How does contact occur at the atomic level?

[Johnny5454]

When we touch something, will there always be a layer of air between us?

 

[sophiecentaur]

Hi and welcome to PF.
Good question. The answer is that it will depend on the time elapsed. Initially there will be partial contact, with gaps (bubbles even) with air in them. After time, the skin will come together and gradually squeeze most of the air molecules out. But there will always be some trace molecules in there, until they are absorbed by the skin and the moisture between the fingers.
This is a phenomenon with touching metal surfaces. In air, they have a very thin layer of air and associated 'grot' which keeps them separate, usually and if they are not scrupulously clean. Metals in a high vacuum, as in spacecraft can weld themselves together once all the surface impurities have evaporated. Hatches can weld shut - which is a potential problem. Scary.

 

[Johnny5454]

Hi and welcome to PF.
Good question. The answer is that it will depend on the time elapsed. Initially there will be partial contact, with gaps (bubbles even) with air in them. After time, the skin will come together and gradually squeeze most of the air molecules out. But there will always be some trace molecules in there, until they are absorbed by the skin and the moisture between the fingers.
This is a phenomenon with touching metal surfaces. In air, they have a very thin layer of air and associated 'grot' which keeps them separate, usually and if they are not scrupulously clean. Metals in a high vacuum, as in spacecraft can weld themselves together once all the surface impurities have evaporated. Hatches can weld shut - which is a potential problem. Scary.

How long will it take before there is air left between my hand and the surface?

 

[sophiecentaur]

How long is "long"? Your question opens a whole can of practical worms.
Also, what level of air are you referring to? Most of it will go in a fraction of a second - as your fingers 'squidge' together. If your fingers are greasy (no water to dissolve the gases), I guess it could be hours before there is no detectable air in there because it will not dissolve.
Try an experiment (Always try an experiment when you possibly can - that's my motto - really) with a drinking glass, held in the fingers and look from the other side from the contact at a very oblique angle. Depending on how dry /wet / greasy your finger is, you can get reflection (total internal) of light at the inner surface. Sometimes you see the finger print ridges and sometimes a silvery pattern, when the trapped air allows total internal reflection.

A glass block is even better, If you can get hold of one. Perhaps some picture glass or a toy prism etc..

 

[Johnny5454]

that is, when I touch the surface, almost all of the air is forced out and the atoms of my fingers come in contact with the atoms of the surface?

 

[Johnny5454]

1) when a person’s fingers touch the surface, will the atoms of the fingers directly interact with the atoms of the surface, or is there a layer of air between them that will prevent contact? Since air atoms are everywhere. 2) How does contact with the surface occur? Does Pauli's principle of exclusion take effect immediately or do electrostatic forces first act?

 

[fresh_42]

No layer of air. The atoms of finger and table interact directly, in the sense that the electron hulls repel each other. There is normally no exchange of electrons, hence Pauli doesn't apply.

 

[PeroK]

1) when a person’s fingers touch the surface, will the atoms of the fingers directly interact with the atoms of the surface, or is there a layer of air between them that will prevent contact? Since air atoms are everywhere. 2) How does contact with the surface occur? Does Pauli's principle of exclusion take effect immediately or do electrostatic forces first act?

Try this:

https://wtamu.edu/~cbaird/sq/2013/04/16/do-atoms-ever-actually-touch-each-other/

 

[A.T.]

 

[hutchphd]

At one point I did some work scattering He atoms from the surface of crystalline graphite. The interaction was remarkable well described by a Lennard-Jones potential. In this potential the attraction is Van der Waals and the repulsion is essentially Pauli exclusion.

https://en.wikipedia.org/wiki/Lennard-Jones_potential

 

[etotheipi]

Does Pauli's principle of exclusion take effect immediately or do electrostatic forces first act?

I think this is a good question; I hope someone can shed some light on it.

When you bring two atoms close to each other, okay maybe there is some energy level splitting because of some exchange interaction (maybe you can find this from the overlap integral?). If the two atoms don't readily bond then perhaps to excite the electrons you need to supply energy and this makes the process unfavourable energetically. Maybe we can lump this into some sort of 'repulsive' term (?).

But often people refer to contact forces as arising from electromagnetic repulsions.

How do these two different explanations mesh together? I apologise if I made a mistake, I don't really know anything about this.

 

[sophiecentaur]

No layer of air. The atoms of finger and table interact directly, in the sense that the electron hulls repel each other. There is normally no exchange of electrons, hence Pauli doesn't apply.

I can't help thinking that, if that were the case, your fingers would actually stick together. But I guess we are talking about different time scales. I'd bet it would take many minutes before all traces of 'air' molecules went away and that would be due to absorption of the skin. Living tissue is a difficult substance to discuss. (Even the proteins in chopped meat will tend to recombine and the chunks can 'stick together'. )

 

[hutchphd]

How do these two different explanations mesh together? I apologise if I made a mistake, I don't really know anything about this.

Good questions. Consider just two atoms
The attraction at long range is due to fluctuating dipolar interactions. Even between two noble gas atoms. Here is a (not very good) explanation for why they are attracted at long range:

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

The repulsion which worries you is really quite subtle. Because these interactions are between electrically neutral atoms, the exact distribution of the charge becomes more important. The net effect of Pauli is to make electrons vacate the space between the nuclei. This makes the repulsive effect of the coulomb repulsion very much stronger at short interatomic distances than it would otherwise be. This is an "exchange mediated" effect.

 

[davenn]

The answer is that it will depend on the time elapsed. Initially there will be partial contact, with gaps (bubbles even) with air in them. After time, the skin will come together and gradually squeeze most of the air molecules out. But there will always be some trace molecules in there, until they are absorbed by the skin and the moisture between the fingers.

