What is Triboluminescence?
Triboluminescence is tribology phenomenon resulting in the generation of light through creating any frictional interaction between the materials. The term for this phenomenon comes from a combination of the greek word for “rub”, tribein, and the latin word for “light”, lumin. Frictional motion refers to the process of rubbing, scratching, crushing or ripping a material; The process leads to contact electrification by separating and reunifying the electric charges. Although this optical phenomenon is widely researched but still there is a long way to go in order to fully understand it.
The term itself was coined in 1888 by Wiedemann and Schmidt to describe ‘an emission of light not due to rise of temperature which occurs on crushing certain substances’ . Francis Bacon (1620) is considered to be the pioneer observer of triboluminescence effect who put forth his notion stating the sparkling effect of sugar crystals when broken. Following this, Robert Boyle (1663) validated Bacon’s work related to triboluminescence and the sparkling effect upon being rubbed or scratched. In the late 1790’s sugar nips were introduced to break sugar in crystalized chunks noticing the visibility of light. Besides all the researches, glowing barometer which was noticed by Picard (1675) caught the attention of all the theorists revealing the glowing effect on the emptied space which lied above mercury. This historic instance led the researchers to discover static electricity and the potential of low pressure air to depict glowing effect. Further research revealed that the sparkling occurs just at place where the crystals are asymmetric due to certain impurities, which facilitate the charge separation.
How can you observe triboluminescence?
A very famous triboluminescence example is the case with the Wint-O-Green Lifesaver™ ‘spark in the dark’. Triboluminescence results from the fracture of (usually) asymmetrical materials. The fracture separates electrical charges, which recombine and ionize the air. The ionization of nitrogen in the air produces ultraviolet light, which is invisible. Triboluminescence can be observed only when there is a material that absorbs the generated ultraviolet light and then emits it in the visible range (fluoresces). Many other materials exhibit triboluminescence. Regular sugar cubes as well as any candy made with sugar (sucrose). Most adhesive tapes also emit light when they are teared off, such as duct tape. Amblygonite, calcite, feldspar, fluorite,quartz and many more are all minerals known to exhibit triboluminescence when mechanically stressed.
An interesting demonstration of triboluminescence effect can be seen on the video below:
The concept of triboluminescence has been widely explored by the researchers and theorists in the field of physics, biology, technology, mechanics, electronics, energy and manufacturing along with explaining its uses in the respective domains. Construction industry is one of them where most of the research has been conducted on cement based products following multiple aspects to understand its application and benefits. Triboluminescent materials are widely used in the construction industry for crack detection as well as damage assessment using image processing and machine learning protocols. This crack detection seems to be highly significant because it acts as a proactive approach to avoid catastrophic damages. Aich, Appala and Saleh  conducted a research in this domain where Mortar cubes of cement were coated with manganese- doped zinc sulfide intending to observe and record the luminescence to initiate image analysis and quantification of cracks. Results of the study revealed a direct relationship between triboluminescenct concentration and electrization.
In general the triboluminescence effect can be used to design smart structural sensors. These sensors can be used for damage detection and monitoring of civil, aerospace, military structures, spacecraft/structures, and aircraft . TL-based sensor systems have the potential for wireless, in situ, and distributed sensing that can enable real-time continuous monitoring and makes them attractive for several industrial applications. They can also be used as stress, fracture, and damage sensors.
 On Triboluminescence,WILLIAM JACKSON POPE, Nature volume 59, pages 618–619, https://doi.org/10.1038/059618a0
 Aich, N., Appalla, A., Saleh, N. B., & Ziehl, P. (2013). Triboluminescence for distributed damage assessment in cement-based materials. Journal of Intelligent Material Systems and Structures, 24(14), 1714-1721.
 Incorporating strongly triboluminescent europium dibenzoylmethide triethylammonium into simple polymers, Ross S Fontenot, William A Hollerman, Kamala N Bhat, Mohan D Aggarwal & Benjamin G Penn, Polymer Journal volume 46, pages 111–116 (2014), https://doi.org/10.1038/pj.2013.78
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