Science Wednesday Prince Rupert’s Drops

I found the "mystery" of Prince Rupert’s drops fascinating for several reasons.  First the story:

Since the 17th century, Prince Rupert’s drops have puzzled scientists. The drops are made by dipping a bead of molten soda-lime or flint glass in cold water, which forms a tadpole-shaped piece of glass. While the head of the drop is incredibly strong and resist everything from a hammer blow to speeding bullets, just flicking the tail of the crystal can cause the whole thing to burst into powder. Now, as David Szondy at New Atlas reports, researchers have finally figured out the secrets behind these drops.

In 1994, researchers used high-speed photography to record and analyze the way the drops shatter, Lisa Zyga reports for Phys.org. They concluded that the surface of the drop has high compressive stress while the interior of the drops is under high tension. While that combo makes the head very strong, it’s not in equilibrium, which means even a slight disruption at the tail causes the whole thing to destabilizing and fall apart. In fact, the cracks move at 4,000 miles per hour, which pulverizes the glass.
But it wasn’t until recent technological advances, however, that could researchers examine the stress distribution in detail. They used a transmission polariscope, a type of microscope to study the tensions within the glass. By sending red LED light through the drop while it was submerged in a clear liquid, they could measure how stresses in the drop slowed down the light, giving them a rainbow-colored optical map of the forces within the drop. Using mathematical models they then calculated the various interior and exterior forces. The researchers published their results last year in the journal Applied Physics Letters.

Check out the video at Smithsonian.

Now as to why this caught my eye.  If you had asked me in my fifth or sixth year of college about this I could have rattled off the answer and it would have taken a few hours to work up the rough math to support my answer.  It never occurred to me that such a phenomena would be considered a mystery.

Second is the use of polarized light to reveal stress.  I did this in my first real job designing aerospace fasteners.  We even made up hundreds of samples to prove the design was not stress inducing and handed them out to disbelieving colleagues.

Regardless, really cool stuff. 

Why mention this today?   Google is celebrating the 115th anniversary of the Antikythera Mechanism's discovery with a new Doodle.