A Glass Drop Shrugs Off a Hammer but Explodes From Its Tail

Drip molten glass into cold water and it freezes into a tadpole shape called a Prince Rupert’s drop — a curiosity that has baffled scientists since the 1600s. Whack the bulbous head with a hammer and nothing happens; the glass simply shrugs it off. But snip the thin tail and the entire drop bursts into fine powder in an instant.

The secret is locked-in stress. As the drop quenches, its outer skin cools and hardens first, then the inside contracts as it solidifies, squeezing the surface into intense compression while leaving the core under tension. Researchers at Purdue and the University of Cambridge measured that surface squeeze at up to 700 megapascals — comparable to some steels — in a layer just a tenth of the head’s width.

Break the tail and you uncork all that pent-up energy at once.

Cracks then race up through the tense interior at more than 3,000 mph, repeatedly splitting as they go. The drop disintegrates faster than the eye can follow — captured only by cameras shooting a million frames a second.