Head of this teardrop-shaped glass can withstand bullets
TO THE casual observer, Prince Rupert’s drops look like miniature glass sculptures of tadpoles. But they’re one of humanity’s most fascinating creations.
They can withstand blows from a hammer, the punishing crush of a hydraulic press and even a bullet speeding from the barrel of a .22-calibre handgun. Their tails, however, which narrow into thin threads, serve as a surprising Achilles’ heel. Snip the tails with a pair of pliers, or simply snap them between your fingers, and with an audible pop, the entire drop shatters into a fine powder, leaving no trace of what existed.
We’ve long known the drops are strong, and we’ve long known why. But researchers at a team from Purdue University, the University of Cambridge and Tallinn University of Technology have quantified just how strong, which they recently published in the journal Applied Physics Letters.
The compressive forces in the drop’s fat head begin at 29 tons per square inch and reach up to reach 50 tons per square inch, they said. That’s as strong as some grades of steel.
For all their mystery, Prince Rupert’s drops are fairly simple to create – anyone can do it.
Just melt some sort of glass with a high thermal expansion coefficient (i.e. glass that expands upon heating), such as sodalime glass (the kind used in most bottles, jars and windowpanes), and let a molten drop fall into cold water.
The water immediately cools the outside of the melted glass into a solid, before cooling the inside. This results in strong compressive (or pushing) forces on the outside and strong tensile (or pulling) forces on the inside.
The resulting tension makes the glass strong. — Washington Post