Prince Rupert’s drops (also known as Dutch or Batavian 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, 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. Research carried out in the 20th and 21st centuries shed further light on the reasons for the drops’ contradictory properties.
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 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: when the tail is snipped, the drop disintegrates explosively into powder, whereas the bulbous head can withstand compressive forces of up to 287,900 newtons (64,700 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.
The process for the production of toughened glass by quenching was probably inspired by the study of the drops, as it was patented in England by Parisian Francois Barthelemy Alfred Royer de la Bastie, in 1874, just one year after V. De Luynes had published accounts of his experiments with them.
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. 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.
It has been suggested that methods for making the drops have been known to glassmakers since 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.