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Researchers Have Finally Cracked 400 Years Old Mystery of Prince Rupert's Drops

In the new study published in Applied Physics Letters, researchers have revealed the secret of Prince Rupert's drops. In 17th century Prince Rupert of Germany brought some tadpole-shaped pieces of glass drops to England's King Charles II, who was fascinated by their strange properties.

Although the head of the drop is so solid that it can resist the power of a hammer, the tail is so delicate that twisting it with your fingers will not only breakdown the tail, but cause the complete droplet to rapidly disintegrate into a fine powder. How it happens is unexplained and is one of oldest physical mysteries.
Prince Rupert’s drop.

These strange characteristics of Prince Rupert's drops has stumped scientists since the early 1600s. But now they have finally solved this.

Aben, Chandrasekar, Chaudhri, and their coauthors of the research paper, examined the stress distribution in Prince Rupert's drops by means of a transmission polariscope, which is a kind of microscope that measures the birefringence in an axi-symmetrical see-through object, such as a Prince Rupert's drop.

In their tests, the researchers placed a Prince Rupert's drop in a pure transparent liquid, and then lightened the drop with a red LED. Using the polariscope, the scientists measured the optical retardation of the light as it journeyed through the glass drop, and then used the data to create the stress distribution throughout the whole drop.
Fringes throughout a Prince Rupert’s drop indicate residual stresses. Credit: Aben et al. ©2017 American Institute of Physics

The outcomes disclosed that the heads of the drops have a much greater surface compressive stress than previously thought—up to 700 megapascals, which is approximately 7,000 times atmospheric pressure. This surface compressive layer is also thin, about 10% of the diameter of the head of a drop.

As the researchers describe in the paper, these values offer the droplet heads a very high fracture strength. In order to breakdown a droplet, it's essential to create a crack that enters the internal tension zone in the drop. 

Since cracks on the surface incline to grow parallel to the surface, they cannot move in the tension zone. Instead, the easiest way to breakdown a drop is to disturb the tail, since a disorder in this locality lets the cracks to enter the tension zone.

The researchers consider that the fallouts finally clarify the great strength of Prince Rupert's drops.

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