Mystery of Prince Rupert's Drop at 130,000 fps - Smarter Every Day 86

Hey, it's me, Destin. Welcome back to Smarter Every Day! Today, we're gonna do awesome science with orbits at Hot Glass here at Lookout Mountain, Alabama. Goggle up; science is about to happen! We're gonna use a high-speed camera and learn about Prince Rupert's drop. It's never been done on the Internet.

Okay, we are here inside the shop with Cal. Cal owns the place. Can you show them how to make a Prince Rupert's drop? So after it cools down, this is what you're left with. It kind of looks like a tadpole, but it has some really interesting mechanical properties. We can actually hit this thing with a hammer, and it won't break.

Okay, ready? Didn't go! Okay, I think Cal is kind of a pansy, so I think you can break it. So we're gonna try again, only this time I'm gonna make you hit it really hard, and I'm gonna record it with high speed. That work? Thank you, very good. All right, let's do it! The challenge is on!

Yeah! [Music] So, do you think you actually broke it? I don't think you did. You think you broke it? Look at a high speed. [Music]

Okay, so the drop broke, but technically it wasn't the hammer that broke it. If you look closer in my speed, you can see that it's the wiggling of the tail that makes it go. This is the mystery of the Prince Rupert's drop. You can try as hard as you can to break the ball, but you can't. But if you even nick the tail, the entire thing will explode—not shatter, but actually explode.

Let's go outside, and I'll show you more. Okay, we're gonna just run an obscene framerate here. We have this Phantom V 1610, so glass breaking occurs so fast that you have to have like hundreds of thousands of frames per second. So we're gonna have a lower resolution, and we have to have a lot of light as well. We're also going to use this mirror over here to run about 3,000 frames per second, so we can get a wide shot to see the whole event as well. [Music]

Okay, now we understand what a Prince Rupert's drop does, but at this point, we don't quite understand why it does it. Let's take a closer look.

Okay, this is called a polariscope, and basically what it is, is a filtered piece of glass that's polarized. Now I have another filter here; you can see if I turn it, then I can block out the light. Now if I put this on the camera that you're looking through here, and then I put the Prince Rupert's drop in between the two pieces of glass, you should be able to see the internal stresses built up inside the Prince Rupert's drop.

So to understand how these stresses got here, let's use the color grey to represent nice and strong solid glass. We'll use red to represent molten glass, and because the thermal expansion coefficient is safe to assume that the higher the temperature, the larger this glass wants to be. Blue represents glass that's cooling off or transitioning between the two states. Because of that same thermal expansion coefficient, this glass is shrinking and basically pulling in on itself.

Think of a Prince Rupert's drop as a bunch of little infinitesimal pieces of glass, with each piece trying to interact with the pieces around it. When the molten glass is first dipped into the water, the outside layer touches the water and immediately solidifies. This locks in that outside shape of the drop. The inside of the drop, however, is still a hot, expanded liquid.

As heat is transferred to the water, that glass on the inside slowly begins to cool down and pulls in against that outside layer. The problem is that because it's already locked in as a round solid, it only compresses tighter against itself. This actually makes it stronger, kinda like how an arch compresses and gets stronger when you put your weight on it. Only this is in all directions now.

Because the cooling glass can't move that outside layer, it begins to pull against itself, causing it to be in an extremely high tension. It then hardens in this state of tension, and there you have it—a Prince Rupert's drop. The outside is in an extremely high compressive stress, and the inside is in an extremely high tensile stress.

If one link in this tension chain is ever cut, it breaks down the line, feeding off of its own stored up energy, just like a chemical explosive does. The difference here is that instead of releasing chemical potential energy, mechanical strain energy is released. This wave of energy is what we call a failure front.

You can directly measure the velocity of that failure front as long as you have a camera fast enough. Let's give the shot at 130,000 frames per second. [Music]

So, a big thanks to Cal from Orbitz Hot Glass. If you found this interesting and you want to support him by buying stuff, go click on the link wherever I put it and go check out his website. Click the cat to subscribe! With the helmet just said, you've never caught the cat on fire, ever? Never! Not even once!

These things are named for a guy named Prince Rupert, imagine that, who lived in Bavaria back in the 1600s. Here, he brought these over as a gift to King Charles II in England, who gave them to his Royal Society to try to figure out. Yep, ready? No upload! Blowed up! [Music]