A glass teardrop that shrugs off a hammer but explodes if you snip its tail.
Drip molten glass into cold water and you get a tadpole-shaped bead. Hit its round head with a hammer and it survives. Nick its thin tail and the whole thing bursts into powder in an instant. The secret is locked-in stress, and it took centuries to fully explain.
Prince Rupert's drops form when blobs of molten glass fall into cold water. The outside cools and hardens almost instantly while the inside is still molten. As the core then cools and shrinks, it pulls on the rigid outer skin. The result is a bead with its surface squeezed in powerful compression and its interior held in tension, like a spring wound tight and frozen in place.
That hidden stress gives the drops two contradictory talents. The compressed surface of the head is extraordinarily tough: measurements in 2017 found surface compression of up to about 700 megapascals, in a layer only around a tenth of the head's diameter thick. Cracks on the surface tend to run parallel to it and never reach the tense core. But damage the tail and a crack gets straight into the tension zone. It races through the drop at roughly 1,450 to 1,900 metres per second, splitting again and again, and the drop disintegrates.
The drops were circulating as curiosities in northern Germany by 1625, and glassmakers may have known the trick far earlier. Prince Rupert of the Rhine did not invent them, but he brought them to England in 1660 and gave them to Charles II, who passed them to the newly founded Royal Society. Robert Hooke discussed them in his Micrographia of 1665, and they became a fashionable party piece mentioned by Samuel Pepys.
A full explanation needed the modern science of how cracks spread, begun by A. A. Griffith in 1920, and high-speed cameras, which caught the shattering in 1994. The drops also mattered practically: studying them probably inspired toughened glass made by quenching, patented in 1874. Similar droplets form naturally in volcanic lava, where they are called Pele's tears.
Source: Wikipedia — Prince Rupert's drop · Text summarised from Wikipedia (CC BY-SA 4.0)
