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I was interested to see this paper, which goes into detail on a chemical phenomenon that many have seen but no one understands very well: triboluminescence. That word meets with a blank stare or instant recognition; there’s not much in between. It means “emission of light when a material is physically broken”, and that encompasses fracturing, scratching, rubbing, and so on.

Many people have noticed this with sugar crystals (Francis Bacon referred to this as “well known” in his Novum Organum back in 1620), and the classic modern example is with wintergreen-flavored candies such as Life Savers. In that case, it’s the sugar that’s giving off the light, and the methyl salicylate that’s fluorescing in response to it, making the whole process easier to see. Often, the light seen in these situations is from similarly excited nitrogen molecules in the air around the crystal, but the molecules of the solid itself can emit in the visible range directly. I should note that triboluminiscence doesn’t have to be in the visible or even ultraviolet range: a famous example is the small-but-detectable emission of X-rays by peeling Scotch tape (in a vacuum, mind you).

There are a lot of materials besides sugar that will do the trick, of course, quartz being perhaps the second most well-known. Some years ago, I had an aromatic amide compound in my lab that turned out to be triboluminescent, which I discovered by accident while scraping the stuff out of a round bottom flask. I wondered at the time what the structural features might be that led to the effect, and thanks to this paper I know more about that now. One thing I didn’t realize is that N-aryl phthalimides are known as a class to be prone to this, and that might have been close to what I was seeing.

The biggest factor is that the crystals have to be in a non-centrosymmetric space group. (You wouldn’t think that we’d get from wintergreen candy to space groups so quickly, but that’s the chemistry of materials for you). Of the seven crystal systems, all of them have one or more space groups with an inversion center in them, and those will not show triboluminescence or the related phenomena of piezoelectricity.There are some apparent exceptions in the literature, but those are thought to be due to impurities leading to local symmetry-breaking. When non-centrosymmetric crystal packing is broken, the newly formed surfaces can end up with notable electric charges on them, and that’s where all of this starts off. The complications come in with the various crystal-packing interactions and orientations ( through stacking, hydrogen bonding, etc.), the conformational freedom of the molecules in the lattice, the electronic structure of the compounds themselves, and so on, which give you a huge variety of possibilities, all the way from “doesn’t emit a thing because the energy all goes somewhere else” to “glows so brightly you’ll drop the flask”.

The authors of this new review note that there are a lot of potential applications for triboluminescence, but our lack of understanding of how to engineer it in a more systematic way has held it back. They say that “it would not be surprising if a sudden expansion of their potential applications in organic electronics (e.g. sensors, smart materials etc.) was to occur in the very near future“, and that would be fun to watch. On several levels!


24 comments on “Triboluminescence”

  1. navarr says:

    I’m not sure why, but the controls for the video are obscuring parts of the text. I using the current version of Chrome.

    1. navarro says:

      Okay, now the video is no longer on top. Please remove my previous comment along with this one. Thanks.

      1. Derek Lowe says:

        Trying to get that in there correctly, but until it works, I’m removing it. . .

  2. David Challener says:

    There was a company that used this effect to make an XRF – there were modules that were plugged in and used triboluminescence to create the x-rays. Unfortunately they appear to have gone out of business. See:

  3. Nick K says:

    Are Lifesavers the same as Polo Mints in the UK?

  4. It’s alleged that frozen pumpkin displays the phenomenon, but experimental verification seems to be lacking.

    1. John Wayne says:

      Challenge accepted

    2. Hap says:

      Apparently not enough people freeze their pumpkins before dropping them off of large buildings. At least one dorm where I went to school did it during Rush every year – I wonder if they would freeze some for dropping. For Science, of course.

  5. OldLabRat says:

    There’s another easy way to see the phenomenon. The adhesive used on the sterile packaging of the adhesive bandages from the company that owns the Scotch tape trademark will show a blue light as the two sides are peeled apart. Do it in a very dark room. If the wrapper isn’t completely opened, you can squeeze it back together, wait a moment and repeat quite a few times. Even kids get a kick out of it.

    1. Gordonjcp says:

      Same with the tape around old 16mm cine film cans, not that you’d find that anywhere these days.

