In the year 1660, Prince Rupert, a nephew of King Charles I of England, gave a very interesting gift to King Charles II. He had brought five glass crystals, each with a bulbous head and a long tapering tail. Though they looked like unremarkable glass baubles and were made of no special material, it soon became obvious that their qualities were far from ordinary.

The head of the glass drops were very strong, strong enough to withstand a blow from a hammer without breaking. However, the long thin tails were almost comically weak.

Just a flick of the tail, or a slight bend with the fingers, would cause the tail to shatter. Not only that, but when the tail broke, it set off a chain reaction that caused the entire drop to suddenly and explosively burst into powdered glass. The king was so mystified that the following year, he turned the remaining drops over to the Royal Society for study. However, they too were unable to provide an explanation for the glass’ behavior.

In honor of the gift that brought these glass enigmas into the light, they were named Prince Rupert’s drops. Scientists and other great thinkers continued to try to uncover their secrets for the next four centuries, but to no avail. It wasn’t until 1994 that the mechanism for their explosive destruction was finally uncovered, and in November of 2016, the rest of the mystery was solved for good.

Prince Rupert’s Drops

It isn’t some special chemical mixture that gives Prince Rupert’s drops their bizarre strength and fragility. Rather, it’s their formation and shape that turn them into something spectacular. A drop is made by dropping a bead of molten glass into cold water (they can form naturally in volcanic lava as well). The glass has to be of a variety that expands upon heating, like soda-lime or flint glass. As it turns out, strange things happen to a material that expands when heated if it doesn’t cool at a uniform rate.

Aben, Anton, and Ois of the Tallinn University of Technology analyzed Prince Rupert’s drops using polarized light. They found that when the drops were submerged in a clear liquid with polarized red light shining through them, the drops lit up in rainbow bands. These bands were a visual map of stresses experienced by the drop. Even when just sitting on a shelf, Prince Rupert’s drops are under a great deal of stress and strain. The outer portions of the drops have strong compressive, or pushing, forces. In direct contrast, the inner portion has tensile, or pulling, forces.

The bands indicate stresses in the drop. Aben et al. ©2017 American Institute of Physics — The bands indicate stresses in the drop. Aben et al. ©2017 American Institute of Physics

The compressive stress at the surface of the drop is a massive 29-50 tons per square inch, as strong as some types of steel. However, this isn’t enough to give the drops their resiliency. It is the interface between the outer compressive stress and the inner tensile stress that is the greatest protection.

Usually when glass is struck, the force of the blow goes straight through the glass, cracking it. When the head of a Prince Rupert’s drop is hit (or even shot with a bullet), the force is deflected sideways, preventing cracks from spreading. As long as the stress forces between the inside and the outside of the drop are in balance, the head of the drop is nigh indestructible. It’s at the tail where we find the drop’s Achilles heel.

The Museum of Glass made this special video showing the drops exploding.

The thinness and fragility of the tail tail provides easy access to the interior tension zone. Not even the balance of forces can prevent a crack from forming here, and the smallest of cracks in the tail can have devastating consequences. As the crack forms, it begins to spread parallel to the axis to the drop, penetrating deep into the head. This releases the balanced forces of compression and tension, creating a chain reaction that results in the drop exploding into powder from the tail to the head. The very same forces that make the head of the drop so strong are the reason that it shatters when the damage reaches the interior.

Why do the drops form with these strange stress patterns? It all comes down to cooling. When the drop of molten glass is plunged into cold water, the surface cools much faster than the interior. Since the molten glass was expanded by heat, it contracts as it cools. The surface contracts faster than the interior, leading to high compressive stress in the outer 10% of the drop, while the inner portions experience the tensile forces instead. Drops that were cooled slowly might look like a Prince Rupert’s drop, but would behave like normal glass.

It’s been a long time since Prince Rupert gave those drops to King Charles II, but our appreciation for them lives on in academic studies, and numerous youtube videos of their glass shattering in slow motion (like a drop being shot by bullets, or drops being crushed by hydraulic presses). In 1663, an anonymous poet in the Ballad of Gresham College wrote an ode to their mystery and ability to confound:

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

What’s next for the Prince Rupert’s drop? A process similar to the one that results in their creation is already used for manufacturing smartphone screens. In the future, perhaps their secrets will improve our production of shatterproof glass even further.

Until then, though the drops are now less inscrutable, they are still delightful examples of physics in action.

- Kate Dzikiewicz, Paul Griswold Howes Fellow