17th-century mystery solved: Prince Rupert’s drops

Professor Srinivasan Chandrasekar writes for the College of Engineering Medium blog.

Srinivasan Chandrasekar displays a Prince Rupert’s drop. (Purdue University photo/Trevor Mahlmann)Srinivasan Chandrasekar displays a Prince Rupert’s drop. (Purdue University photo/Trevor Mahlmann)

Scientific curiosity is alive and well in today’s world — just witness the 65 million views of a short video I posted on YouTube about Prince Rupert’s drops. Everyone loves a mystery, and the combination of this 17th century conundrum with the scientific and engineering sleuthing required to crack the case proved irresistible to all those who clicked on the link.

Once upon a time, in Germany, there was a prince named Rupert. Around 1660, he brought these seemingly magical drops to England and presented them as a gift to King Charles II, who handed them off to his newly founded (and now esteemed) Royal Society for study.

The Royal Society Fellows, especially Robert Hooke, made some inroads into unraveling the mystery based on the available scientific knowledge and experimental methods of the age. But it wasn’t until 2016 that we — myself; Munawar Chaudhri of Cambridge University; Hillar Aben of the Tallinn University of Technology in Estonia; and Koushik Viswanathan, currently at the Indian Institute of Science in Bangalore (India) — fully pieced together the puzzle.

Prince Rupert’s drops are small glass structures that look like tadpoles. The mystery is less in how they are created — lowering molten glass drops into cold water, which quickly cools in a process called quenching — than in their mix of both unusual strength and lack of it (fragility).

Integrated photoelasticity of a Prince Rupert’s drop (Purdue University photo)Integrated photoelasticity of a Prince Rupert’s drop (Purdue University photo)

You can bang on the head of the “tadpole” with a hammer repeatedly, but it will not break. Yet when you apply even light finger pressure to the tail, the entire structure shatters instantly, with a sharp popping noise, into fine powder.

In 1994, Munawar Chaudhri and I employed high-speed photographic techniques to view the disintegration of the drop. Imaging at some 1 million frames per second, we observed that any cracks initiated in the tail sped toward the head at more than 3,000 miles per hour, while repeatedly bifurcating, thus explaining the explosive disintegration. In 2016, we solved the mystery of the head’s ability to resist shattering. Turns out this toughened glass exhibits high fracture resistance due to internal compressive stresses — some 50 tons per square inch — making it as strong as many types of steel.

1994 high-speed photography images of an exploding Prince Rupert’s drop (Purdue University photos)1994 high-speed photography images of an exploding Prince Rupert’s drop (Purdue University photos)

Our discoveries could only have been made through new technologies and experimental methods unknown to the Royal Society in the late 17th century. We were able to determine that the molten drop’s surface, as it solidifies, cools faster than its interior. This produces an unsteady equilibrium — a mix of compressive stresses on the surface, and (compensating) pulling stresses in the interior. It is these latter, tensile stresses, that typically cause materials to fracture — in this case, initiated by breaking of the tail. A single crack that initiates and accelerates does the trick.

The newest findings rely on a technique called integrated photoelasticity, courtesy of the pioneering work done by Professor Aben. We used a transmission polariscope — an optical measurement device that measures stress — to map the light as it traveled through the glass drop and ascertain the complex stress distributions. We then used mathematical analysis to reconstruct the data and understand the stress patterns.

While we believe we have solved the mystery, in science and engineering, it is always possible that new elements of Prince Rupert’s drops, and our explanation of the phenomenon, may be called into question. The motto of the Royal Society is “Nullius in verba” — “Take nobody’s word for it” — and there will always be something else to challenge, and another mystery to solve.

That’s part of the allure of science and engineering: At its heart, it is detective work, driven by an unquenchable curiosity to study and solve mysteries.

Srinivasan Chandrasekar, PhD

Srinivasan Chandrasekar, PhD

Professor of Industrial Engineering

Director, Center for Materials Processing and Tribology

College of Engineering

Purdue University