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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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.
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