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breaking - unbreaking


michel123456

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Question

 

Why, when I break a glass for example and then assembling the pieces that fit perfectly together, why do the pieces not stick together just like that and reform the glass solid as it was without glue? Has the structure been modified so that the pieces correspond perfectly but the structural bonds are gone?

 

I hope my question is clear.

Edited by michel123456
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The same opportunity for bonds is no longer present, but the details may differ from material to material.

 

One common problem is that the molecular force is repulsive before it becomes attractive, and simply pushing the parts together doesn't get the surfaces close enough. Some materials oxidize quickly and that changes what bonds could be made. Some breakage involves deformation of the surface and modifies the structure. Surface bonds differ from bulk material bonds. And probably more.

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That's about atoms.

I thought it should have something to do with molecular bonds.

 

---------------------

(edit)

thanks for the article anyway.

It does not explain why the process is not reversed when putting back the broken parts though.

The same opportunity for bonds is no longer present, but the details may differ from material to material.

 

One common problem is that the molecular force is repulsive before it becomes attractive, and simply pushing the parts together doesn't get the surfaces close enough. Some materials oxidize quickly and that changes what bonds could be made. Some breakage involves deformation of the surface and modifies the structure. Surface bonds differ from bulk material bonds. And probably more.

 

Thanks.

Ignorant as I am, I looked at this wikipedia article which is not so clear as your answer is.

 

Do you mean that it is a same force that changes sign as the distance decreases?

Edited by michel123456
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Even though the pieces appear, to the naked eye, the same after the break as before, they are not.

 

When you break something into parts you create new surfaces. This takes energy input. Every part with a new surface has more energy than before, by the amount of this additional surface energy.

 

The theory of fracture mechanics is based on this fact and evaluating this energy and its effects.

Edited by studiot
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Even though the pieces appear, to the naked eye, the same after the break as before, they are not.

 

When you break something into parts you create new surfaces. This takes energy input. Every part with a new surface has more energy than before, by the amount of this additional surface energy.

 

The theory of fracture mechanics is based on this fact and evaluating this energy and its effects.

That is totally new to me.

And counter-intuitive. i would have thought that many parts have less energy than a solid block. I would have thought that you need energy to construct a rock, more than you need to have sand.

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Do you understand how surface tension arises in liquids?

Yes i understand.

 

But still it is counter-intuitive in terms of energy.

It is bizarre to think that the ocean for example has less energy as a whole than if it was entirely spread in droplets. That a statue has less energy than a sandbox.

I'll read more.

Lots of atoms bonded together make a molecule, so what's the difference?

Well, I was thinking that when you break something, you do not deal with atomic forces but with molecular forces.

Edited by michel123456
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Yes i understand.

 

That's good since the same situation applies to all matter in all states.

 

The particles at the surface are not in equilibrium.

The interior particles are.

 

And yes an ocean has less energy than all the water spread out into droplets and the larger the free surface of anything the more energy it has, to maintain this disequilibrium.

 

This is why droplets try to pull themselsves into balls - to minimise the surface area to volume ratio.

 

Breaking and reforming are the converses of this manifestation of surface energy.

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That's good since the same situation applies to all matter in all states.

 

The particles at the surface are not in equilibrium.

The interior particles are.

 

And yes an ocean has less energy than all the water spread out into droplets and the larger the free surface of anything the more energy it has, to maintain this disequilibrium.

 

This is why droplets try to pull themselsves into balls - to minimise the surface area to volume ratio.

 

Breaking and reforming are the converses of this manifestation of surface energy.

Oh, that's not gravity's job?

Edited by michel123456
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Might be simpler to look at it fron a physics point of view...

 

A rock has a given amount of energy.

You need to give it more energy ( in the form of a swinging sledgehammer ) to pulverise it.

Ergo the solid rock has less energy than the rock debris.

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Might be simpler to look at it fron a physics point of view...

 

A rock has a given amount of energy.

You need to give it more energy ( in the form of a swinging sledgehammer ) to pulverise it.

Ergo the solid rock has less energy than the rock debris.

Let's say yes.

But then, following the same logic, when you put the debris together you should have a release of energy. Which is not the case.

 

No, drops are spheres in zero g.

Right.

But i had the (false) impression that the sphere was caused by gravity, not by molecular or atomic interactions.

Anyway "zero g" is a situation where acceleration from Earth is not felt (free fall), not a situation where objects have no mass. I mean, a drop of water still has its own mass even under zero g.

And also i had the impression that gravity is invoked for the formation of planets in spherical shape. Not molecular or atomic forces.

 

That's good since the same situation applies to all matter in all states.

 

The particles at the surface are not in equilibrium.

The interior particles are.

 

And yes an ocean has less energy than all the water spread out into droplets and the larger the free surface of anything the more energy it has, to maintain this disequilibrium.

 

This is why droplets try to pull themselsves into balls - to minimise the surface area to volume ratio.

 

Breaking and reforming are the converses of this manifestation of surface energy.

