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 Sunday at 07:55 PM2 days 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 ?

6 hours ago, joigus said:

Dimensional analysis suggests this to be an exaggeration.

Maybe 'flow' was the wrong word to use.
I believe the specs for the Mt Palomar reflector/mirror are available, and I would love to see a 'back of the envelope' analysis of the amount of sag ( is that better ? ) in the mirror that the glass would experience between the vertical, and the low horizon, positions. And how does the sag increase with even larger mirrors ?
I believe Mt Palomar is still the largest ground based mirror assembly for the last 75 years; why have no larger ones been built ?
Otherwise, why go to segmented mirrors, or adaptive/deformable mounts ?

1 hour ago, MigL said:

Otherwise, why go to segmented mirrors, or adaptive/deformable mounts ?

The mirror segment actuators are controlled by software which can constantly adapt their configuration in real time to the prevailing seeing conditions affected by localized changes in atmospheric density. They aren't set-and-forget optics. The ground-based adaptive telescope views can apparently rival space-based telescopes in image quality.

Edited Monday at 04:51 AM1 day by StringJunky

9 hours ago, studiot said:

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.

Thanks and good point re article or how it is written. I have undertaken some more digging and found some better links to the research

firstly from the Dressel university news page

https://drexel.edu/news/archive/2026/March/liquid-breaking-point

Secondly to the published paper

https://journals.aps.org/prl/abstract/10.1103/t2vy-32wr

On Physical review letters

Hope fully these help a little more,

Paul

Would monomolecular layers. refer to Graphene,? which if I understand it is a single layer of carbon atoms, even though this is also an allotropic form of carbon.

6 hours ago, paulsutton said:

Thanks and good point re article or how it is written. I have undertaken some more digging and found some better links to the research

firstly from the Dressel university news page

https://drexel.edu/news/archive/2026/March/liquid-breaking-point

Secondly to the published paper

https://journals.aps.org/prl/abstract/10.1103/t2vy-32wr

On Physical review letters

Hope fully these help a little more,

Paul

Would monomolecular layers. refer to Graphene,? which if I understand it is a single layer of carbon atoms, even though this is also an allotropic form of carbon.

Where in these articles are monomolecular layers referred to?

6 hours ago, paulsutton said:

Thanks and good point re article or how it is written. I have undertaken some more digging and found some better links to the research

firstly from the Dressel university news page

https://drexel.edu/news/archive/2026/March/liquid-breaking-point

Secondly to the published paper

https://journals.aps.org/prl/abstract/10.1103/t2vy-32wr

On Physical review letters

Hope fully these help a little more,

Paul

Would monomolecular layers. refer to Graphene,? which if I understand it is a single layer of carbon atoms, even though this is also an allotropic form of carbon.

Thank you for the extra links.
Unfortunately the second is paywalled to me.
There is, however a useful abstract, that does not uses better technical language like fractures, not breaks.

I have however been reconsidering the subject.

Abstract

Solids fracture under critical stress, while liquids exhibit continuous deformation. Viscoelastic liquids can fracture like solids when the deformation rate is high enough that the material’s storage modulus 𝐺′ is approximately the same or higher than its loss modulus 𝐺′′. Here, we present direct experimental evidence of solidlike fracturing in hydrocarbon blend liquids whose 𝐺′′ is an order of magnitude greater than 𝐺′ for the rates probed. In our experimental setup, high-viscosity simple liquids were subjected to shear and extensional flow. Fracture occurred at a total critical stress of 2⁢(±1)  MPa for hydrocarbon blends and styrene oligomers, which interestingly remained independent of viscosity, chemical composition, temperature, and velocity. Near the crack-onset velocity, ductile cup-cone fractures are observed, whereas at higher deformation rates the fracture becomes predominantly brittle. To test the generality of this phenomenon, we repeated the experiments using a different simple liquid, styrene oligomer, at different temperatures and observed the same fracture behavior. Our findings suggest that fractures in liquids can occur independent of the relative dominance of viscous or elastic behavior. These findings expand our understanding of fracture mechanics beyond purely elastic-induced phenomena. Further research is warranted to explore the underlying mechanisms and practical implications.

