Today was Fun Physics Day at work, as often happens the afternoon after a whole morning is killed by meetings. I have no idea how we got on the subject, but today’s project ended up being Prince Rupert Drops. Prince Rupert Drops are tear-drop shaped glass balls with long, trailing tails. The base of the bulb is shockingly strong, but the tail can be snapped fairly easily. When this happens, the entire drop explodes into a fine dust. The short explanation:
When molten glass hits cold water, its outer surface cools rapidly and shrinks as it solidifies. Since the center is still fluid, it can flow to adjust to the outer shell’s smaller size. As that center eventually cools and solidifies, it also shrinks, but now the outer shell is already solid and can’t change its shape to accommodate the smaller core.
The result is a great deal of internal stress, as the center pulls the outside in from all sides. Like a tightly wound spring, the glass is set to release a lot of energy. If you break the thin glass at the tail, a chain reaction travels like a shock wave through the drop. As each section breaks, it releases enough energy to break the next section, and so on, shattering the whole drop in less than a millisecond.
Paradoxically, the same tension also makes the Prince Rupert’s drop stronger. Glass breaks when tiny scratches pull apart and spread into fractures. Since the surface is compressed by internal stress, scratches can’t grow, and the glass is very difficult to break.
A professor at Purdue University clocked the fractures at over 4,000mph. Unfortunately my university doesn’t carry the journal he published in, so I don’t have anything more than that abstract to share.
My coworkers made about 5 of these things using glass stirring rods, pipettes, and an ordinary propane torch, although it was a massive pain in the ass. It took forever, and not every tear-shaped glass drop is a Prince Rupert Drop. But when it works, it’s fantastic. My coworker said it felt like a bomb going off in his hand when our drop became a rough silt. It’s also possible to make the tail too fine, so that you have to snap off more than one section to get the explosion. The explanation I read, that I can’t seem to find right now, suggested that the diameter of the glass has to be sufficiently large to allow the cracks to propagate, i.e. if you snap too thin a piece off, the rest of the structure can just shrug it off. If you’ve got a nice traily tail, this means you can flirt with danger by flicking at the flexible bit at the end and dramatically snapping off several pieces before the whole thing disintegrates. Of course, you’d have to play with a lot of them before you got a feel for where that point is.
You can also make your own glass fibers by heating the middle of a glass rod until it softens and glows red, then removing it from the flame and pulling really fast. If your rod was solid, you’ve just made a fiber optic cable. If your rod was a tube, you now have a very small capillary tube.
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