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Is there friction between rough surfaces only (figure below)?

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If we have two perfectly (mathematically) smooth surfaces, will there be friction between them (figure below)? enter image description here

lucas
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    There is no such thing as a mathematically smooth surface, at best we can get down to atomic layers in crystalline materials. However, there is a phenomenon called cold welding that happens when very clean and even surfaces are brought together under vacuum conditions: https://en.wikipedia.org/wiki/Cold_welding. In this case the inter-atomic or inter-molecular forces of the materials will make similar bonds as inside the bulk of the material. This is usually suppressed by water, oil and other "dirt" sticking to the surfaces and acting as a lubricant film. – CuriousOne Jul 07 '16 at 09:39

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If the two surfaces are real and perfectly smooth at an atomic level, and made of the same material, there will be chemical bonding between them. They will fuse together as though they had always been the same object [1]. Even different metals can fuse together if left in contact for sufficient time, or if forced together, due to mutual diffusion of atoms across the interface [2]. It is only the presence of impurities, contaminants and irregularities in the surface structure which keeps them from fusing together.

[1] https://en.wikipedia.org/wiki/Cold_welding - courtesy of CuriousOne
[2] https://en.wikipedia.org/wiki/Diffusion_bonding

sammy gerbil
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  • Thank you because of your attention! "there will be chemical bonding between them" As you can see in the second figure, there is a distance between two surfaces. I think the reason of this distance is electrostatic repulsive forces between two surfaces. Can you prove that this distance always is less than the distance required for occurring the chemical bonding? – lucas Jul 07 '16 at 10:17
  • What is causing the gap? Are the two object electrically charged or magnetized? If it is repulsion between the outer electrons of atoms, then there would be no friction even in the 1st diagram, and no chemical bonding even within each object. If the surfaces are "mathematically smooth" then there is no reason why atoms cannot come close enough to form the same bonds as those within the objects. – sammy gerbil Jul 07 '16 at 10:26
  • There is a gap between two surfaces in the first diagram, too. (I didn't find a suitable picture) I think friction force is the resultant of electrostatic forces and the surfaces never touch each other. – lucas Jul 07 '16 at 10:34
  • Repulsion which obstructs asperities from sliding past each other is only one mechanism of friction. Other mechanisms are possible, such as cold welding between asperities eg at the red circles. – sammy gerbil Jul 07 '16 at 10:45
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Friction is a result of asperity-tops adhering together (as if they melted together) through chemical bonding. The more asperities that stick to the other surface, the larger the friction. To make them slide across each other, friction must be overcome, meaning that these "glued-together" tops must be broken.

I gave this explanation to another question not long ago.

If the surface is perfectly smooth, it is "one big asperity" that 100 % adheres to the other surface. Friction is here enourmous, and we no longer call it friction but instead cold-welding.

About perfect surfaces of extremely low friction, it is though possible to "trap" a perfectly uniform and extremely thin layer of air between the surfaces. They will Thus "float" over one another with practically no friction on this aerodynamic bearing mechanism. The air can in this way be trapped, because the surfaces being perfectly flat and parallel will cause vacuum if the air molecules try to escape, which keeps them in place.

If pushes hard enough together to squeeze out the air, the surfaces will be cold-welded together and theoretically impossible to separate again.

Steeven
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