When surfaces are smooth enough there is an effect called 'wringing' where they essentially form a bond, why does this occur?

Question

I have seen two answers to why surfaces 'wring', ie stick together when below a specific level of roughness. This includes two general answers. The first is that they are acting like a suction cup, and therefore the force holding the objects together is the surrounding atmospheric pressure. The other answer though I have heard is that once the atoms get that close together across the entire surface, a bonding between the surfaces happen because of the attraction of the atoms to each other.

These answers seem very different, and yet I hear both anytime the question is brought up. Is there any testing that has been done of the the 'wringing' effect in a vacuum to know if there is an attractive force because of the atoms, that has nothing to do with the atmospheric pressure, and thus the effect would also occur in space for example? Or perhaps is both things going on, and combining for a larger effect say then either by themselves?

Answer 1

I presume you are referring to the "wringing" as present in gauge blocks, as to "wring" in that sense is quite specific technical vocabulary. The actual mechanism involved is still a matter of debate (see sources 5, 6 on the linked Wikipedia article); possible explanations include:

(1) the air pressure acting from outside, as you say, as well as

(2) the surface tension between the two blocks due to the oil between them (in the same way that a water-glass might lift up the coaster it rests on, if there is a layer of water causing an adhesive force between them), and

(3) the possibility that the individual molecules or metal cations in each surface are binding the surfaces together through intermolecular forces between them - as if the objects behind the two surfaces had merged into one object.

I will not put forward an opinion on this because I am not familiar with these effects in enough detail to have formed a professional opinion; however, it may be useful to know that the question you are asking is still unanswered in the academic community. I recognize the sources in the Wikipedia article are from 2009; I did a search on the ArXiv for the phrase "gauge block", as well as for "wringing", and "wring", and found no new work.