page_title: Prince Rupert's drop
section_title: Description
hierarchical_section_title: Prince Rupert's drop / Description
caption_attribution_description: English: Figure from "Account of the Glass Drops" by Sir Robert Moray, as given to The Royal Society in 1661.
context_page_description: Prince Rupert's drops 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, and are known as Pele's tears.
  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.
context_section_description: Prince Rupert's drops are produced by dropping molten glass drops into cold water. The water rapidly cools and solidifies the glass from the outside inward. This thermal quenching may be described using a simplified model of a rapidly cooled sphere. Prince Rupert's drops have remained a scientific curiosity for nearly 400 years due to two unusual mechanical properties: when the tail is snipped, the drop disintegrates explosively into powder, whereas the bulbous head can withstand compressive forces of up to 15,000 newtons (3,400 lbf).
  The explosive disintegration arises due to multiple crack bifurcation events when the tail is cut – a single crack is accelerated in the tensile residual stress field in the center of the tail and bifurcates after it reaches a critical velocity of 1,450–1,900 metres per second (3,200–4,300 mph). Given these high speeds, the disintegration process due to crack bifurcation can only be inferred by looking into the tail and employing high speed imaging techniques. This is perhaps why this curious property of the drops remained unexplained for centuries.
  The second unusual property of the drops, namely the strength of the heads, is a direct consequence of large compressive residual stresses—up to 700 megapascals (100,000 psi)—that exist in the vicinity of the head's outer surface. This stress distribution is measured by using glass's natural property of stress-induced birefringence and by employing techniques of 3D photoelasticity. The high fracture toughness due to residual compressive stresses makes Prince Rupert's drops one of the earliest examples of toughened glass.
caption: A figure describing a Prince Rupert's drop, from Account of the Glass Drops (1661) by Sir Robert Moray.
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