Prince Rupert's Drop

Prince Rupert's Drops

Prince Rupert's Drops (also known as Dutch tears^[1]) are glass objects created by dripping 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 core of the drop is in a state of tensile stress. It is a type of toughened glass.

The very high residual stress within the drop gives rise to counter-intuitive properties, such as the ability to withstand a blow from a hammer on the bulbous end without breaking, but experiencing explosive disintegration if the tail end is even slightly damaged.

Description

While the head of the drop can be hit with a hammer without breaking, 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 break the entire structure into flakes and powder.^{[lower-alpha 1]}

An examination of the shattering of Prince Rupert's Drops by the use of high speed video^[2][3] 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]).

Because of the transparency of glass, the internal stress within these objects can be demonstrated by viewing them through polarizing filters, a technique used in the study of photoelasticity.^[4]

History

A scholarly account of the early history of Prince Rupert’s Drops is given in the Notes and Records of the Royal Society of London.^[5] Most of the early scientific study of the drops was performed at the Royal Society.

The drops are reliably reported to have been made in Mecklenburg in North Germany, at least as early as 1625. However, it has been claimed that they were invented in Netherlands, hence a common name for them in the 17th century was larmes bataviques or lacrymae Batavicae (Dutch tears). The secret of how to make them remained in the Mecklenburg area for some time, although the drops were spread across Europe from there, for sale as toys or entertainments.

The Dutch scientist Constantijn Huygens asked Margaret Cavendish, Duchess of Newcastle to investigate the properties of the drops; her opinion after carrying out experiments was that a small amount of volatile liquid was trapped inside.^[6]

It seems clear that Prince Rupert did not discover the drops, but played a role in their history by being the first to bring them to Britain, in 1660. He gave them to King Charles II, who in turn delivered them in 1661 to the Royal Society (which had been created the previous year) for scientific study. Several early publications from the Royal Society give accounts of the drops and describe experiments performed. Among these publications was Micrographia of 1665 by Robert Hooke, who later would discover Hooke’s Law.^[7] His publication laid out correctly most of what can be said about Prince Rupert’s Drops without a fuller understanding than existed at the time, of elasticity (to which Hooke himself later contributed) and of the failure of brittle materials from the propagation of cracks. A fuller understanding of crack propagation had to wait until the work of A. A. Griffith in 1920.^[8]

Scientific uses

It has been known since at least the 19th century that formations similar to Prince Rupert's Drops are produced under certain conditions in volcanic lava.^[9] More recently researchers at the University of Bristol and the University of Iceland have studied the glass particles produced by explosive fragmentation of Prince Rupert's drops in the laboratory to better understand magma fragmentation and ash formation driven by stored thermal stresses in active volcanoes.^[10]

Literary references

Because of their use as a party piece, Prince Rupert’s Drops became widely known in the late 17th century — far more than today. It can be seen that educated people (or those in “society”) were expected to be familiar with them, from their use in the literature of the day. Samuel Butler used them as a metaphor in his poem Hudibras in 1663,^[11][12] and Pepys refers to them in his diary.^[13]

The drops were immortalized in a verse of the anonymous Ballad of Gresham College (1663):

And that which makes their Fame ring louder,
With much adoe they shew'd the King
To make glasse Buttons turn to powder,
If off the[m] their tayles you doe but wring.
How this was donne by soe small Force
Did cost the Colledg a Month's discourse.^[1]

1. ^ Stimson, Dorothy (July 1932). "'Ballad of Gresham Colledge'". Isis. 18 (1): 103–17. doi:10.1086/346689. JSTOR 224481. 

Sigmund Freud, discussing the dissolution of military groups in "Group Psychology and the Analysis of the Ego" (1921), notes the panic that results from the loss of the leader: "The group vanishes in dust, like a Prince Rupert's drop when its tail is broken off."

In the 1940 detective novel There Came Both Mist and Snow by Michael Innes (J.I.M. Stewart), a character incorrectly refers to them as "Verona drops"; the error is corrected towards the end of the novel by the detective Sir John Appleby.

In his 1943 novella Conjure Wife, Fritz Leiber uses Prince Rupert Drops as a metaphor for the volatility of several characters' personalities. These small-town college faculty people seem to be placid and impervious, but "explode" at a mere "flick of the filament".

Peter Carey devotes a chapter to the drops in his 1988 novel Oscar and Lucinda.

Alfred Jarry's novel Supermale references the drops in an analogy for the molten glass drops falling from a failed device meant to pass eleven thousand volts of electricity through the supermale's body.

See also

• Thermal stress

Notes

References

Further reading

• Albergotti, Clifton (1989). "Prince Rupert's drops in literature". The Physics Teacher. 27 (7): 530–2. Bibcode:1989PhTea..27..530A. doi:10.1119/1.2342858. 

External links

• " Prince Rupert's Drops" Popular Science Monthly Volume 8 Wikisource January 1876 ISSN 0161-7370 
• Mystery of the Prince Rupert's Drop - Smarter Every Day 86 Video demonstrating the creation, processes, explanation, and multiple high speed recordings of a Prince Rupert Drop.
• Prince Rupert's Drop Video demonstrating the creation, strength, and explosive fragility of a Prince Rupert Drop.
• Video showing the making and the breaking of Prince Rupert's Drops from the Museum of Glass
• Popular Science article with video detailing Prince Rupert’s Drops
• Corning Inc. "The Glass Age, Part 2: Strong, Durable Glass". Youtube. Retrieved 2015-03-24.  Adam Savage and Jamie Hyneman demonstrate Rupert's Drops, including diagram of internal stresses