Will a solid mix with a different solid? I apologize at first if this question is naive since I am not a physics major student... 
In my understanding, the basic building block of matter is the atom, and according to my knowledge from basic physics, the atoms are keep moving around, although the atoms of solid matter can not move too much around, but they are not fixed. 
Here is my question: 
assume two solid matters are put together (adjacent to each other). Then, would it be possible that certain amount (maybe extremely small) of atoms of those two matters, at the adjacent layer, mix with each other?
If yes, could you provide some reference? If not, can any 'thing' smaller than atom has such property? photons? 
Thank you!
 A: 
Here is my question: assume two solid matters are put together (adjacent to each other). Then, would it be possible that certain amount (maybe extremely small) of atoms of those two matters, at the adjacent layer, mix with each other?

It is possible for two solids to bond, rather than mix, and before I started this answer, I was confident I could immediately think of two solids that could bonded together, but perhaps needing a certain amount of pressure. However, pressure causes heat, so there is a high probability that you will be joining a liquid to a solid, which is not your question.
If you want two solids to mix, as far as I know, you need to apply some process that establishes a liquid interface, even if only on a temporary basis, before it cools back to solid. Welding two different metals together is an example of this.
The atoms in a solid tend to attract each other, more than they are  attracted by another substance, which I suppose is one definition of a solid.
So strictly speaking, atoms from one solid will not get caught up in the atomic bonding structure of another solid just by placing them together.
But the bonding of a solid to another solid is actually the cause of  friction.  An example of this is the temporary bonding of my shoes to the pavement and, although wasteful of energy, friction is also vital to ordinary life, as it allows me to walk.  The friction occurs  from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination.
The part that interests us here is inter-surface adhesion.
SOURCE: Dipoles And Friction

Electromagnetic interactions at the atomic scale are a source of friction, but atoms and molecules are neutral so this answer sounds odd. However, if two atoms come close together then the repulsion between their electron clouds can cause the clouds to want to maximise their distance of separation. However there is also an attraction the cloud feels from the other atom's nucleus and so what happens is that the atoms polarise each other, creating a scenario where electrons are cluster in a small region of the atom to minimise repulsive forces. Resulting in a region of strongly negative charge but the nucleus also provides a region of positive charge, this gives rise to an induced dipole. 


If you imagine one ball is an electron from the top surface, and the other ball is a proton from the bottom surface, then they be held together for a very short time, by electrostatic forces, and that's where the photons come in, as they establish the attracting force between the two particles.

You can think of a dipole as a bound system of two charges of equal magnitude and opposite sign. The axes (if we consider axis direction as going from the positive charge to the negative charge) of the dipoles induced this way by adjacent atoms are anti-parallel, meaning that their interaction is attractive (as the electron cloud of one atom is closer to the other atom's nucleus than it is to other electron cloud). Attraction between dipoles that are induced by atoms at the boundaries of the two surfaces moving past each other is the simplest of the interactions that results in friction.

There are also interactions where bonds are created between atoms or molecules in the two surfaces, creating an adhesive friction.
There are many kinds of friction, but it is still a subject not fully understood today. 
A: You may want to look up mechanical alloying.  Smashing solid powders together at high pressures does allow solids to mix, albeit in a very limited way in comparison to the mixing of liquids.