A question arises as to what is meant by 'breaks' ?

Detachment of part of the body of material or just (nonelastic) deformation?

Both of these phenomena can be observed simply by going down to the sea shore and watching the waves roll in.

The results are not called breakers for nothing.

But the abstract implies something rather different.

Forced flow in a tube or pipe, with perhaps a nozzle, with a slow moving highly viscous material.

It is possible they were counting the rate of drop formation (detachment).

Many materials are tested in this way.

10 hours ago, MigL said:

I believe the specs for the Mt Palomar reflector/mirror are available, and I would love to see a 'back of the envelope' analysis of the amount of sag ( is that better ? ) in the mirror that the glass would experience between the vertical, and the low horizon, positions. And how does the sag increase with even larger mirrors ?

Yes, the word "flow" threw me a bit off track. I should have understood that in your use of quotation marks. Sorry.

I also said something incorrect. Namely, that fluids do not resist shear, which is incorrect. It's only correct for so-called ideal fluids. Viscosity is defined in a way analogous to a shear.

23 hours ago, paulsutton said:

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.

56 minutes ago, studiot said:

A question arises as to what is meant by 'breaks' ?

Detachment of part of the body of material or just (nonelastic) deformation?

Both of these phenomena can be observed simply by going down to the sea shore and watching the waves roll in.

The results are not called breakers for nothing.

By 'fracturing' and such I suppose they mean the velocity field experiences a discontinuous change. I'm inferring that from the text. But they don't explicitly say that.

I don't know what other thing they might be referring to.

2 hours ago, joigus said:

By 'fracturing' and such I suppose they mean the velocity field experiences a discontinuous change.

So gases ( like air ) can 'fracture' also, at the shock line between supersonic and subsonic flow.
( just glad to be discussing something other than made-up 'theories of everything', or drug induced 'consciousness' in QM )

10 hours ago, StringJunky said:

The mirror segment actuators are controlled by software which can constantly adapt their configuration in real time

True, but they also adapt to reflector 'astigmatism' induced by differing mirror orientations.
( astigmatism is different curvature along different radial axis; I have plenty on my corneas due to scar tissue from many operations )

Edited Monday at 03:12 PM1 day by MigL

18 minutes ago, MigL said:

( just glad to be discussing something other than made-up 'theories of everything', or drug induced 'consciousness' in QM )

+1

No I disagree about an abrupt change in a velocity profile defining fracture as the object/material may be stationary, but still break or not as the case may be.

However I do agree that the motion of the object is a condition to be taken into account, as with the marine breaker I mentioned, where the top of a wave is travelling faster than the retarded bottom, although there may be a smooth rather than an abrupt variation in the velocity profile.

16 minutes ago, MigL said:

So gases ( like air ) can 'fracture' also, at the shock line between supersonic and subsonic flow.
( just glad to be discussing something other than made-up 'theories of everything', or drug induced 'consciousness' in QM )

In fact the authors describe a sonic effect too, reminiscent of what happens in the case you mention.

I agree on TOEs and quantum hallucinations, etc. This is much more interesting.

x-posted with @studiot

Edited Monday at 03:33 PM1 day by joigus
minor addition

I think the authors of the paper talk about stress regimes, rather than velocity regimes.

They also seem to limit their definition of breakage or fracture to what happens with metals.

But the real world is a heck of a lot more complex than solid, liquid or gas.

So is a carbon fibre fishing rod a solid ?

How about a four by two piece of 'solid' oak ? (Why do the americans insist on two by four?)

Both are fibrous, not crystalline.

But what about soil ?

The mechanics of soil and its response to stress depends upon load sharing between the granular solid particles and soil pore water.

@exchemist mentions bitumen.

