When a drop of molten glass is put into water, it rapidly cools – the outside cools and contracts, but the inside has no room to contract. So it “pushes” against the outside, creating internal pressure.
So when something else (i.e. hammer) tries to push from the outside, the pressure from the inside is enough to offset it.
If the tail is snapped, however, that creates a structural weakness, and that lets the pressure inside find a “way out”, so to speak – and the whole thing explodes.
Fun fact: PRD are responsible for the discovery and development of tempered glass.
It is an example of prestressing. The drop has high radial tension which is in the opposite direction from any external blow or impact. Thus an impact tends to reduce rather than create mechanical stress.
Glass is an extremely strong material, but it cannot stretch at all. Any microscopic defect in its surface creates a crack when stretched, the glass breaks.
If you can keep the surface from stretching, you can get fantastic performance out of glass. In a PRD, the core shrinks as it cools, and so the outside of the drop is being squeezed with tons of force. Before it can stretch at all, that force must be overcome, and so it is very strong.
When glass is in tension, a tiny crack can easily grow, when it’s in compression, that crack will just be squeezed back together.
In a prince rupert’s drop the outside freezes and cools first, and the inside is still molten, so when the glass on the inside shrinks as it cools the inside ends up in tension, and the outside ends up in compression. If you can get past the compressed shell(for example by breaking the tail) all that stored stress gets released at once, shattering it to dust.
Basically glass can only fail by tension. The way the PRD cools, it has a high compressive layer on the outside.
This means that for the glass to break, it needs to experience a tensile force that overcomes that compressive layer + the regular tensile strength of the glass.
To give a TLDR of that video, it’s all about preload. Putting tension on a junction of materials increases the strength of that joint significantly.
So the science here is pretty cool! But during the cooling process of a prince ruperts drop there is a lot of tension! Which preloads the drop so much that in order to break the head, you must overcome the overall tension of the joints inside the drop, or the bonds between different glass molecules!
So technically you can break the head of a PRD, but you must achieve a transfer of MORE energy than is already being enacted on the drop by internal tension.
Now the reason it explodes when you hit the tail is because the tail has a lot less tension and requires must less energy to break it, and this is true all the way up the drop. It’s a taper. So as the drop breaks towards the tail, it releases stored tension. This release of energy is enough to break the next portion of the drop working towards the head, so on and so forth till it explodes!
It’s not the drop that is strong, glass is. Glass is very very hard. Harder than steel. But it is brittle, it has structural frailty, making it not resistant in all directions. In a prince Rupert drop, that structural strenght is in all directions at the “sack” of the drop, while the weakness is only at the tail. Which is why when you hydraulic press the Rupert drop at the sack, it dents the press metal. If, however, the press had a material harder than the glass as its plate, the drop would explode.
Think of this way. You want to crush a soft baked cookie, it’s easy because the material is soft and pliant.
Now imagine a hard cookie, it’s hard to break, but it’s also full of air so crushing it simply requires breaking the solid bits until they crush into the open air.
Now imagine you’re crushing a piece of rock. It can’t squish because it’s not malleable, but if you try hard enough, you can compress it into what little free space it has to crush it.
A Rupert’s drop is so extremely dense and so highly compressed, the force goes directly from its source to its destination without impacting the drop itself because it simply can’t move.
They can be crushed, but only by applying force that is far stronger than anything in its surrounding.
Comments
When a drop of molten glass is put into water, it rapidly cools – the outside cools and contracts, but the inside has no room to contract. So it “pushes” against the outside, creating internal pressure.
So when something else (i.e. hammer) tries to push from the outside, the pressure from the inside is enough to offset it.
If the tail is snapped, however, that creates a structural weakness, and that lets the pressure inside find a “way out”, so to speak – and the whole thing explodes.
Fun fact: PRD are responsible for the discovery and development of tempered glass.
https://www.reddit.com/r/explainlikeimfive/s/OcSR0Cs7r9
It is an example of prestressing. The drop has high radial tension which is in the opposite direction from any external blow or impact. Thus an impact tends to reduce rather than create mechanical stress.
Glass is an extremely strong material, but it cannot stretch at all. Any microscopic defect in its surface creates a crack when stretched, the glass breaks.
If you can keep the surface from stretching, you can get fantastic performance out of glass. In a PRD, the core shrinks as it cools, and so the outside of the drop is being squeezed with tons of force. Before it can stretch at all, that force must be overcome, and so it is very strong.
When glass is in tension, a tiny crack can easily grow, when it’s in compression, that crack will just be squeezed back together.
In a prince rupert’s drop the outside freezes and cools first, and the inside is still molten, so when the glass on the inside shrinks as it cools the inside ends up in tension, and the outside ends up in compression. If you can get past the compressed shell(for example by breaking the tail) all that stored stress gets released at once, shattering it to dust.
So, how about this follow up question. What is, if any, practical application of a PRD, in particular?
There are dozens of video’s on Youtube that explain it.
Basically glass can only fail by tension. The way the PRD cools, it has a high compressive layer on the outside.
This means that for the glass to break, it needs to experience a tensile force that overcomes that compressive layer + the regular tensile strength of the glass.
Maybe a helpful read, with deeper explanation and pictures: https://msestudent.com/prince-ruperts-drops-the-exploding-glass-teardrop/
Well unironically I think you should watch this video on nuts and bolts:
https://youtu.be/XLzTB4KLCxU?si=gmirv9hU8EIgEypG
To give a TLDR of that video, it’s all about preload. Putting tension on a junction of materials increases the strength of that joint significantly.
So the science here is pretty cool! But during the cooling process of a prince ruperts drop there is a lot of tension! Which preloads the drop so much that in order to break the head, you must overcome the overall tension of the joints inside the drop, or the bonds between different glass molecules!
So technically you can break the head of a PRD, but you must achieve a transfer of MORE energy than is already being enacted on the drop by internal tension.
Now the reason it explodes when you hit the tail is because the tail has a lot less tension and requires must less energy to break it, and this is true all the way up the drop. It’s a taper. So as the drop breaks towards the tail, it releases stored tension. This release of energy is enough to break the next portion of the drop working towards the head, so on and so forth till it explodes!
Because it shrinks and the middle of it goes sucky sucky so that the outside can’t escape
It’s not the drop that is strong, glass is. Glass is very very hard. Harder than steel. But it is brittle, it has structural frailty, making it not resistant in all directions. In a prince Rupert drop, that structural strenght is in all directions at the “sack” of the drop, while the weakness is only at the tail. Which is why when you hydraulic press the Rupert drop at the sack, it dents the press metal. If, however, the press had a material harder than the glass as its plate, the drop would explode.
Think of this way. You want to crush a soft baked cookie, it’s easy because the material is soft and pliant.
Now imagine a hard cookie, it’s hard to break, but it’s also full of air so crushing it simply requires breaking the solid bits until they crush into the open air.
Now imagine you’re crushing a piece of rock. It can’t squish because it’s not malleable, but if you try hard enough, you can compress it into what little free space it has to crush it.
A Rupert’s drop is so extremely dense and so highly compressed, the force goes directly from its source to its destination without impacting the drop itself because it simply can’t move.
They can be crushed, but only by applying force that is far stronger than anything in its surrounding.