The Pitch Drop Experiment

Hi,

Glass does indeed flow over time. I used to work in a lab and there was a room set aside for glassblowing. About 20 years before I got there, the staff used to make glass coils for a gas chromatograph from straight lengths of thin wall soda glass tubing (OD about 4mm) which were about 6 feet long. These were stored in a wooden box which was about 5 feet long, so one end was in the box while the other end was supported by the top of the box. After 20 years or so, each of the tubes was permanently curved, with the middle of them being at least 6 inches away from a straight line between the two ends. This was presumably due the plastic deformation (flow) of the glass over that time.

If you want to see a really long term experiment, have a look at this:

https://en.wikipedia.org/wiki/Oxford_Electric_Bell

Truly amazing.

Regards,

Alan

Alan

I don't know the mechanism affecting the coiled glass but I do know that a similar effect can be seen in suspended lengths of most metals, plastic, wood etc. The thinner the section and the longer the unsupported length the greater the resulting bend.

The force acting of course is gravity but how the molecular structure responds ( differently I suspect in each of the above materials ) is beyond my mostly retired pay grade.

Pero

My contention that glass does not flow and that it does not fit the definition of a supercooled liquid is supported by the science (where have I heard that before?) as I understand it so reading your post made me think.

I suspect that the glass was not flowing as if that were the case it would also be thicker at the bottom and develop a 'club foot' so some other mechanism might be at work. I consider this to be confirmed firstly by thinking about wood and similar materials which if stored as you suggest would likely also the have developed a set as you describe with no thought of them flowing.

Certainly bowing of a thin object could be caused by elastic and extraelastic deformation, but this also occurs in solids where the molecules, unlike in a glass, are bound in a lattice and the lattice itself is distorted. For flow to occur the molecules would probably need to move relative to each other.

Hi Nick,

I don't think your assertion that glass is not a supercooled liquid is correct. If you look at the ternary phase diagram of the system Na2O - CaO - SiO2 (soda glass) there are a whole host of different compounds which can crystallise from it, most notably devitrite - Na₂Ca₃Si₆O₁₆. If molten glass is slowly cooled from above the liquidus temperature and held at or just below it, the mass will crystallise. The fact that it hasn't in your windows, is that the rate of cooling below the liquidus is such that the viscosity quickly rises to a level which prevents the atoms in the glass from migrating to take their places in a crystal lattice. Normally, a supercooled liquid is formed when the liquid is completely free of impurities which can act as seeds for crystal nucleation, and the system remains liquid below the liquidus temperature shown in its phase diagram. I would suggest that glass is in the same state - i.e. amorphous (non - crystalline) below the liquidus temperature.

As far as other materials (steel or wood for example) also becoming curved when supported at their ends, I would make the following argument.

I believe that liquid flow is the result of the elements within the substance rearranging themselves at an atomic or molecular level. This would be the case with an amorphous material such as glass. It would not however be the case in steel, where the rearrangement would involve crystalline components sliding past each other or wood which would involve the fibres sliding past each other.

Of course my argument is predicated on my definition of liquid flow which could be a load of bollocks.

Regards,

Alan

Thanks for your comments Alan, however I don't think they affect the now commonly held view that Glass is not a supercooled liquid.

Taking the first one - I quite agree that glass is amorphous, but that as an amorphous solid the molecules do not flow and rather than having a high viscosity as would be the case above the glass transition temperature, the molecules are joined together but not in a regular pattern.

Working at a more practical level - lenses and ancient glass objects would be expected to show some flow and windows would not only be thicker at the bottom but would exhibit flow over frames and the nails holding the panes into the frame, which I have never seen suggested. Hence I am of the belief that the thicker at the bottom concept is due to manufacturing rather than flow. The comment one sometimes sees that glass becomes more solid with age and very old objects are less likely to flow also appears to be at variance with the facts as it is the oldest building glass that is said to exhibit the most flow.

Referring to your second point, that the glass in your lab had to flow to change shape. You have explained how this cannot be the case with wood or steel but both of these materials do change shape and while I agree that it is not due to flow - I cannot agree that glass is a special case and has bent for a unique reason.

Please consider what I have written - I am trying to summarise information garnered over the last 25 years or so since the first arguments against the 'glass is a supercooled liquid' theory were discussed in the scientific press. However it is the accepted theory at present - but the one thing I hope is not to fall out with anyone here, but rather to present accepted and justified (most important that!) scientific thought as appropriate.

Take care,

Nick