Prince Rupert's drops (also known as Dutch tears or Batavian tears) are toughened glass beads created by dripping molten glass into cold water, which causes the glass 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 examples of 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 unraveling of the principles of their unusual properties probably led to the development of the process for the production of toughened glass, which was patented in 1874. Research carried out in the 20th and 21st centuries shed further light on the reasons for the drops' counterintuitive properties.
Description
Prince Rupert's drops are produced by dropping molten glass drops into cold water. The glass rapidly cools and solidifies in the water from the outside inward. This thermal quenching may be described by means of 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. First, when the tail is snipped, the drop disintegrates explosively into powder, whereas the bulbous head can withstand compressive forces of up to 664.3 kilonewtons (149,300 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 a high-speed camera. 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. Their high fracture toughness due to residual compressive stresses makes Prince Rupert's drops one of the earliest examples of toughened glass.
History
It has been suggested that methods for making the drops have been known to glassmakers since at least the times of the Roman Empire. Sometimes attributed to Dutch inventor Cornelis Drebbel, the drops were often referred to as lacrymae Borussicae (Prussian tears) or lacrymae Batavicae (Dutch tears) in contemporary accounts. Verifiable accounts of the drops from Mecklenburg in North Germany appear as early as 1625. The secret of how to make them remained in the Mecklenburg area for some time, although the drops were disseminated across Europe from there for sale as toys or curiosities. 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. Although Prince Rupert did not discover the drops, he was responsible for bringing 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. His publication correctly laid out 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) or 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.
In 1994, Srinivasan Chandrasekar, an engineering professor at Purdue University, and Munawar Chaudhri, head of the materials group at the University of Cambridge, used high-speed framing photography to observe the drop-shattering process and concluded that while the surface of the drops experiences highly compressive stresses, the inside experiences high tension forces, creating a state of unequal equilibrium which can easily be disturbed by breaking the tail. However, this left open the question of how the stresses are distributed throughout a Prince Rupert's drop. In a further study published in 2017, the team, in collaboration with Hillar Aben, a professor at Tallinn University of Technology in Estonia, used a transmission polariscope to measure the optical retardation of light from a red LED as it traveled through the glass drop, and used the data to construct the stress distribution throughout the drop. This showed that the heads of the drops have a much higher surface compressive stress than previously thought—at up to 700 megapascals (100,000 psi)—but that this surface compressive layer is also thin, only about 10% of the diameter of the head of a drop. This gives the surface a high fracture strength, which means it is necessary to create a crack that enters the interior tension zone to break the droplet. As cracks on the surface tend to grow parallel to the surface, they cannot enter the tension zone, but a disturbance in the tail allows cracks to enter the tension zone. 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, where much of the early scientific study of the drops was performed.
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