Strong But Fragile: A Review on Prince Rupert’s Drop
- Madelyn Lee
- Nov 28, 2022
- 3 min read
Don’t let the size and material sway you from the truth! These tiny little droplet shapes of glass are actually stronger than you can imagine. They can withstand the impact of a hammer and even the formidable impact of a bullet. That is until you touch its tail.
Source: MaeBurke/STUNLAW/Blogger
What are Prince Rupert’s drops? These scientific wonders, also known as Dutch tears, are toughened glass beads, shaped like droplets. They are characterized by extremely high residual stresses, causing such opposite properties. While the head of the droplet can withstand blow after blow without any breakage, the thin tail shatters the droplet from even the slightest bend. These counterintuitive properties have confused scientists and researchers for centuries.
Prince Rupert’s drops were first introduced in the 17th century as a gift to England’s monarch. Prince Rupert from Germany had brought these glass drops to King Charles II of England because the king was curious about its unique abilities. The best scientists and physicists from this era were stumped by its unusualness. It was not until the birth of modern technology came out when the droplets’ properties were finally revealed.
In 1994, Srinivasan Chandrasekar from Purdue University and M. Munawar Chaudhri from the University of Cambridge conducted groundbreaking experiments on Prince Rupert's drop using high-speed framing photography. They meticulously analyzed the droplet's shattering process, discovering that cracks propagated along the droplet at speeds reaching up to 1,900 m/s. Their research revealed that the droplet's surface endures intense compressive stresses, while its interior experiences significant tensile forces. This unique stress distribution causes the droplet to be in an unstable state of equilibrium, making it highly susceptible to fragmentation from even minor disturbances at the tail. Despite uncovering this distinct stress pattern, the detailed distribution of stress within the droplet remained an unresolved mystery.
In a more recent experiment leveraging advanced technology, scientists employed a transmission polariscope—a specialized type of microscope—to investigate the stress distribution within Prince Rupert's drop. By illuminating the glass with a red LED light while it was submerged in a clear liquid, researchers were able to produce a vivid, colorful map of the internal forces within the droplet. This technique provided a visual representation of the stress patterns, offering deeper insights into the complex mechanics at play within the glass structure.
Using these images, scientists and researchers applied mathematical models to quantify the forces within Prince Rupert's drop. They discovered that the compressive stress around the head of the droplet ranges between 29 and 50 tons per square inch, comparable to the strength of steel. Remarkably, this extremely strong layer constitutes only about ten percent of the droplet's total volume, encasing a region of significantly lower compressive stress. The images also revealed a sensitive interior that is closely connected to the delicate tail end. This means that disturbances at the tail propagate through the interior, causing the glass to shatter explosively from the inside out.
The method of creating Prince Rupert’s drops greatly contributes to its unique characteristics. To make these droplets, blobs of molten glass are dropped into water. When this rapid cooling happens, the surface of the drops cool faster than the interior glass. This causes the exterior to obtain combinations of compressive stresses while tensile stresses develop in the interior of the droplet.
Named after the Prince who gifted these marveling glass pieces to the king of England, Prince Rupert’s drops continue to fascinate people worldwide. Many people challenge the physics behind the droplets by attempting to smash the heads with hammers, putting them inside a hydraulic press, and even shooting them with bullets.
Ultimately, Prince Rupert’s drop showcases how rapid cooling can create remarkable physical properties in materials. This unique glass structure not only highlights the intricate relationship between material science and physics, but also serves as a demonstration of how seemingly simple phenomena can lead to intriguing insights into the behavior of materials under stress. I hope you learned something new! Keep a lookout for the next post!
References
Aben, H., Anton, J., Õis, M., Viswanathan, K., Chandrasekar, S., Chaudhri, M. M. (2016 Dec. 5). On the extraordinary strength of Prince Rupert's drops. Applied Physics Letters. 109 (23): 231903. Retrieved November 24, 2022, from https://doi.org/10.1063/1.4971339
Daley, J. (2017 May 15). The 400-Year-Old Mystery of These Bullet-Shattering Glass Drops May Finally Be Solved. Smithsonian Magazine. Retrieved November 24, 2022, from https://www.smithsonianmag.com/smart-news/researchers-crack-400-year-old-mystery-prince-ruperts-drops-180963308/
McRae, M. (2017 May 10). We've Finally Cracked The Secret of Prince Rupert's Drops. Science Alert. Retrieved November 24, 2022, from https://www.sciencealert.com/we-ve-finally-cracked-the-secret-of-prince-rupert-s-drops
Zyga, L. (2017 May 9). Scientists solve 400-year-old mystery of Prince Rupert's drops. Phys.org. Retrieved November 24, 2022, from https://phys.org/news/2017-05-scientists-year-old-mystery-prince-rupert.html
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