Today while I was watching a video on youtube by Smarter Every Day, in which the man was shooting bullets on a Prince Rupert’s Drop to see how strong it is and something weird happened, on the site of impact, we could see sparks or bright flashes. This phenomenon is called triboluminescence or mechanoluminescence or fractoluminescence, and is the production of light due to rubbing, shearing, tearing, breaking, or crushing a substance. Substances like hard sugar candies (like polo or wintergreen lifesavers), minerals (opal, quartz, feldspar, calcite) and other common substances like the duct tape. So how does this crazy thing happen?
The first written record of triboluminescence was by English scholar Francis Bacon in 1620 in his Novum Organum, where he quoted,“It is well known that all sugar, whether candied or plain, if it be hard, will sparkle when broken or scraped in the dark.” But it is believed that the Uncomphagre Ute Indians of Colorado used the light from quartz crystals through triboluminescence or mechanoluminescence much before 1620. The word Triboluminescence is made from two parts, tribo- meaning ‘rub’ in latin and luminescence- meaning ‘producing light’. Which clearly explains what it is. But why does it happen? Honesty, NO ONE KNOWS.
There are currently two theories around, to explain this phenomenon, that are probably closest to the real answer. The first one deals with breaking of chemical bonds. According to this theory, when you break a triboluminescent material, like a hard sugar candy, to do so, you break the chemical bonds of the atoms or molecules in it. These bonds act like glue that holds the candy together, and when this glue breaks energy is released. This energy is the order of tens and hundreds of kilocalorie. To put into perspective, one calorie is the amount of heat required to raise temperature of one gram of water through one degree celsius at one atmospheric pressure. This energy excites the electrons in the substance, and when they calm down, they release radiations, and when these radiations are in the visible range for us, we see them as ‘light’. VIOLA! The radiations can be in X-ray range too, like for quartz. The visible light can also vary in colour, depending on its characteristic(wavelength), it can be blue for the polo or lifesaver candies ( these contain a chemical compound called ‘oil of wintergreen’ or methy salicylate, a flavouring agent, whose bond cleavage (or bond breaking; scientists are creative with names) is thought to be the cause of the triboluminescence in the candy), or it can be orange like for Opal, or even white, as teh one you see when you peel off a duct tape.
The second theory, which involves some extra steps, points towards a current due to a net flow of electrons, as the cause of the phenomena. Let me explain. Substances that exhibit triboluminescence are or contain a components that is ‘asymmetric’, meaning it has negative and positive charges that can be separated. When the structure is disturbed, like hammered, rubbed, or peeled off(as in duct tape). the positive and negative charges( the nucleus and the electrons respectively), separate from each other. In the attempt to rejoin with a nucleus, the electrons move or flow towards it due to attraction. Now since electric current is nothing but a flow of electrons, a net flow of electrons in the crystal or substance, causes an electric current. Now while moving in through the air gap created by the crack or fracture, or the air pockets in the minerals, the electrons collide with the molecules of air, (mostly Nitrogen), and impart energy to them, thus exciting them. When these electrons collide with these gas molecules, they slow down. And just like in the previous explanation, when these electrons calm down, they release radiations, which is seen as light when in the visible range.
We don’t know yet whether any of the above explanation is true or not, but nevertheless I found this phenomenon very interesting, and amusing. You can yourselves see this, that too without and fancy equipment. Just hammer some polos, or more appropriately Wintergreen Lifesavers, in a dark room(after your eyes have grown accustomed to the darkness), and you will be able to see tiny blue sparks if you look closely. You can do this to a duct tape too, just pull it off from its roll, and see what happens when you pull it of at different pressures and speeds. But even if you can’t or are too lazy to do it, you can look at the gif posted above from the original Smarter Every Day video!
Triboluminescence has various uses like embedding a triboluminescent substance in concrete, can help know about early fractures and cracks in the structure using light sensors to detect the light. It can also be used to sense earthquakes in a somewhat similar process as above, or to sense when a car crashes.
Physics or in general Science never fails to astonish me, and always has something in store to riddle us. Till the next time, with another science stuff, Happy Reading!
-The Cosmogasmic Person
Cover Picture Credit: http://www.sciencephotography.com