Saturday, March 23, 2013

Exploding glass: slowed down

This is not a blog entry about state-of-the-art physics but it is about mechanics of solids which is a part of physics which is why it may have its place on TRF.

Prince Rupert's Drops are not named in this way because a prince invented them but because a prince brought them from North Germany or Bavaria or Holland to England in 1660.

They are created by rapidly cooling molten glass in water. This makes a tadpole-shaped structure.

This piece of glass turns out to be remarkably resilient because the pressure deep inside the drop is higher. It isn't easy to break it, unlike most of the floorball sticks I have bought in the last 5 years (not to mention a rib). Well, if you try, it does break.

But if you look what's happening each eight microseconds – if you look by a camera running at 130,000 fps – you will discover that it's not the hammer that breaks the glass. It's a seemingly innocent vibration spreading from the tail of the tadpole. The brand new video above kind of clearly and visually explains what's going on. Not even Prince Rupert could have seen such a clear history of his exploding glass.

Those 130,000 fps may sounds like a lot. But note that the LHC detectors effectively have 40,000,000 fps – there are about 600 million collisions per second but it doesn't hurt too much if some of them are clumped together. Well, there are also some other differences in the ATLAS/CMS "cameras" that make each of these detectors cost a billion of dollars, more than the camera used to create the video above. :-)

If you know how to explain that the tail matters so much and why the explosion ultimately runs in the outward directions – and not just back into the tail – you are invited to post your comment. ;-) For more similar videos, see the Smarter Every Day channel.


  1. Amazing. Does the vibration created by the hit warms up the tail ?.. and then the difference between the cold and the heat still inside the drop triggers the explosion ?

  2. Tempered glass has been made this way for decades. “Pyrex” products once were made of expensive borosilicate glass but are now made from tempered soda glass, which is much cheaper.
    Glass and all amorphous solids fracture via crack propagation and cracks can propagate over a wide range of speeds, from zero up to the speed of sound in the material, SQRT(E/d), where E is the elastic modulus and d the density. The propagation rate depends on the local stress and the chemistry going on at the tip of the crack. Foreign fluids, notably water, tend to assist propagation.
    It is energetically difficult to initiate a crack within the volume, i.e. away from a surface. In the extreme example of the video there are doubtless millions of micro-cracks internally, despite this difficulty, but they cannot propagate to the surface because they ultimately reach the compressive zone and the stress becomes zero. Once any portion of the surface experiences significant tensile stress (due to flexure in the video) all the energy is released explosively.

  3. Good point about PYREX. It "explodes" too. Back in the 1950s I remember my grandmother bewilder over a Pyrex container that had disappeared while it was sitting on the kitchen counter with hot food, the lid sitting over the spilled food. She kept saying "I did not do anything, I did not do anything".


  4. When I saw that the origin of the Prince Rubert's Drops was not known for certain, I wondered. "why not Bohemia, with it's rich glass blowing tradition?" ... and sure enough Wikipedia tells me that Rupert was in fact born in Prague, but then reading on I see that his residence in the area ended (rather dramatically) soon after his birth.

  5. Right, he was an ethnic German but he aligned himself with the revolting Czech protestant nobility which is also why he lost the 1620 Battle of White Mountain with "us" - we generally view it as the darkest moment of the non-modern Czech history.

    Just to be sure, once again, Prince Rupert didn't invent this glass fun.

    Most of the glass industry in Bohemia has traditionally been done by ethnic Germans, too.

  6. Real (borosilicate) Pyrex does not explode. The coefficient of thermal expansion of borosilicate glass is very small; it does not need to be tempered. That’s how Pyrex products got started. It is only when they cheapened the product by substituting tempered soda glass that the explosions started.
    My mother had a couple of real Pyrex cooking pans which she put directly on the gas burners with no problems.

  7. I once had a darkroom enlarger made in Czechoslovakia, doubtless in Bohemia. The glass lenses were excellent despite its commie origin.

  8. Lubos, you said "because the pressure deep inside the drop is higher" which I think is wrong. The surface of the drop is under compressive stress, while the inside is under tensile stress. That's because the outside hardens first, fixing the shape and volume. The inside wants to contract as it cools later, but is prevented, and thus comes under tension. Right?