Rupert's drop

The Rupert's 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. Also known as Dutch tears or Batavian tears.

Understand

This is a 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. 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: when the tail is snipped, the drop disintegrates explosively into powder, whereas the bulbous head can withstand compressive forces of up to 664,300 newtons or 67,740 kg.

Features

Rupert's drops have two unique features. First one is 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 meters 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.