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The fascinating properties of the Prince Rupert’s Drop

The Prince Rupert's Drop is a piece of glass with unique properties which both serves as an excellent demonstration of physics, and shows the strength and fragility of glass.

To the casual observer, the Prince Rupert’s Drop looks like an ordinary piece of swirling glass. First brought to the public eye by, you guessed it, a prince named Rupert, the swirling glass piece is a lot more than meets the eye. Rupert himself gave one to the Royal Society of London to discover its secrets. You see, once you start playing around with it, you’ll discover some remarkable properties that demonstrate just how intricate things we take for granted really can be.

You see, the glass drop is simultaneously incredibly tough and immensely fragile. The fat blob-like part of it can be hammered or squeezed with pliers with considerable force without breaking. However, the thin tail, if even slightly stressed, will send a wave-front through the structure exploding it into a million pieces in a violent and beautiful display.


Exploding Prince Rupert's Drop


The reason for this strange behavior has to do with how the drop is created. It’s made by quenching, or rapidly cooling, glass that has just been taken out of the smelter. Quenching is a method of treating materials that almost always results in a change of the material properties due to the redistribution of the inner structure.


 In the case of glass, quenching it in ice water rapidly cools the outer layers of the glass, while the inside remains molten. Later, as the next layer of glass begins cooling, the outside is already a solid and completely rigid. Glass contracts when it is cooled, and because of this, the tension between the already solid layer and the currently cooling layer grows. This new layer freezes in a tense position with pretty large amounts of energy stored within. Then the same thing happens with the next layer, and the next layer, all the way into the center. The end result is a glass structure that is under massive compressive stress from the outside, and equal tensile (pulling) stress from the inside.


This combination of pulling and pushing makes the glass incredibly sturdy; as strong as steel even. This means you can hammer away at it with tools and considerable force without damaging it. The tail of the Prince Rupert’s drop however, doesn’t go through the same intricate cooling process. Since it is much thinner, the cooling doesn’t pass through as many layers as the main body of the drop, and the more elongated shape of the tail makes it susceptible to damage from much smaller forces. The tail is in other words much easier to break.


Through a polarizing filter, the quenched layers become visible


It is the combination of these two different attributes that leads to the drop’s explosive properties. If tensile stress or compressive stress acted on the drops alone, they would destroy it; it is only through the balance of compression and tensing that the drop remains solid. The built up stress energy is stored inside the material structure of the glass, and if that structure is broken, the balance is upset. Thus, if a fragile part of the structure, such as the tail, is cut it will send a wave front of explosive nature down through the entire drop, breaking it into a million shimmering pieces.


If you want to check it out in amazing slow motion, Smarter Every Day has a video showing the process at 130,000 frames per second, right here below:

David F.
A grad student in experimental physics, David is fascinated by science, space and technology. When not buried in lecture books, he enjoys movies, gaming and mountainbiking

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