Prince Rupert's Drops

Prince Rupert's Drops 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 exterior, while the core of the drop is in a state of tensile stress. It can be said to be a kind of tempered glass.

The very high residual 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 the tail end is damaged, the large amount of potential energy stored in the drop's crystalline or amorphous atomic 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 made 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, a number that in air would be Mach 5.5).