Interesting article from 'Interesting Engineering'

https://interestingengineering.com/science/scientists-discover-liquids-can-fracture

So a simple question on this, what makes a liquid, a liquid in terms of viscosity, given that water is free flowing (if put on a tray and the tray is moved around the water will move around freely). However, if I put cooking oil on the tray and move the surrounding tray requires more tilt to move the oil, (it also depends on friction from the tray I guess (smooth vs rough surface).

So do we think about solids and liquids differently ?

By fracturing are they suggesting that the bonds in the molecules break or are they referring to the forces that hold the molecules in state where the state would be classed as a liquid. I think oils have long chain, so is it the chain that is pulled for forced apart.

Paul

1 hour ago, paulsutton said:

Interesting article from 'Interesting Engineering'

https://interestingengineering.com/science/scientists-discover-liquids-can-fracture

So a simple question on this, what makes a liquid, a liquid in terms of viscosity, given that water is free flowing (if put on a tray and the tray is moved around the water will move around freely). However, if I put cooking oil on the tray and move the surrounding tray requires more tilt to move the oil, (it also depends on friction from the tray I guess (smooth vs rough surface).

So do we think about solids and liquids differently ?

By fracturing are they suggesting that the bonds in the molecules break or are they referring to the forces that hold the molecules in state where the state would be classed as a liquid. I think oils have long chain, so is it the chain that is pulled for forced apart.

Paul

This immediately made me think of bitumen. Bitumen blocks will slowly flow under their own weight, given enough time, yet you can shatter them with a hammer. Technically bitumen is a liquid, but one that is so viscous it behaves in practice more like a solid. So I suppose what they have done is explore this for liquids of relatively low viscosity and found there can come a point at which the liquid is unable to flow fast enough to react to the force pulling it apart, whereupon it breaks like bitumen.

I would imagine in molecular liquids the bonds being broken will be just the intermolecular forces rather than the bonding within the molecules.

My understanding was that any amorphous solid is technically a 'liquid', albeit a very slow flowing one, due to intermolecular bonds that are too weak to form a regular crystalline structure.
Examples being glass or amorphous Aluminum alloys, and certain other inorganic glass ceramics.

15 minutes ago, MigL said:

My understanding was that any amorphous solid is technically a 'liquid', albeit a very slow flowing one, due to intermolecular bonds that are too weak to form a regular crystalline structure.
Examples being glass or amorphous Aluminum alloys, and certain other inorganic glass ceramics.

This is sort of interesting. I thought this, but someone told me that is wrong because window glass, for example, is below something called the "glass transition temperature". So, according to this, a glass is an "amorphous solid" rather than a very viscous liquid.

However, when I look up what a glass transition temperature is, it appears it is an imprecisely defined range of temperature, rather than a specific value. Furthermore, when making this "transition", there don't seem to be any of the features one associates with a genuine phase change, such as release of latent heat, change in order at the molecular level, etc.

So now I am left with the suspicion that the distinction is a bit bogus and based only on how it seems best to treat the material in practice, rather than any physical or chemical change in state. But I'd be interested in a comment from anyone better informed.

46 minutes ago, exchemist said:

This is sort of interesting. I thought this, but someone told me that is wrong because window glass, for example, is below something called the "glass transition temperature". So, according to this, a glass is an "amorphous solid" rather than a very viscous liquid.

However, when I look up what a glass transition temperature is, it appears it is an imprecisely defined range of temperature, rather than a specific value. Furthermore, when making this "transition", there don't seem to be any of the features one associates with a genuine phase change, such as release of latent heat, change in order at the molecular level, etc.

So now I am left with the suspicion that the distinction is a bit bogus and based only on how it seems best to treat the material in practice, rather than any physical or chemical change in state. But I'd be interested in a comment from anyone better informed.

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

I see here changes in heat capacity with increasing temperature on curve on the right graph..

3 hours ago, paulsutton said:

So do we think about solids and liquids differently ?

1 hour ago, MigL said:

My understanding was that any amorphous solid is technically a 'liquid', albeit a very slow flowing one, due to intermolecular bonds that are too weak to form a regular crystalline structure.

48 minutes ago, exchemist said:

This is sort of interesting. I thought this, but someone told me that is wrong because window glass, for example, is below something called the "glass transition temperature". So, according to this, a glass is an "amorphous solid" rather than a very viscous liquid.

