Recently, researchers have been finally able to answer a question that stumped scientists ever since the early 1600s, being Why are the heads of the tadpole-shaped pieces of glass, called “Prince Rupert’s drops” so strong?.

Back in the 17th Century, Prince Rupert from Germany brought some of these glass drops to the King of England, Charles II, who was intrigued by their unusual properties. The mystery lies in the fact that while the head of the drop is so strong that it can withstand the impact of a hammer, the tail is so fragile, that bending it with your fingers will not only break it, but cause the entire droplet to disintegrate in an instant, into a fine powder.

The Prince Rupert’s drops are easily made, by dropping red, hot blobs of molten glass into water. Despite the efforts made for many years by the scientists to understand the causes that trigger the unusual properties, it was only until recently, that modern technology has allowed researchers to investigate them thoroughly.

In order to uncover the mystery around the droplets, back in 1994, two scientists from the Purdue University, along with the University of Cambridge, used high-speed framing photography to be able to observe the drop-shattering process. On the basis of their experiments, it was concluded that the surface of each drop experienced highly compressive stresses, while the interior experienced high tension forces. The drop reaches a state of unstable equilibrium, which can be easily disturbed by breaking the tail.

In order to get a proper understanding of the functionality of the drop, the scientists investigated the stress distribution using a transmission polariscope, which is the type of a microscope that measures the birefringence in an axi-symmetrical transparent object. A Prince Rupert’s drop was suspended in a clear liquid, and then it was illuminated with a red LED. Using a polariscope, the researchers measured 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 results showed that the heads of the drops have a much higher surface compressive stress than previously thought, being up to 700 megapascals, that is 7,000 times atmospheric pressure. The surface compressive layer is thin, about 10% of the diameter of the head of a drop.

According to the findings, the values presented above provide the droplet with a very high fracture strength, as in order to break it, it is necessary to create a crack that enters the interior tension zone in the drop. But the easiest way to break the drop is to disturb the tail, since a disturbance in this location allows cracks to enter the tension zone.