The science behind the Prince Rupert’s Drop
Prince Rupert’s Drop Original Photo by Michael Grogan, released to the public domain.
These days most people have no problem finding a source of entertainment. Every man and his dog owns a smartphone with a soul-sucking application that you could dedicate your life to mastering. I’m looking at you Candy Crush.
During the 17th century, the average European didn’t have access to the amusement we take for granted. In fact, most people at the time were starved of entertainment. Cards, chess, backgammon and other board games were the favoured pastimes but this can get pretty repetitive and boring after a while. But every now and then a new invention would come along and leave the population fascinated.
Such was the case when in 1660, Prince Rupert, who was an incredibly interesting fellow, brought to King Charles II a teardrop-shaped piece of glass with a long tail that showed some interesting properties. Firstly, the head of the teardrop was incredibly strong and could withstand very harsh blows from hammers or other hard instruments. Secondly, the tail of the glass teardrop was incredibly brittle and could be easily snapped. Thirdly, and most astounding, is that as soon as a break occurred along any part of the tail the entire drop would explode in a shower of glass shrapnel.
Don’t try this at home
To understand why this happens we need to know how these incredible drops form. Known now as a Prince Rupert’s Drop, the enigmatic glass object was originally produced in Mecklenburg, Germany as a Dutch tear. The creators guarded the secret of the Dutch tear while distributing them across Europe as toys for children and adults.
We know now that the drop is formed by pouring molten glass into cold water. The outside of the drop cools rapidly while the inside remains molten and cools more slowly.This is the key behind the mystery of the Prince Rupert’s Drop. The outer molecules of glass crystallise incredibly quickly, leaving a thin layer of hardened glass around the remaining molten chamber. This chamber will slowly contract as it cools due to the effect of thermal expansion, which causes liquids to expand when heated and contract when cooled.
As this chamber slowly cools it pulls at the surrounding wall of already solidified glass, which experiences high compressive stress especially around the drop’s head. As the molten glass in the chamber crystallises the tensile stress builds up as a chain stretching from the head of the drop to its tail.
The glass eventually crystallises and locks itself in this state of high tension. If any part of the chain is broken, the tensile stress is released resulting in the release of the stored energy along the chain. This causes the drop to explode from the inside out and from the tail to the head. You can actually see the movement of failure in this video as well as a great visual representation of what occurs within a Prince Rupert’s Drop. It will probably be the most interesting video you watch today.
Here’s another one for good measure because it’s just so cool.
Prince Rupert’s projectile?
Volcanic Bomb Original Image by Mark Wilson, released to the public domain.
What’s even more interesting is that this phenomenon can occur naturally in lava and volcano ejecta, which crystallise rapidly as they fly out of the volcano.
The production of shot balls, which are lead projectiles used in shotguns and cannons, use the same principle to help harden the lead ball. Molten lead is dropped down a shot tower, like the one in Melbourne Central, into a pool of water. Because these projectiles are spherical, they do not have an exposed tail to destroy but rather all the tensile force is concentrated into its core. Also lead is far less brittle than glass and it would not shatter in the same manner.
Unfortunately for King Charles II he never discovered the secret behind the Prince Rupert’s Drop. However, if he did it would have spoiled a sense of wonder and curiosity that a large portion of 17th century Europeans experienced thanks to its amazing mechanical properties.