One of the apparent mysteries of physics is what the famous machinist Joseph Whitworth called the "adhesive force" by which two very flat objects will adhere to each other as long as the air clinging to them is squeezed out. This force can easily be over 100 PSI for steel blocks that have been well crafted.

According to the most recent sources known to me (eg, Krar, Steve F.; Gill, Arthur R.; Smid, Peter (2005), Technology of Machine Tools (6th ed.), McGraw-Hill Career Education, ISBN 978-0-07-723225-2, p. 98) the cause and exact nature of this force is unknown, other than it appears to be the result of molecules having some attraction to each other when brought in close proximity.

Has there been any progress on this problem in recent years?

  • $\begingroup$ Depending how much one want to dig in their behaviour and modelling intermolecular forces are indeed somehow elusive but for sure they do exist. I am not sure is a mystery, and normally taken to be of electrostatic nature via dipole- dipole / dipole induced / and transient. While I do admit their are elusive as for my indeed elusive listing, perhaps mysterious is too much. Note that they can extend to metallic bond as in welding into vacuum, where ideally each atom doesn't know to which block another atom brought in contact to it should belong. $\endgroup$ – Alchimista Apr 22 at 7:23

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