Mystery behind Prince Rupert’s Drop

Did you ever break a glass? I hope most of you did. But do you know, there is a glass which is unbreakable? Even a hammer or a bullet didn’t break that glass. Want to try to break it….

Welcome to Engineering Master. In this article we will tell you all about Prince Rupert’s drop and tell you how to break it.

Prince Rupert’s Drop is a small piece of glass which is created by dripping molten glass into cold water. A tadpole shaped glass with a thin tail is obtained after it solidifies.

Credits- Smarter Everyday

The head of the drop is so strong that it can withstand the impact of hammer or a bullet. But the tail is brittle. So, if you bend the tail with your fingers than it will break and the entire drop is disintegrating into fine powder.

Let’s talk about the history of this drop. In 1660 Prince Rupert of the Rhine gave a number of glass bubbles to King Charles 2 of England as a gift who passes them to the Royal Society of London to conduct a few studies.

The mystery behind this property is unknown till recent years.  In 1994, S. Chandrasekhar at Purdue University and MM Chaudhri at the University of Cambridge used high speed framing Photography to observe the drop splitting process. From their experiment, they concluded that the surface of each drop experiences highly compressive stresses, while the interior experiences high tension forces. So the drop is in a state of unstable equilibrium, which can be easily disturbed by breaking the tail.

Now the question arises, how stresses are distributed throughout the Prince Rupert’s drop? Chandrasekhar and Chaudhri collaborate with Hillar Aben, a professor at Tallinn University of Technology in Estonia. They have investigated the stress distribution in Prince Rupert’s Drop using a poloariscope (a type of microscope that measures the birefringence in an axi-symmetrical transparent object).

In this experiment, the researchers suspended a Prince Rupert’s drop in a clear liquid and then illuminated the drop with a red LED. Using the polariscope, the researchers measures the optical retardation of the light as it travelled through the glass drop and then used the data to construct the stress distribution throughout the entire drop.

The result showed the heads of the drop have much higher surface compressive stress up to 700 MPa, which is nearly 7000 times the atmospheric pressure. The surface compressive layer is also thin- 10 % of the diameter of the head of a drop.

These values give the droplet heads very high fracture strength. In order to break the drop, it is necessary to create a crack that enters the interior tension zone in the drop. Since cracks on the surface grow parallel to the surface, they cannot enter the tension zone. Instead, the easiest way to break a drop is to disturb the tail. This disturbance allows cracks to enter the tension zone.
Researchers believe that this theory/research explains the strength of Prince Rupert’s Drop.

44 Replies to “Mystery behind Prince Rupert’s Drop”

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  2. Nice piece of information you have shared in your own unique style.Though not been from engineering background I understood the article.Good luck and keep sharing such articles.

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