Prince Rupert's Drop

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Prince Rupert's Drop.

Prince Rupert's Drops (also known as Rupert's Balls, Dutch tears^[1], or Czar's Tears^{[dubious – discuss]}) 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).