If I have two perfectly smooth planar surfaces of two solid objects made of the same element and make them touch, why do they not stick and form a single bigger object? By 'touch' I mean bring the pieces so close together that the two most outer layers of atoms of the two object are as close as the second most outer layers of each object to its most outer layer. How do the atoms know to which object they belong? Is it imperfection and dirt or is my intuition wrong?
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$\begingroup$ Duplicate? physics.stackexchange.com/q/87107/104696 and a few more other posts after putting "cold welding" into the search engine and read the last bit of section 12-2 Friction in the Feynman lectures feynmanlectures.caltech.edu/I_12.html $\endgroup$– FarcherCommented Feb 22, 2017 at 17:01
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$\begingroup$ There are no perfectly smooth planar surfaces. If they were, they would stick together (as in sticky tape). $\endgroup$– DschoniCommented Feb 22, 2017 at 17:08
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On the surface usually oxide or contamination layers are present. Very well prepared (atomically) flat surfaces (e.g. in ultra high vacuum) actually stick together. However, this occurs by cohesion because even a perfectly plane crystal surface without contamination usually doesn't have free dangling bonds due to the phenomenon of surface reconstruction so that there is no continuation of the usual crystal lattice at the interface after joining. Such a continuation can, however, be achieved by heating the interface for a restructuring of the interface bonds.
answered Feb 22, 2017 at 17:15
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$\begingroup$ Gold foil is a good example for stick on many surfaces, be this a metall or wood or plaster. $\endgroup$ Commented Feb 22, 2017 at 19:07
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$\begingroup$ So if stuff does stick together in a vaccuum. How do spaceships avoid being "welded" to a space station when docking? $\endgroup$ Commented Apr 13, 2018 at 8:57
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1$\begingroup$ @GillesLesire Vacuum is only one of the requirements for this to happen. As freecharly said, the surfaces must be "well-prepared" (cleaned of solid impurities and smoothed). $\endgroup$– SpencerCommented Jun 22, 2018 at 15:02
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$\begingroup$ @Spencer So a rocket would be "coated" in air wich makes it impossible for it to weld to the space station? $\endgroup$ Commented Jul 18, 2018 at 10:17