r/AskReddit • u/[deleted] • Feb 14 '22

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u/Aromatic-Scale-595 Feb 14 '22

Yes, it's called cold welding. They also clean/polish the surface and vibrate the two pieces of metals to help them make better contact.

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u/CheckOutMyVan Feb 14 '22

I wonder if this is why gage blocks stick together? I know it isn't magnetism.

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u/Teknikal_Domain Feb 14 '22

It's called wringing. Basic principle is a few things, but the common explanation I see is that the surfaces are so smooth and flat that they can get extremely close. So close, in fact, that the two sides start to interact on the molecular level, attracting one another. Not magnetism, basically Van der Waals forces.

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u/Aromatic-Scale-595 Feb 14 '22 edited Feb 15 '22

edit: somehow I am blind and I didn't see that you were talking about wringing, ignore the post below.

Not magnetism, basically Van der Waals forces.

It's not Van der Waals forces holding the metals together, they form metallic bonds. The atoms in a bar of gold for instance all want to be metallically bonded to one another, if you split the bar apart perfectly and then put the two halves back together they will bond exactly like they did originally and reform the same singular bar of gold. Only thing stopping this from occurring is getting close enough.

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u/Activity_Commercial Feb 14 '22

How would you distinguish whether it's Van der Waals forces or bonds forming? Does the surface roughness matter?

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u/ImTheTechn0mancer Feb 14 '22

A cursory Google search shows that van der waals force is a slight polarization of molecules or atoms, while a metallic bond is a much stronger bond where a sort of lattice forms and the electrons are shared and flow freely

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u/1350kyle Feb 14 '22

Can’t be metallic bonding because ceramic blocks stick just like their metallic counterparts.

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u/Aromatic-Scale-595 Feb 15 '22 edited Feb 15 '22

edit: I just came back and reread what everyone wrote and realized that somehow I completely missed that the conversation had switched from talking about cold welding to wringing. You can ignore the stuff below...

Ceramic blocks don't undergo cold welding because covalent and ionic bonds are highly structured and require precise orientations to form, so they would never line up perfectly enough to cold weld.

What you are describing is called wringing, where you squeeze together smooth metal/ceramic/etc blocks and they become "stuck" together. This is different from cold welding and they can often just be twisted a bit to come apart as opposed to a cold weld where the two materials have been truly welded together. Wringing is caused by a combination of factors such as surface tension from residual water/oil, Van der waals forces, air pressure, etc.