Have you heard of the wild, almost indestructible Prince Rupert’s Drop? Science Alert recently unearthed a video from Smarter Every Day that shows this strong-as-hell glass marvel actually shattering a bullet at a beautiful 150,000 frames per second.
To make a Prince Rupert’s Drop, a glass handler lets molten glass drip and fall into a vat of water. What results is like a tadpole with an extra long ropey tail, and the drop part is almost unparalleled in strength. Glassmakers have played with this oddly strong glass for hundreds of years, and the combination of ultra-high-speed cameras and scientific knowhow have made the 21st century a new golden age for ol’ Prince Rupert.
The drop of glass forms a classic science mindbender. The tough, strong bulbous end can literally break bullets, but one snap along the very long and thready tail and the entire Prince Rupert’s Drop won’t just break—it’ll vaporize, and so quickly that only a high-speed camera can show how the break begins at the tail and makes its way forward. So what’s going on inside this improbable piece of glass?
Adam Savage and Jamie Hyneman of Mythbusters discussed the Prince Rupert’s Drop in a 2014 miniseries for Corning. Since the 1800s, Corning has made specialty glass and used contemporary science to improve glass technology, from industrial and military applications to temperature-tolerant Corning Ware for cooking and baking.
The drop is a novelty in the way it’s presented and destroyed, but the lessons inside it are broadly applicable for improving glass over hundreds of years:
Savage lets molten glass drop into a tank, where it sizzles in ultra-slow-motion. He explains that what’s happening involves the rapidly cooling surface and the still-molten interior of the drop, which form an overall structure with an indestructible exterior and an interior held taut with residual stress.
The outside of the molten glass drop cools very fast and forms a shell that the rest of the glass inside must continue to fill, even as it, too, cools and wants to retract.
As the entire structure of the drop retracts as much as is physically possible, the shape and the tension inside form an incredible shock absorber. “I feel like we’ve been swindled,” Hyneman says, watching a hammer fail to break the drop. “Swindled we have not,” Savage replies. “We’ve experienced the power of compressive strength!”
But the Prince Rupert’s Drop has what Savage calls an Achilles’ heel. If you cut through the long tail of the drop—and it can be a meter or more long—all the compression and tension are released at once.
The drop explodes like a kinetic stick bomb, but the competing forces mean the glass vaporizes almost in place at first rather than truly exploding outward. The nature of the reaction underlines how much surface tension on the rapidly cooled exterior of the drop has been holding the whole thing together.
Picture: Screenshot from video