Prince Rupert's Drop

Prince Rupert's Drops (also known as Dutch tears) are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail. These droplets are characterized internally by very high residual stresses, which give rise to counter-intuitive properties, such as the ability to withstand a blow from a hammer or a bullet on the bulbous end without breaking, while exhibiting explosive disintegration if the tail end is even slightly damaged. In nature, similar structures are produced under certain conditions in volcanic lava.

The drops are named after Prince Rupert of the Rhine, who brought them to England in 1660, although they were reportedly being produced in the Netherlands earlier in the 17th century and had probably been known to glassmakers for much longer. They were studied as scientific curiosities by the Royal Society and the unravelling of the principles of their unusual properties probably led to the development of the process for the production of toughened glass, patented in 1874.

Prince Rupert's drops are produced by dripping molten glass into cold water. The water rapidly cools and solidifies the glass on the outside of the drop, while the inner core remains molten. When the glass on the inside eventually cools it contracts inwards, producing significant compressive stresses on the surface of the drop while the core solidifies in a state of tensile stress. The quasi-spherical shape of the bulbous head gives it great strength, such that it can be hit with a hammer or struck with a bullet without breaking while the quasi-cylindrical shape of the tail makes it fragile and easily fractured. When any portion of the tail is damaged, the large amount of potential energy stored in the internal structure is released, causing fractures to propagate through the glass toward the head at very high speeds which breaks the entire structure into flakes and powder.

An examination of the shattering of Prince Rupert's Drops by the use of high speed video 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 speed (1.45 to 1.9 km/s [0.9–1.2 mi/s]).

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