If the core is contracting and causing the surface particles to push on each other - shouldn't that mean that the core is in compression (since it is shrinking) while the surface is in tension (as the surface particles are pushing on each other).
I watched Smarter Every Day's video on Prince Rupert's Drop, and I still didn't understand, so help would be appreciated.
The core is in tension because the surrounding glass prevents it from shrinking as much as it naturally would if the surrounding glass was not present.
In 1994, Chandresekhar from Purdue did experiments to try and finally "crack" the mystery of these drops.
From what I've learned about these drops over the years, along with intermolecular forces and the way they behave and interact, it's not about what happens as the glass cools, but rather the way that those internal tension forces and compression stresses distribute after cooling.
His experiments showed that the surface is under high compression stresses and internally it is experiencing major tension forces. Just this fact alone means that the drop is in a kind of unstable equilibrium. This makes it ripe for fracture.
So why couldn't the hammer break it? Because his experiments also showed that the head of the drop has 700 megapascals of surface compressive stresses. It's about 7,000 times atmospheric pressure; much higher than originally thought.
Usually, cracks on the surface will create fractures that are parallel to the surface--hence, why the hammer couldn't break the drop.
By tweaking the tail, this creates a fracture crack that will actually enter the drop perpendicular to the surface and puncture the internal tension zone.
If interested, you can read the abstract that he and his team published in Applied Physics Letters here: https://aip.scitation.org/doi/abs/10.1063/1.4971339
First, appreciate that glass expands as it heats and shrinks as it cools. This is different than water and ice, where water expands as it forms ice.
Second, compression means that the environment surrounding a material is pushing inward on it (making the equilibrium position of the atoms closer together than they would be without the external compression), and tension means that the environment is pulling outward on it (making the equilibrium position of the atoms farther apart than they would be without the external tension).
Third, glass deforms plastically at high temperatures and gradually becomes more rigid as its temperature drops. A plastic deformation will not result in internal stresses (compression or tension), but applying strain to a rigid material will.
Now, picture a pane (or a drop) of glass, initially at a uniformly high temperature, as it cools from the outside. The outer layer cools first, hence shrinks first, plastically at first, then rigidly. The inner layer stays hotter and hence plastic for longer, so it plastically deforms to adjust to the now-cooler (and shrunken) outer layer. Then, the inner layer cools and contracts, but it is no longer able to deform plastically. So, as it contracts, it pulls on the outer layer (parallel to the surface), putting the outer layer in compression and the inner layer in tension.
I have an excellent human-scale example of this. My wife loves to have her back stretched out (put in tension). Often when she hugs me, she also lifts her feet off the floor, hence my back is supporting both her weight and mine. This puts my back in compression (analogous to the outer layer of tempered glass) and her back in tension (analogous to the inner layer of tempered glass).
Then, since glass is stronger when it is in compression, tempered glass is overall stronger than untempered glass. Although if a crack reaches the inner layer, it propagates very quickly, resulting in near-complete shattering of the entire pane of glass.