  6. Stephen says:

    Its one of these fun phenomena that will probably never find any practical use – I’ve refereed a few papers on the subject and still struggle to think what to do with it.

    1. Scott says:

      A damage sensor comes to mind for a practical use. Say you have an aircraft wing. Most of the inside is going to be sealed to light. If you put a small camera into each space, you can look for sudden additions of light.

    2. Barry says:

      Although I struggle to see any use for it, George Whitesides has found uses for what others dismiss as useless throughout his distinguished career.

  7. eyesoars says:

    Triboelectric discharge is likely what was responsible for the rather spectacular explosion of the Amos-6 rocket during a refueling exercise in 2016. Frozen oxygen is known to be triboelectric, and LOX apparently froze when the carbon fiber helium pressure bottles in the propellant tanks were loaded with supercritical helium. When carbon fiber (a suitable fuel) crushed oxygen crystals in the helium tanks… boom.

    1. Chris Phoenix says:

      The (some? all?) helium bottles in the Falcon 9 rocket are inside the liquid oxygen tank, and are immersed in liquid oxygen. The Falcon 9 uses densified propellant – cooled below the boiling point. The carbon fiber overwrap of the tank was apparently a bit porous / void-y. I don’t know if the helium was cooled below solid-Ox temperature, or if it was a combination of very cold LOX and pressure, that apparently solidified the LOX that seeped into the winding of the He pressure vessel.

      So the LOX got in, and the tank was (normally) stressed while fueling, and the LOX solidified. I had not heard the triboelectricity theory – I had seen speculation that that SOX crystals caused some carbon fiber strands to break, which could have released enough heat from the recoil to ignite when immersed in LOX. But now a web search for “falcon amos triboelectric” finds articles from 2016 that propose triboelectricity.

      Either way, it was quite a shame.

      1. eyesoars says:

        To my surprise, the helium loaded into the tanks was supercritical (transcripts of the loading process note this), well below the freezing point of oxygen, and this was done because it can be loaded much faster than warm helium.
        Though details are scant, the public version of the released incident report noted that the explosion could be reproduced in a test rig and that changing the ‘loading conditions’ could prevent them from happening. At a guess, that was by loading warm helium so that no solid oxygen formed during loading.

        1. zero says:

          Supercritical helium has the unusual property of cooling as the pressure increases, at least within a certain relevant regime. That means the fast load procedure actually caused further cooling of the helium, potentially dropping low enough to hit solid O2 temps on the outside of the bottle.

          This appears to be the accepted explanation for cause of SOX formation. Once formed there were several mechanisms for oxygen crystals to have caused ignition.

  8. Chris Phoenix says:

    If triboluminescence is related to piezoelectricity, that could be very useful. The main piezoelectric material used today in the electronics industry is PZT, where the P stands for lead. They actually got an exception from the lead-free RoHS standard because it’s the only thing that works well. (just search for “rohs pzt”)

    If better piezoelectric systems can be developed, then we can stop including lead in a variety of components. Also, piezoelectrics are useful already in speakers, motion/strain sensors, and specialized actuators (including scanning probe microscopes) – better ones could be more broadly useful.

    1. eub says:

      And seems like triboluminescence itself could be useful as part of a sensor — to transduce from mechanical behavior into light, which then can be turned into an electronic signal.

      Triboluminescent mechanical actuators would be fun if that can be arranged…

      1. Nameless says:

        But the sensor would be not reuseable. To emit light the structure hast to be changed (damaged). A self repairing gizmo seems to be out of reach of current technology.

        1. eub says:

          Does it necessitate a dissipative change then, not a slip or other reversible shift? I have to admit I didn’t understand the explanation of “where the energy comes from”.

  9. Dominic Ryan says:

    I’m sure many of you have seen this but it seems relevant to this thread. They exploit a piezoelectric effect from a lithium niobate crystal to produce VLF radio waves with a very short (10cm) antenna (instead of sizes that can be huge). There is a link to but that seems not to be working this morning.

  10. Simon Auclair says:

    Shake uranyl nitrate crystals in the dark for yellow green flashes.

    Be sure to say “Behold the Power of the Atom!”

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