Tuesday, October 30, 2018

Smashing (Frozen) Pumpkins: Celebrate Halloween with Triboluminescence!

Pumpkins have a very important role this time of year. Not only can they be used as decorations, jack-o-lanterns, and in pumpkin pies, but we can use them for some unique autumnal science, to demonstrate the phenomenon of triboluminescence: when solid objects emit light under physical strain. This isn’t like putting a candle inside a jack-o-lantern, mind you—the pumpkin itself can glow...but only for a split second, before it stops being a pumpkin altogether.

As part of an annual tradition at Rhodes college and other universities around the world, pumpkins are frozen solid with liquid nitrogen, and then dropped from the top of the physics building to shatter at the feet of the crowd below. Why do we drop frozen pumpkins? They have to shatter, rather than splatting, in order to see this weird physics phenomenon in action. It's not a bright flash—which is why the whole ordeal takes place at night—but if you watch closely enough, they say you can see the light! There are other ways to see triboluminescence, like with tape, candy, and even by hitting the right kind of rocks together—but watching pumpkins fall from the top of a physics building is really fun.

So what is triboluminescence? It’s the light produced when a solid object—or anything held together by chemical bonds—bends or breaks. The pumpkin breaking is due to mechanical stress, which comes from the object is pushed, pulled, squeezed, or physically manipulated. Mechanical stress on objects with the right kind of structure can produce an electric field, a property called piezoelectricity that's closely related to triboluminescence.

Not all solid objects produce light when they break; they have to be asymmetrical at a molecular scale. Think about crystals: a crystal might be symmetrical in one direction, but not in others. Imagine you've got a bar magnet, with north and south poles, that's flexible enough to fold. You can fold it either lengthwise—so the north pole stays near the north pole—or crosswise, so the opposite sides end up next to each other: the magnet is symmetrical along one axis, but not along the other. In a similar way, we could say that there's a “north” and a “south” in the structure of a crystal. When a break in the structure separates the "north" end from the "south", electrons find themselves jolted free from their home atoms by the fracturing of the material and the electric field it generates. These energetic electrons eventually settle back down and reattach to an atom, but to do so they have to be less energetic—which means releasing some energy in the form of light!

How do pumpkins come into this? Although they don't taste very sweet on their own, pumpkins are full of sugars that can solidify into just the right kind of crystal structure for triboluminescence. But being wet and squishy, pumpkins aren’t great for triboluminescence without some help—that’s why we freeze them. An ordinary freezer would work fine, but liquid nitrogen can solidify the pumpkin in minutes, rather than the hours that it would take otherwise, making the phase transition seem almost like a magic trick.

Once a pumpkin is frozen with liquid nitrogen, you definitely can’t carve it anymore; all of the pulp that you take out when you carve a jack-o-lantern becomes solid like a popsicle. Rather than smashing into a goo, frozen pumpkins shatter like glass when they fall. If frozen properly, the pulp that shatters will release electrons with a flash of light! It's only when something shatters that chemical bonds are broken in a sudden, sharp way. Since that's what produces triboluminescence, the integrity of the solid that's broken is important—the stringy, sticky splatter of a thawed pumpkin won't do!

There is a much easier way to go about getting triboluminescence without waiting until Halloween and getting liquid nitrogen, though. Wint-O-Green Lifesaver mints produce triboluminescence when you crush them; you can see this when your friend chews one with their mouth open. Similarly to the pumpkins, the sugar molecules inside the candy release light when they are compressed or pulled apart. You can also find triboluminescence when you pull tape off the roll! The chemical bonds that make tape sticky are strong enough to produce light when they break.

Striking pieces of quartz together can also produce triboluminescence, as fractures form in the crystal. Quartz isn't hard to find—a few minutes of hunting turned up these pieces at the side of a creek near our offices!
Image Credit: APS Physics
It's not bright enough to be seen in the light, but in a darkened room the flashes are easy to see!

The key to triboluminescence is the change in charge between the air and the pumpkin, crystal, candy, or tape. And this can have some useful properties. For example, Dr. Ross Fontenot and his team at Alabama A&M have found a triboluminescent material bright enough to be seen in daylight, which they hope to use for smart sensors. This would allow for real-time warnings of structural damages to infrastructure by producing light! If a new bridge is being built and it has this triboluminescent material (EuD4TEA) was embedded into the structure, it would produce light signaling that there was a problem. How cool would it be if the same phenomenon that illuminates frozen pumpkins on Halloween shows civil engineers where there are problems in bridges, plumbing, and interstates?

Phoebe Sharp

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