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Name: Calvin
Status: student
Grade: 6-8
Country: USA
Date: Fall 2013


Question:
Why does a Prince Rupert's Drop withstand barrage until it is cut and then it shatters?



Replies:
Calvin,

If you mean "Prince Rupert's Drop", a bulb of glass with a long stem (like a tear drop shape), then the reason for its unusual qualities - the bulb can withstand a blow from a hammer, but the whole object disintegrates if the neck or tail is cut - comes from how the object is formed. The object is made by dripping molten glass into cold water. As the drop enters the water the outside of the bulb cools very quickly, but the inner part of the bulb remains quite hot. The outside part of the drop hardens into its shape, while the inside is still somewhat molten. Since glass contracts as it cools, the inside part has trouble shrinking because the outer part is already solid. As a result, it is as if the outer part is a hard shell while the inner part is trying to shrink inward, while the outer shell is trying to stretch the inside parts. Think of it as though the inside part is made of a bunch of very stiff strands of rubber that has been stretched with its ends attached to the outer shell.

If the outer shell is hit, since the outer shell is hard glass (which is strong in its own right) but is also trying to shrink inward (due to the inner part pulling it in), then the blow to the outer shell only helps to reduce the tugging from the inside and is therefore not very harmful to the drop. ON the other hand, cutting the tail or stem creates a fracture that reduces the effectiveness of the resistance of the inner shell to the shrinking pull of the inner parts. As a result, the cracks will propagate through the outer shell (high speed cameras have actually captured the cracks traveling from the stem all the way to the tip of the bulb: http://www.youtube.com/watch?v=xe-f4gokRBs) and it seems like the bulb disintegrates because the cracks propagate so fast as the resistance of the outer shell to the pull from the inside diminishes dramatically.

Greg (Roberto Gregorius) Canisius College


Hi Calvin,

Thanks for the question. If you look up "Prince Rupert's Drop" on the Internet, you will find some good explanations, but they may be a bit more advanced. Basically, molten glass is poured directly into water. The glass cools immediately and does not have time to anneal--which is when the atoms move about to release stress. Without the stress being released, the glass will shatter if not handled with the greatest of care. I should mention that glass particles (from the shattering) can fly into your eye and cause severe eye damage. So, please be careful around glass that is not annealed.

I hope this helps. Thanks Jeff Grell


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