#1219 What is Prince Rupert’s drop?

What is Prince Rupert’s drop? Prince Rupert’s drop is a small, tadpole-shaped glass bead that is incredibly strong but incredibly fragile.

Prince Rupert’s drops have been around for a long time and there is some thought that they were first discovered by Roman glassmakers two thousand years ago. They were first brought to Great Britain in 1660 by Prince Rupert of the Rhine, who was an English-German noble, soldier, and scientist. He gave them to King Charles II, who in turn gave them to the Royal Society to be studied. They became known as Prince Rupert’s drops and the name has stuck. King Charles II was very interested in science and he was the patron of the Royal Society, which had only been started a few years earlier.

Prince Rupert’s drops have very special properties. They are made by dripping molten glass into cold water and they set in a tadpole shape, with a bulbous head and a longish tail. The head of the tadpole is incredibly strong and can withstand hammer blows and bullets. They have been tested to be able to withstand a force of 664,300 Newtons, which is 67,740 kg. Yet, despite this awesome strength for something so small, if you even slightly chip the tail of the tadpole, the whole Prince Rupert’s drop will just explode and become a powder, instantly.

To make a Prince Rupert’s drop, you have to have a ball of molten glass and glass melts at between 1,400 to 1,600 ℃, so that’s pretty hot. When you have the molten glass ready, let it drip into a bucket of cold water. The cold water will absorb the heat energy from the molten glass as soon as they come in contact and the outside of the molten glass drop will cool and harden pretty quickly. The heat inside the drop will take longer to be absorbed and the very center will be the last part to harden. You will be left with a solid glass tadpole drop shape which you can then hit with a hammer. Just be careful of the tail.

The scientists of the Royal Society had several theories about why the glass was so strong and yet so brittle at the same time, but they were unable to come up with the complete answer. In fact, that answer has only been discovered in the last few years, and that is only because of modern technology. It took a high-speed camera that could shoot at a million frames a second and something called a transmission polariscope, which can measure optical radiation.

The high-speed camera that could take a million frames a second allowed scientists to see what was happening when the tail was chipped. Cracks appear at the point where the tail was damaged and they rapidly spread, forming more and more cracks, until the whole piece of glass is reduced to powder. It took a high-speed camera to see this because the cracks promulgate at a speed of roughly 1,800 meters per second, which is about 6,500 km/h. So, why is a Prince Rupert’s drop so strong, yet it explodes if you chip its tail?

The reason for both of these things are the same. When the molten glass is lowered into the cold water, the heat energy held in the molecules passes into the water because heat energy always goes from a hot place to a cold place. When the molecules have heat energy, they move more and they take up more space. When they lose this energy, they stop moving and they contract, taking up less space. If the Prince Rupert’s drop was cooled from the inside out, the center would contact and then the succeeding layers would contract until it was all slightly smaller. However, it cools from the outside first. The outside layer very quickly cools and becomes hard. The inside layers start to cool and contract, and they pull on the outside layer, but it has already solidified and doesn’t move. Each successive layer that cools, pulls on the already solid layer above it, and all they do is pull it down, putting it under tension. This makes each layer much stronger than if it wasn’t being pulled down. It is similar to an arch bridge that gets stronger when it has weight on it because the weight pushes it down and the forces are distributed along the curve of the arch.

This high tension makes the Prince Rupert’s drop very strong, but it also makes it a tensile stress time bomb waiting to go off. All of the molecules are pulling on the ones in front of them, which gives it an enormous amount of potential energy. Any break in the chain of molecules will release all of that energy, which will gradually increase, like a mass of dominos after the first one is knocked over. The reason why a chip in the tail is where this energy starts to get released is because it is much thinner there and there is not so much strength. It is much easier to break the chain and release the energy, which races forwards at several times the speed of sound, leaving nothing but powder. And this is what I learned today.

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Sources

https://www.ck12.org/flexi/chemistry/heating-and-cooling-curves/what-is-the-temperature-at-which-glass-melts

https://royalsocietypublishing.org/doi/pdf/10.1098/rsnr.1986.0001