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hubie writes:

Mechanoluminescence, also known as triboluminescence, is the production of light as the result of mechanical action taken on a solid, such as by squeezing or bending it. It was first discovered in 1605 by Sir Francis Bacon when he saw light given off as he was scraping a lump of sugar with a knife (and you may have seen it yourself if you've seen the green spark given off when biting into a Wint-O-Green Lifesaver in the dark). If you stress a solid elastically, meaning that it recovers its original shape when you remove the stress, and it gives off light, this is known as elasto-mechanoluminescence. There are very few materials known with this behavior and they are a topic of interest because there are some useful things one could imagine making with such a material, such as visual stress sensors and other specialty lighting.

Researchers in France found that a certain kind of glass containing mechanoluminescent crystalline particles not only gives off light when subject to mechanical stress, but it behaves in a manner suggestive of a photonic sponge. If the glass is initially charged with exposure to UV light, it emits green light as a stress is applied to it. When the applied pressure stops changing and reaches a steady-state, the light disappears. The authors call this a photonic sponge because it is analogous to after one fills a sponge with water, the water only comes out while you are squeezing it to wring it out. What was also surprising in their results was that as they removed the stress, additional light was emitted as the stress was released. The total amount of light emitted didn't depend upon the total static force applied, but rather on the change in the force, known as the deviatoric part of the stress. They used a Li,Na metaphosphate glass embedded with SrAl₂O₄ particles. Provided with the paper is a video showing the experiment in action.

Dubernet, M., Bruyer, E., Gueguen, Y. et al. Mechanics and physics of a glass/particles photonic sponge. Sci Rep 10, 19495 (2020). (DOI: 10.1038/s41598-020-75504-9)

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