However, when I look up what a glass transition temperature is, it appears it is an imprecisely defined range of temperature, rather than a specific value. Furthermore, when making this "transition", there don't seem to be any of the features one associates with a genuine phase change, such as release of latent heat, change in order at the molecular level, etc.

So now I am left with the suspicion that the distinction is a bit bogus and based only on how it seems best to treat the material in practice, rather than any physical or chemical change in state. But I'd be interested in a comment from anyone better informed.

This has always been my understanding too, for a long time. But I hesitate to tell students glass is a liquid in which every molecule more or less keeps in place, just because there is no recognizable spatial pattern. Another criterion I seem to remember is liquids do not resist shear, which amorphous "solids" do.

I would be more favourable to enriching the classification of different phases.

It's been a long time since we know of liquid crystals. Planetary science introduces all kinds of different phases of either amorphous or crystalline materials too. Water alone can crystallise in many different systems.

If I remember correctly, second-order phase transitions do not have a discontinuous change of entropy (which gives rise to a latent heat) and therefore are continuous. Yet they are still considered phase transitions in some sense.

38 minutes ago, Sensei said:

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

I see here changes in heat capacity with increasing temperature on curve on the right graph..

Aha, thanks, so there is a progressive kink in the heat capacity curve, suggesting more degrees of freedom become available as the temperature rises through this transition range.

Perhaps, thinking about this, part of issue for me is that glasses are non-equilibrium states. Just about all the chemistry I learnt was really about equilibrium states. The statistical mechanics of non-equilibrium states will be quite a bit more complex.

I do remember reading that one of the reasons for space based telescopes, other than mitigation of atmospheric effects, was that large glass mirrors, like the 200 inch Mt Palomar, will 'flow' over time,due to gravity, and give rise to optical defects.

52 minutes ago, MigL said:

I do remember reading that one of the reasons for space based telescopes, other than mitigation of atmospheric effects, was that large glass mirrors, like the 200 inch Mt Palomar, will 'flow' over time,due to gravity, and give rise to optical defects.

Dimensional analysis suggests this to be an exaggeration. At room temperature, glass viscosity is somewhere around 10²⁰-10²² Pa·s. Atmospheric pressure is of the order 10² Pa. The dimensional quantity with units of time that results is the order of 10²⁰ s, which exceeds the age of the visible universe by 3 orders of magnitude. Gravity being implied in atmospheric pressure.

With desert temperatures it becomes a tiny bit more credible, but still...

Edit:

Sorry, atmospheric pressure is not 10². It' rather 10⁵ in Pa (different format for numbers here, I'm sorry). But still...

Edited 10 minutes ago10 min by joigus
correction

6 hours ago, paulsutton said:

Interesting article from 'Interesting Engineering'

https://interestingengineering.com/science/scientists-discover-liquids-can-fracture

So a simple question on this, what makes a liquid, a liquid in terms of viscosity, given that water is free flowing (if put on a tray and the tray is moved around the water will move around freely). However, if I put cooking oil on the tray and move the surrounding tray requires more tilt to move the oil, (it also depends on friction from the tray I guess (smooth vs rough surface).

So do we think about solids and liquids differently ?

By fracturing are they suggesting that the bonds in the molecules break or are they referring to the forces that hold the molecules in state where the state would be classed as a liquid. I think oils have long chain, so is it the chain that is pulled for forced apart.

Paul

First let me say thank you Paul for bringing this to our attention. +1

However the linked article seems more like the hyped up writings of a sensationalist journalist, than an august Professor.

What is meant, for instance, by pulling a liquid apart ? or by breaking a liquid ?

The classification into solids, liquids and gases was known 100 years ago to be seriously simplistic and unable to describe the behaviour of most materials in the real world.

A good question to ask is what causes solids to break ?

In fact solids have several different breakage mechanisms available, including one where the solid cannot break at all as it is so confined.

So a second good question is what gives them their strength ?

A third one might be what is a solid ?

A pile of dry sand or wheatgrain follows the laws of fluid mechanics.

What about a tube of toothpaste ?

All these issues and many more are largely ignored by physicists, but taken up by Rheologists, Chemists, Pharmacists, Geologists and others.

There is significant modern research into the properties of monomolecular layers.

So where would you like to start ?