Intermolecular-forces and droplet coalescence

[Ahmed Abdullah]

When two liquid droplets meet they coalesce to form a bigger droplet- this can happens partly because of the intermolecular force and partly because liquid has no definite geometric structure (so that they can flow and respond to the external disturbance).
Now, when two solid material - say two iron rod come into contact they don't coalesce to form a single rod. Why?
Is this because the iron rod has a definite structure and the intermolecular force is not large enough to break this structure and mould into a new one?
When these two rods collide with sufficient kinetic energy, can't we expect them to weld together!
Please explain in detail.

 

[Danger]

Sorry, but I can't explain in detail. In fact, I can't 'explain' at all. I'll just mention a couple of things that might be relevant.
To start with, two droplets will only merge if they're compatible. A drop of motor oil will not merge with a drop of methonol. I'm pretty sure that surface tension plays a large role in the merging, but I don't know for sure how it works.
If rammed together hard enough, two iron rods will merge to some extent. The heat of compression will liquify or even vaporize them, and they'll be bonded upon solidification.

 

[Claude Bile]

I suppose if you have two very flat, polished (i.e. clean) surfaces and the crystalline structures of each surface were aligned just right the metal surfaces could fuse together. You definitely wouldn't see it in normal scenario because a) the surfaces aren't clean enough, b) the surfaces aren't flat enough and c) energetic collisions will likely invoke a) or b).

I've ignored the possibility of the metals melting and welding together because I think that we all agree that that mechanism can and does lead to metals sticking to one another.

Claude.

 

[Loren Booda]

Liquids and solids rely on resonances set up upon impact (like the genesis of the Moon and the Earth, or the shock of a meteorite) to determine the integrity of the final state. Head-on collisions are more likely to convert kinetic energy into heat (which might aid in welding) and have more symmetric resonances as with singular, coalesced drops of liquid. Mixing upon impact is much like that in more controlled conditions: substances having an affinity for each other tend to form whole products of collisions, as Danger suggests, due to mutual surface tension (i. e., relative meniscus).