A Prince Rupert's drop looks like a glass tadpole from a beginner's crafts festival, but it's so strong it can take a hammer hit without breaking. That would be impressive enough, but if you break its tail, which can be done with finger pressure, the drop explodes into powder. The reason for this has mystified scientists for 400 years, but a team from Purdue University, the University of Cambridge, and Tallinn University of Technology in Estonia finally has an answer.

Also called Batavian tears, Prince Rupert's drops were discovered in the 17th century and became famous when Prince Rupert of Bavaria presented five of the curiosities to Charles II of England. These were turned over to the Royal Society for study in 1661, yet despite four centuries of research, the secret of the drop's combination of great strength and self-destructive fragility remained elusive.

The drops are made by taking red hot blobs of molten glass with a high thermal expansion coefficient, like soda-lime or flint glass, and dropping them into a jar of cold water. The molten glass instantly solidifies into the characteristic tailed drop shape in a quenching process similar to that used to make the tempered glass for modern smartphone screens, which wasn't discovered until the 19th century.

Along with Purdue professor of industrial engineering Srinivasan Chandrasekar, team leader Hillar Aben of Tallinn University used integrated photoelasticity to investigate the drops. This is a technique where a transparent 3D object is suspended in an immersion bath and polarized light is passed through it. The alterations in the light's polarization inside the object show up as rainbow bands that correspond to stress lines.

Previous work by Chandrasekar and Cambridge physicist Munawar Chaudhri in 1994 showed that by filming an exploding drop at almost one million frames per second, it could be seen disintegrating as cracks propagated within it at over 4,000 mph (6,437 km/h) when the tail was snipped.

Focusing on the head of the drop instead of the tail, the current study found that the compressive stresses in the glass are about 50 tons per square inch, which gives it the strength of some steels. According to the team, this is because the outside of the drop cools faster than the inside. This turns the outside into a layer of powerful compressive forces pushing inward. These are balanced out by the tensile or pulling forces inside the drop.

So long as these forces remain in balance, the drop remains stable and can withstand tremendous punishment. Normally, because glass is a supercooled liquid rather than a solid, any cracks in the surface propagate at the speed of sound through a glass object, breaking it.

But in a Prince Rupert's drop, the interface between the inner and outer regions deflects the forces sideways, so the crack can't propagate. However, if the tail is broken, The shallow cracks in the tail shoot parallel to the axis of the drop, deep into the head, and into the interface. The damage is so great that the balanced forces are released, causing the drop to explode.

1994 high-speed photography images of an exploding Prince Rupert's drop(Credit: Purdue University)

"The tensile stress is what usually causes materials to fracture analogous to tearing a sheet of paper in half," says Purdue postdoctoral associate Koushik Viswanathan. "But if you could change the tensile stress to a compressive stress, then it becomes difficult for cracks to grow, and this is what happens in the head portion of the Prince Rupert's drops."

The research was published in Applied Physics Letters and its findings are discussed in the video below.

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