Prince Rupert's drops (also known as Dutch or Batavian tears) are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail. These droplets are characterized internally by very high residual stresses, which give rise to counter-intuitive properties, such as the ability to withstand a blow from a hammer or a bullet on the bulbous end without breaking, while exhibiting explosive disintegration if the tail end is even slightly damaged. In nature, similar structures are produced under certain conditions in volcanic lava, and are known as Pele's tears.
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 near-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. Glass melts between 2,552 degrees and 2,912 degrees Fahrenheit. When a glassmaker drops a molten glass bead into a tank of water, its surface cools rapidly, quickly forming a hardened shell around the liquid interior, similar to an egg white cooking faster than the yolk inside. The 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 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 MythBusters pair in the Corning video above demonstrates 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.
The glass is made to be tough through a process called ion exchange. During manufacturing, small ion particles form at the surface. These are usually electrically-charged sodium atoms. To toughen the surface, the 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.
You must be logged in to post a comment.