Indian-American-led team of researchers say they’ve solved centuries-old riddle of indestructible Prince Rupert’s Drops

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Srinivasan Chandrasekar, a Purdue University professor of industrial engineering, displays a Rupert’s drop. (Purdue University image/ Trevor Mahlmann)

An Indian-American professor along with an international team of researchers says they have solved the riddle of what makes small glass structures created in the 17th century, withstand immense blunt force trauma.

Srinivasan Chandrasekar, a Purdue University professor of industrial engineering and director of the university’s Center for Materials Processing and Tribology, along with the team has pinpointed the source of the bizarre shatter-resistant behavior behind Prince Rupert’s drops, a press release from the university said. The work was a collaboration of researchers from Purdue University, the University of Cambridge in the U.K. and Tallinn University of Technology in Estonia, and included several scientists of Indian origin.

These small glass nodules that can withstand the blows of a hammer and yet burst into powdery dust by simply snipping their threadlike tails, have proved a mystery for scientists and philosophers for 400 years.

Germany’s Prince Rupert brought five of the drops to England and presented them to King Charles II, who became interested in their extraordinary properties.

“On one hand, the head can withstand hammering, and on the other hand, the tail can be broken with just the slightest finger pressure, and within a few microseconds the entire thing shatters into fine powder with an accompanying sharp popping noise,” Chandrasekar is quoted saying in the press release.

According to the university, the new research is an extension of work performed more than 20 years ago by Chandrasekar and Cambridge physicist Munawar Chaudhri, who is the corresponding author on the Applied Physics Letters paper. In their 1994 work, Chandrasekar and Chaudhri showed evidence that explained the explosive disintegration when the tail was snipped off.

Prince Rupert’s drops, glass structures resembling tadpoles, can withstand the blows of a hammer and yet burst into powdery dust at the slightest touch of their threadlike tails. Now researchers understand why. (Purdue University image/ Trevor Mahlmann)

While the 1994 research paper focused on the tail, the new research concentrates on the head’s shatter-resistant behavior.

In new findings, the researchers used a technique called integrated photoelasticity, pioneered by Tallinn University scientist Hillar Aben, who is lead author of a paper published in the journal Applied Physics Letters (doi: http://dx.doi.org/10.1063/1.4971339). A YouTube video is available at https://youtu.be/lt-zvsGvtqg.

The measurements the team made reveal the complex stress distribution in the drop as rainbow-colored bands when viewed through polarizing filters. Mathematical techniques, similar to those used in reconstructing 3-D information from medical CT scans, are then used to precisely recover the stresses based on the band patterns.

Findings showed the high strength of the head comes from compressive stresses calculated at around 50 tons per square inch, making them as strong as some grades of steel.

The drops, also known as Batavian tears, are made of glass having a “high thermal expansion coefficient,” which is needed to create the compressive residual stresses that provide the shatter-resistance. They are produced by dropping red hot blobs of molten soda-lime or flint glass into cold water, quickly cooling in a process called quenching. It is similar to processes used to make shatter-resistant glasses like those in today’s cellphone screens.

“The first patents to strengthen glass were in the 19th century, so there was a time lag of more than 200 years,” Chandrasekar said.

On cooling down and solidifying, the drops form in a tadpole shape with bulbous head and long threadlike tail, which combined, are about four inches long. The surface of the drops cools faster than the interior, producing a combination of compressive stresses on the surface, and compensating tensile – or pulling – stresses in the interior of the drops.

“The tensile stress is what usually causes materials to fracture analogous to tearing a sheet of paper in half,” said Purdue postdoctoral associate Koushik Viswanathan, a co-author of the paper. “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,” he said.

As recently as 2013, published findings have proposed incorrect solutions to the riddle of the drop’s shatter-resistant strength, the university said.

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