Prince Rupert's Drop

Have you heard of the ferocious, nearly indestructible Prince Rupert's Drop?

Prince Rupert's drops (also known as Dutch or Batavian tears) are toughened glass beads produced by dripping molten glass into cold water, where it solidifies into a tadpole-shaped droplet with a long, thin tail.

These glass drop structures exhibit remarkable material properties, demonstrating that glass is more than just a brittle, easily broken material.

Smacking the droplet's head with a hammer, pressing it with up to 20 tonnes of force, or even shooting it with a gun won't do much damage.

To break it, however, you only need to tap its tail, which will cause the entire object to disintegrate into a fine powder.

The drops are named after Prince Rupert of the Rhine, who brought them to England in 1660, although they were reportedly produced in the Netherlands earlier in the 17th century and were likely known to glassmakers for much longer. The Royal Society studied them as scientific curiosities. The unraveling of the principles of their unusual properties most likely led to the development of the process for producing toughened glass, which was patented in 1874.

Prince Rupert's drops are simple to make; they're nothing more than molten glass. Dropping red hot blobs of molten glass into water is a simple way to make Prince Rupert's drops. Although researchers have been trying to figure out what causes these drops' unusual properties for many years, it wasn't until recently that modern technology allowed them to investigate them thoroughly.

The answer is found in the internal balancing act of compressive and tensile stresses. After dropping the glass into the water, the outside layer of the glass cools while the inside remains relatively hot.

When the drop's glass core cools, the molecules inside have nowhere to shrink because the outer layer is already set, so they pull toward each other, creating a super high tension inside the bulb that eventually hardens. 

However, because the outside of the glass is under extremely high compressive stress and the inside is under extremely high tensile stress, if one link breaks, the entire structure explodes, feeding off its stored internal energy.