhysicsWeb.
• Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape. Nature 455, 1089-1092 (23 October 2008) | doi:10.1038/nature07378