Prince Rupert’s Drop

prince ruperts drops

Prince Rupert’s Drops (also known as Rupert’s Balls or Dutch tears) 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 hot. 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 speeds. Due to glass’s inherent transparency, the internal stress within these objects can be demonstrated by viewing them through polarizing filters.

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