That is soo interesting. It is like saying that geometry is the cause.

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And also i had the impression that gravity is invoked for the formation of planets in spherical shape. Not molecular or atomic forces.

 

For a planet it is, because there is sufficient mass to overcome the other forces. But objects that are small enough are not spherical. (e.g. asteroids)

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So there's no difference between breaking the bonds between atoms and breaking the bonds in a molecule (in most cases- notably those cases under consideration such as glass)

Atomic and molecular forces are the same thing.

The bonds between molecules are the same that bonds between atoms?

And the bonds inside molecules are the same too?

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You took my example a little too literally Michel123456.

 

OK, take a REALLY big rock, say in space, and swing a HUGE sledgehammer ( Nuclear bomb ) at it.

You have added energy and the rock breaks up.

You now leave to its own and see the pieces come back together gravitationally to its lowest energy state, as near as it can get to spherical.

No energy added !!!

 

You won't see a regular rock do this because of the entropy of irreversible processes. So you have to expend more energy to physically bring the pieces back togrther

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But i had the (false) impression that the sphere was caused by gravity, not by molecular or atomic interactions.

 

I deliberately used the word particles, not molecules.

 

 

The particles at the surface are not in equilibrium.

The interior particles are.

 

This brings up several interesting related points.

 

Gravity does indeed cause aggregation of particles. Some cosmology theory relating to heavenly body formation and growth relates to this.

 

However gravity is not as strong as molecular bonds. If this were not so then any object you picked up would fall apart under gravity.

Conversely gravity is not strong enough to put them back together either.

 

John Cuthber has pointed out that breaking many 'solids' (particularly crystalline ones) involves breaking molecular bonds.

 

Many materials are mixtures and indeed the lesser-than-molecular forces that hold these together will allow separation under gravity.

 

So I said particles because particles includes molecules and larger (though still minute) material aggregates.

 

Which comes back to surface energy and fracture reconstruction.

 

Yes you would need to extract energy to perform reconstruction. You would also need a mechanism to perform this.

 

Remember also that, as swansont has already pointed out, oxides form rapidly on fracture surfaces.

This is because if you do break a molecular bond in fracturing, then you have an unattached bond end seeking a home to balance the valency of the molecule and the ever present oxygen in the air happily supplies this.

Edited by studiot
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You took my example a little too literally Michel123456.

 

OK, take a REALLY big rock, say in space, and swing a HUGE sledgehammer ( Nuclear bomb ) at it.

You have added energy and the rock breaks up.

You now leave to its own and see the pieces come back together gravitationally to its lowest energy state, as near as it can get to spherical.

No energy added !!!

 

You won't see a regular rock do this because of the entropy of irreversible processes. So you have to expend more energy to physically bring the pieces back togrther

There you have mixed gravitation. and gravitation puts back the pieces in a spherical shape when the object is large enough. which is not the case (if i understand clearly) with a drop of water in zero g.

 

As if there were 2 kind of forces both regulated by geometry. Gravity through the curvature of spacetime and atomic/molecular bonds (is that E.M. interaction?) through the curvature of the surface of material objects.

 

Or have I missed something?

Edited by michel123456
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I deliberately used the word particles, not molecules.

 

 

This brings up several interesting related points.

 

Gravity does indeed cause aggregation of particles. Some cosmology theory relating to heavenly body formation and growth relates to this.

 

However gravity is not as strong as molecular bonds. If this were not so then any object you picked up would fall apart under gravity.

Conversely gravity is not strong enough to put them back together either.

 

John Cuthber has pointed out that breaking many 'solids' (particularly crystalline ones) involves breaking molecular bonds.

 

Many materials are mixtures and indeed the lesser-than-molecular forces that hold these together will allow separation under gravity.

 

So I said particles because particles includes molecules and larger (though still minute) material aggregates.

 

Which comes back to surface energy and fracture reconstruction.

 

Yes you would need to extract energy to perform reconstruction. You would also need a mechanism to perform this.

 

Remember also that, as swansont has already pointed out, oxides form rapidly on fracture surfaces.

This is because if you do break a molecular bond in fracturing, then you have an unattached bond end seeking a home to balance the valency of the molecule and the ever present oxygen in the air happily supplies this.

Would it be possible to perform an experiment in void where a broken solid material can be put together? And release energy? (since the material would have less energy).That would be cool.

Like ice?

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Would it be possible to perform an experiment in void

 

I have my doubts that it would ever be possible to perfectly preserve fracture surfaces and then exactly align them.

 

However I seem to remember cold fusion bonding being achieved in ultraclean labs between two samples of suitable material.

 

Not broken material, but specially prepared material.

 

So that is probably the nearest we will ever get.

 

Sorry I don't have a proper reference, but I will try to find one.

 

And like I said there is the problem of the energy released. You do not want this to go into the newly formed lattice since this is exactly the right energy to break a bond. So it must be removed somehow.

Edited by studiot
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