Prince Rupert's Drop

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Prince Rupert's Drops (or Rupert's Balls) are a glass curiosity created by dripping hot molten glass into cold water. The glass cools into a tadpole-shaped droplet with a long, thin, tail. The water rapidly cools the molten glass on the outside of the drop, while the inner portion of the drop remains significantly hotter. When the glass on the inside eventually cools, it contracts inside the already-solid outer part. This contraction sets up very large compressive stresses on the surface, while the interior of the glass is placed under tension. It can be said to be a kind of tempered glass. The very high stress within the drop gives rise to unusual qualities, such as the ability to withstand a blow from a hammer on the bulbous end without breaking, while the drops will disintegrate explosively if the tail end is even slightly damaged. When this happens, the large amount of potential energy stored in the drop's crystalline structure is released, causing fractures to propagate through the material at very high speed.

Recently an examination of the shattering of Prince Rupert's Drops by the use of extremely high speed video (or so called "stop motion" techniques) [1] done by Dr. Srinivasan Chandrasekar at Purdue University has revealed that the "crack front" which is initiated at the tail end, propagates in a disintegrating drop within the tensile zone towards the drop's head at a very high velocity (~ 1450-1900 m/s (or up to ~4,200 miles per hour)).

Due to glass's inherent transparency, the internal stress within these objects can be demonstrated by viewing them through polarizing filters.

The drops were supposedly discovered around the 1640s by Prince Rupert of the Rhine (1619–1682), grandson of James I and the nephew of Charles I of England. Legend has it that the king would often use the drops as a practical joke in his court. He would give a drop to a courtier and then break the tail, causing a small explosion in the hand of the then surprised person.

The drops were apparently investigated by The Royal Society, as immortalized in the Ballad of Gresham College (1663):

And that which makes their Fame ring louder,
With much adoe they shew'd the King
To make glasse Buttons turn to powder,
If off the[m] their tayles you doe but wring.
How this was donne by soe small Force
Did cost the Colledg a Month's discourse.^[2]

As they were invented in the Netherlands, the French term for these creations is larme Batavique. They are a very early precursor of toughened glass.

[edit] Trivia

Prince Rupert's Drops were used as a symbolic device in the novel and movie Oscar and Lucinda.

Despite being set in 1492, over a century before Prince Rupert's birth, these drops were also used, and called by that name, in the film Secret Passage (2004) [3].

These drops were also known as "Czar's Tears".