The OP was asking "at the atomic level"

 

[etotheipi]

The repulsion which worries you is really quite subtle. Because these interactions are between electrically neutral atoms, the exact distribution of the charge becomes more important. The net effect of Pauli is to make electrons vacate the space between the nuclei. This makes the repulsive effect of the coulomb repulsion very much stronger at short interatomic distances than it would otherwise be. This is an "exchange mediated" effect.

Interesting! So it's a combination of the PEP and the Coulomb interactions. But I imagine any work that results from the interaction is still electrostatic in nature, then.

I don't know if this is too far from the OP, but what about for something like electron degeneracy pressure? As far as I know, this would also be an exchange mediated effect (and Wikipedia tells me Dyson showed its not because of electrostatic repulsion). Perhaps one can do some calculations with the De Broglie wavelength to see that the average momentum increases if you confine the electrons to a smaller volume, but does the 'repulsive interaction' (don't know if this is the correct wording!) demonstrated here have any relation to the effect you described, between the electrically neutral atoms? Or does the pressure result from a different type of interaction?

 

[davenn]

I hope someone can shed some light on it.

Did you watch the video in post #9 ?

 

[etotheipi]

Did you watch the video in post #9 ?

Yes, that was where I got the idea for the question from. In any case, I would like to learn a bit more about the details . Plus, it was sort of uncomfortable when they started bickering at the end.

 

[davenn]

Yes, that was where I got the idea for the question from. Plus, it was sort of weird when they started bickering at the end. In any case, I would like to learn a bit more about the details .

well their discussion and "arguments" pretty much tells you the issues being dealt with.
I remember first watching that video some 8 years ago.
for other really cool physics topics, you should search on youtube for sixty symbols

 

[hutchphd]

electron degeneracy pressure?

As far as I know, this would also be an exchange mediated effect (and Wikipedia tells me Dyson showed its not because of electrostatic repulsion)

fred
I guess I would call this a "direct exchange effect" but let us not quibble about semantics.
So I would simply say that both effects exist and their relative importance depends upon the circumstance.
I presume you understand the gist of Dyson's argument. I think it is essentially that
$\frac { \partial E_f } {\partial Volume }$ is a pressure ?

 

[etotheipi]

fred
I guess I would call this a "direct exchange effect" but let us not quibble about semantics.
So I would simply say that both effects exist and their relative importance depends upon the circumstance.
I presume you understand the gist of Dyson's argument. I think it is essentially that
$\frac { \partial E_f } {\partial Volume }$ is a pressure ?

You are right, I also found a derivation along those same thermodynamics lines here. He suggests that this pressure is not linked to any of the fundamental interactions, so there's no 'force' in the usual sense. Thanks for your help!

Anyway I have the feeling I'm derailing OPs thread, so I'll stop for a bit now .

 

[ZapperZ]

When we touch something, will there always be a layer of air between us?

Don't we have a FAQ on this question already? If we don't, we should.

Zz.

 

[berkeman]

Don't we have a FAQ on this question already?

I thought so too. Maybe an Insights?

 

[Johnny5454]

Don't we have a FAQ on this question already? If we don't, we should.

Zz.

when a person touches another person, do the atoms of his fingers really come into contact with the atoms of the skin of another person or will there always be an air layer between them, because air is everywhere. Some say that skin atoms can stick together, but if they come in contact directly?

 

[berkeman]

when a person touches another person, do the atoms of his fingers really come into contact with the atoms of the skin of another person or will there always be an air layer between them, because air is everywhere. Some say that skin atoms can stick together, but if they come in contact directly?

You've gotten a lot of good replies in this thread already. Have you read them? In particular, did you watch the video posted by @A.T. in post #9?

 

[Johnny5454]

I can't help thinking that, if that were the case, your fingers would actually stick together. But I guess we are talking about different time scales. I'd bet it would take many minutes before all traces of 'air' molecules went away and that would be due to absorption of the skin. Living tissue is a difficult substance to discuss. (Even the proteins in chopped meat will tend to recombine and the chunks can 'stick together'. )

I would like to clarify, when a person touches another person, do the atoms of the hand come into contact with the atoms of the skin of another person, or will there be a thin layer of air around all the objects that will impede the contact between the atoms of the hand and the atoms of the skin of another person? Or will almost all of the air be pushed out and there will be direct contact? thank

 

[ZapperZ]

I would like to clarify, when a person touches another person, do the atoms of the hand come into contact with the atoms of the skin of another person, or will there be a thin layer of air around all the objects that will impede the contact between the atoms of the hand and the atoms of the skin of another person? Or will almost all of the air be pushed out and there will be direct contact? thank

It's like talking to a wall.

Zz.

 

[Johnny5454]

It's like talking to a wall.

Zz.

Hello, you are a very good physicist and I respect you and respect your answers.

It’s just that I received a not very clear answer to my question, which is whether the layer of air separates us when we touch something or do we directly interact with the atoms of the object that we touch?
thank

 

[Johnny5454]

I can't help thinking that, if that were the case, your fingers would actually stick together. But I guess we are talking about different time scales. I'd bet it would take many minutes before all traces of 'air' molecules went away and that would be due to absorption of the skin. Living tissue is a difficult substance to discuss. (Even the proteins in chopped meat will tend to recombine and the chunks can 'stick together'. )

Can I clarify? That is, between the hand and the skin there will always be some molecules of air or water, but there will be places where the skin atoms will directly interact with the atoms of the hand without any air between them?