There is the bitumen drop test, an experiment that has being going for nearly 100 years.

Then there is Griffith's theory of strength and fracture and J E Gordon's experiments with alleged flows of old window glass.

@paulsutton

Yes graphite can be considered as a stack of monolayers of carbon.

In fact I know of 4 allotropic forms of carbon viz Diamond, Graphite, Amorphous (soot, coke, charcoal, etc) and Buckeyballs.

There are also many potentialy infinite planar lattices of aluminosilicate materials.
Some of these make up the clay minerals group with characteristic clay failure modes and the atterberg limits test.

10 minutes ago, studiot said:

I think the authors of the paper talk about stress regimes, rather than velocity regimes.

They also seem to limit their definition of breakage or fracture to what happens with metals.

But the real world is a heck of a lot more complex than solid, liquid or gas.

So is a carbon fibre fishing rod a solid ?

How about a four by two piece of 'solid' oak ? (Why do the americans insist on two by four?)

Both are fibrous, not crystalline.

But what about soil ?

The mechanics of soil and its response to stress depends upon load sharing between the granular solid particles and soil pore water.

@exchemist mentions bitumen.

There is the bitumen drop test, an experiment that has being going for nearly 100 years.

Then there is Griffith's theory of strength and fracture and J E Gordon's experiments with alleged flows of old window glass.

@paulsutton

Yes graphite can be considered as a stack of monolayers of carbon.

In fact I know of 4 allotropic forms of carbon viz Diamond, Graphite, Amorphous (soot, coke, charcoal, etc) and Buckeyballs.

There are also many potentialy infinite planar lattices of aluminosilicate materials.
Some of these make up the clay minerals group with characteristic clay failure modes and the atterberg limits test.

The micas and talc (soapstone) spring to mind.

7 minutes ago, exchemist said:

The micas and talc (soapstone) spring to mind.

The difference is that clay is a soil, mica and talc are rocks.

Clay has its peculiar properties due to a complex (electrostatic) interaction between soil water and the clay minerals, whcih carry charges.

Graphite sheets themselves are of course basically electrically neutral, although intersheet forces are Van Der Waals electrostatic.

17 minutes ago, studiot said:

I think the authors of the paper talk about stress regimes, rather than velocity regimes.

They also seem to limit their definition of breakage or fracture to what happens with metals.

But the real world is a heck of a lot more complex than solid, liquid or gas.

I don't know, as the abstract doesn't mention what this phase change consists in. I would be surprised that it didn't somehow involve a discontinutity in the velocity field. Eg: They don't mention domain walls either, but I'm sure they are involved as well. Upon further reading, you may be right that it's more about density than velocity.

As to the materials, it is my understanding that the fluids they use are mostly polymers, while they also claim the phenomenon is quite universal. I don't see where metals are involved, but again, I could be wrong, of course.

10 minutes ago, studiot said:

The difference is that clay is a soil, mica and talc are rocks.

Clay has its peculiar properties due to a complex (electrostatic) interaction between soil water and the clay minerals, whcih carry charges.

Graphite sheets themselves are of course basically electrically neutral, although intersheet forces are Van Der Waals electrostatic.

I thought talc was sometimes classed as a clay mineral though. It too has sheets only bonded by van der Waals attraction, I think. With mica I think there is a cation between the sheets.

1 hour ago, studiot said:

But what about soil ?
The mechanics of soil and its response to stress depends upon load sharing between the granular solid particles and soil pore water.

That's interesting.
Granular solids can be made to act like liquids.
We regularly 'float' Sulphur prills, or flakes, on a cushion of N2 pressure, so it acts 'liquidy', and we can suck it under vacuum into a reactor for dithionation processes ( flakes need different N2 pressure than prills ) at my work.
This effect is also seen in avalanches and land-slides.

By the way, @studiot , there is no such thing as a true 'two by four'.

Edited Monday at 06:25 PM1 day by MigL