Don’t be fooled by this delicate drop of glass. It’s actually really hard to destroy.
Formed of glass teardrops that elongate into a skinny tail, a Prince Rupert’s Drop looks like a delicate, clear tadpole. It’s easy enough to snap the tails, but the rounded drop at the top itself? It seems unbelievably strong. You can’t break it with a hammer blow.
If a glassmaker drips molten glass into cold water, it freezes into a bead with an extra long tail. Like Sauron’s One Ring, you can’t break it with a heavy blow from something as strong as an axe. You actually need the speed and force of a bullet to shatter it. The crazy part is that even a bullet can break apart when it hits the drop. Look at the video at the top of the story from SmarterEveryDay.
When the bullet grazes the bulb of the drop itself, you can see a line of white take off from the tail and race down the drop. This is a traveling shockwave. Two factors, compressive strength built into the drop during its formation, and the residual stress of the impact, cause the nearly instantaneous destruction of the smooth glass. A fraction of a second later—a stunning sequence visible at 100,000 frames per second—the entire object vaporizes into sparkly smoke. Rather than exploding in all directions, the glass shatters in place because the compression and tension are released all at once.
The bullet itself shatters due to the glass head releasing its compressive force. Bullet fragments fly off in a wide array, embedding themselves into the wall behind the setup.
Here’s the explanation for the magic
Glass melts between 2,552 degrees and 2,912 degrees Fahrenheit. When a glassmaker drops a molten glass bead into a tank of water, as shown in the second video, its surface cools rapidly, almost instantly forming a hardened shell around the liquid interior, similar to an egg white cooking faster than the yolk inside.
Molten glass inside continues to cool and retract, pulling the outer layer inward as much as possible. The inner core becomes tightly compressed, which generates an incredible amount of tension in the drop. This is the compressive force that binds together the layers, forming a strong shock absorber made of glass. Surface tension helps hold the drop together.
Prince Rupert’s Drops may be a fun to break, but they also present an opportunity to strengthen useful objects. For example, a special type of tough glass could cover our mobile devices.
The pair in the Corning video demonstrate Gorilla Glass, a thin sheet of seemingly delicate glass that’s actually super strong. Corning developed the glass using the same principles of compressive strength that make the Prince Rupert’s Drop so hard to break. It’s made to be tough through a process called ion exchange. During glass manufacture, small ion particles form at the surface. These are usually electrically charged sodium atoms. To toughen the surface, glass is dipped into a salt solution, causing larger, potassium ions to swap places at the surface of the glass with the smaller ions. These larger particles generate pressure on the interior parts of the glass sheet, compressing it tightly.
Unlike the Prince Rupert’s Drop, Gorilla Glass can be modified during manufacture to control the tension inside the glass, so that it doesn’t compete too much with the compressive force. With the forces in balance, the glass gains strength. It’s still not unbreakable, but it could add a protective layer to objects like cell phones in the future. If you drop your phone on the pavement, you can breathe easy if it’s wearing a thin layer of Gorilla Glass.