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Solving mystery of Rupert's drops (Image 1)

Srinivasan Chandrasekar holds a Rupert's drop

Srinivasan Chandrasekar, a Purdue University professor of industrial engineering, displays a Rupertís drop. First discovered in the 17th century, Rupert's drops are small glass structures resembling tadpoles that can withstand the blows of a hammer, yet burst into powdery dust by simply snipping their threadlike tails. [See related image Here.]

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An international team of researchers has pinpointed the source of the bizarre shatter-resistant behavior behind Prince Rupertís drops, small glass structures that look like tadpoles and can withstand the blows of a hammer and yet burst into powdery dust by simply snipping their threadlike tails.

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

"Since the 17th century, famous scientists and natural philosophers have been trying to understand the exceptional properties of these drops," said Srinivasan Chandrasekar, a Purdue University professor of industrial engineering and director of the universityís Center for Materials Processing and Tribology. "Rupertís drops have been a curiosity for about 400 years. 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."

Measurements taken using a technique called integrated photoelasticity revealed 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, were then used to precisely recover the stresses based on the band patterns.

Previous research by Chandrasekar and a colleague from 20 years earlier in which they used high-speed photographic analysis of drop disintegration at nearly 1 million frames per second revealed individual cracks accelerating from the dropís tail toward the head at more than 4,000 mph. This explained their explosive disintegration when the tail was snipped off.

While this earlier research examined the tail, the current research focused on the headís amazing shatter-resistant behavior. Findings from the newer research 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.

This research was supported in part by the National Science Foundation (grant CMMI 15-62470).

To learn more about this research, see the Purdue news story Research solves centuries-old riddle of Prince Rupertís drops. (Date image taken: 2017; date originally posted to NSF Multimedia Gallery: July 25, 2017)

Credit: Purdue University image/ Trevor Mahlmann
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