Something unusual happens when a drop of molten glass falls into water. As it cools, it creates a crystal clear tadpole-like droplet that’s bulletproof on one end, but impossibly fragile on the other. We’ve known about these droplets for 400 years, but scientists have only recently figured out what makes them almost indestructible.

In a video posted just a few weeks ago, SmarterEveryDay’s Destin Sandlin fired a .22 Magnum bullet with a full metal jacket at a Prince Rupert’s Drop, so-named after Germany’s Prince Rupert gifted five of the unusual glass objects to England’s King Charles II back in 1661. The baffling results of that experiment, with the bullet disintegrating as it collides with the thicker end of the glass teardrop, are no longer a mystery.

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Making a Prince Rupert’s Drop is easy, you just need to melt a glass with a high thermal expansion coefficient—glass that expands when heated—like soda-lime or leaded glass, and then drop a molten blob of it into a container of cold water, which immediately cools it down in a process known as quenching. And that’s where all the magic happens.

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As the drop quickly cools, its outer layer experiences a temperature drop faster than its interior, which results in extreme compressive forces on the outside, but strong tensile (pulling) stresses on the inside. These forces work together to give the Prince Rupert’s Drop its incredible strength. Those inner tensile forces are usually what makes glass so fragile, but the compressive forces on the outside counteract what’s happening inside, to make it all but impossible for cracks to grow when the head of the glass is hammered, or even shot with a bullet.

In a paper published in the journal Applied Physics Letters, scientists at Purdue University’s Center for Materials Processing and Tribology describe how they figured this all out using photography that involves polarizing filters to visually reveal the stress lines in the teardrops. Those images were then processed using complex mathematical techniques that allowed the researchers to calculate the compressive stresses inside the drops, which are close to 50 tons per square inch, making the drops as strong as some steels—but only in their heads.

The other end of a Prince Rupert’s Drop features a long thin tail that’s incredibly fragile by comparison. In 1994, similar research determined that breaking off the end of a Prince Rupert’s Drop results in cracks that travel up the length of the drop towards its head at speeds of over 4,000 miles per hour, releasing all of those compressive stresses at once, and instantly turning the glass object into dust.

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So what does this research mean to you? Other than cool YouTube videos of seemingly indestructible glass? Well, when you’re poking at the thin sheet of glass covering your smartphone or tablet all day, the last thing you want is it shattering. Understanding how Prince Rupert’s Drops are created could lead to even stronger glass materials being engineered in the future. Maybe one day, your phone’s screen will actually survive a tumble to the ground.

[Purdue University via New Atlas]