Why don’t almost all solid bodies merge into one body when they come in contact?

Question

Macroscopic bodies in solid state when they touch each other do not connect in one monolithic body, because of electrostatic repulsion of their electron clouds. Okay.

Say, have two iron ingot, we make them touch each other, and nothing happens. Now we melt them - they connect in one fluid. Then we cool it and it becomes being a single object.

I understand, that the heat is the atomic vibration, and if the amplitude of vibration is too high, connection among them loses.

And I even understand intuitively, that when the body is solid “the doors are closed”, and the more we heat it the more “doors open - new guests can come in, current can come out”.

But how to explain it more scientifically? What do atoms of already melted body have, to connect with another bodies atom, that solid state body atoms don’t have?

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I think it’s because, when the body is melted, its energy levels are “blured” a lot, so the list of energies, atom can accept is more larger. But, at the same time, electrons of another body have(about) the same energies. Why can’t they be attracted then? I’m confused

Also, why plasticine connects in one?

Answer 1

As others have noted, in the case of two perfect iron ingots in a vacuum, they will actually connect and become a single lager object. This is called “vacuum welding”.

The reasons that it does not typically happen with two random lumps of iron in open air include:

• The surfaces are not entirely flat. Unlike plasticine, it’s difficult for a human to squish iron together hard enough to eliminate even hair-sized imperfections.

• Contamination of the surface by rust, oil, and even air. This means that in some places there is a layer of not-iron separating the iron from one block from the iron of the other block. In the case of plasticine this still happens (though the strength of the effect depends on the specific chemistry), but you don’t notice because it’s so easy to pull the plasticine into two lumps anyway.

Answer 2

There is vacuum welding: https://en.wikipedia.org/wiki/Vacuum_cementing

That happens when really clean iron surfaces touch. But it cannot happen in air. Then surfaces are covered with a layer of oxides, hydroxides, adsorbed water, carbon dioxide, carbohydrates and other organics and other crud.

Answer 3

That is chemical reaction - chemical bonds are broken or/and formed during the process. Reaction will happen if potential energy become lower (become kinetic energy of atom) during the process and there is enough initial kinetic energy to collide so activation evergy can be overcome.

For two body to be glued together, enough chemical bonds need to be formed between them.

(1) The contacted surface need to be smooth enough to let more atom to contact with each other to react. If it is not smooth, then only the atoms on the tips and bumps get contacted. Higher pressure right angle to the contact surface can deform and compress the tips and bumps to make more atoms get contacted - the softer the material, the easier to compress.

(2) Atoms on the one surface need to be able to react with atoms on the other surface. Metal may react with each other and form metal bonds, but if it get contaminated like get oxidized orc overed by some organic molecules that can't react with the metal, then there won't be reaction happen.

(3) You need high enough temperature for that reaction to happen, high temperature means high kinetic energy of atom, easier to overcome activation evergy. The temperature needed depends on the reaction to happen.

(4) The bond need to be strong enough. Weak bond won't hold the to part tightly

This is how glue work: it react with both surface and forms strong bonds, it fill most of the gap between the surfaces to form more bonds - you press the to body against each other to let the glue fill the gaps better, it only need room temperature to start reaction to be easy to be used.

Whether to body can form one mainly depends on the above conditions.

Answer 4

Why and how do atoms/molecules form a solid?

Because the negative electron orbitals about the atoms/molecules allow spaces where the positive attraction of the nucleus can get through, thus allowing the possibility of forming lattices, a bit like LEGO blocks lock on each other. There are various ways this can happen, and depending on the material, the temperatures etc these bonds can be very strong.

As discussed in the other answers, the same molecular structures, iron for example, can bind if there is no air or impurities that will not allow the lattices to join. For different atomic content materials the lattices will have to be of the same energy level type to make a continuous blend.

Glue has the possibility to adjust to different molecular contents and make one solid of two surfaces ( though I do not know the exact physics of it. It all has to do with attractive forces that exist and allow forming lattices)