Ultraflat materials can be stuck together via wringing

Originally published at: https://boingboing.net/2020/11/02/ultraflat-materials-can-be-stuck-together-via-wringing.html


My assumption would be Van der Waals force.


AvE did this awhile ago. It is a pretty cool.


According to this chap, you’re right. Plus, the super smooth surface means that more atoms can influence the dipole of others.


Is this a different phenomenon than what keeps screen protectors attached to cellphones?

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My guess is that the uncertanty comes from the fact that ordinarily van der Waals forces have an incredibly short range: on the order of 10 nm. Even for amazingly flat surfaces it is hard to get them in contact at the 10 nm level for a large fraction of the area.

However if there is any residual oil or water in the gap, then surface tension can be a big factor and a tiny amount of liquid can fill in a gap of 10s of nm.


It is probably the same thing at the molecular scale. There are a few different mechanisms for things to stick to each other, and it’s not always known what is most relevant in a given situation, but van der Waal’s forces will certainly be a factor there, and probably the main one.

At a larger (but still microscopic) scale, it’s a bit different because the screen protector’s surface is soft enough to conform to the surface of the glass, which is why it doesn’t need to be especially flat.

It is also possible for plastic to stick to glass when it carries a charge – like when a balloon sticks to your hair – and that is a different mechanism.

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I am pretty sure screen protectors are mostly based on electrostatic attraction – the surface of the film is treated to hold a large net charge which in turn induces an opposite net charge on the screen and they attract. van der Waals is an attraction between electrically neutral objects including conductors such as metal.


So this is somehow distinct from cold welding?


Many years ago when I read RAH’s Between Planets I remember him saying Venusian engineers wrung microscopic joints. Guess we know what the dragons were doing now

Could one describe an atom forming a dipole as technically the smallest magnet possible?

Also, is this a factor in space welding?

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I can’t remember where, but I saw a video where someone tried this in a vacuum chamber, and the blocks still stuck, which rules out air pressure.

Van der Waals force is what allow geckos to walk on most surfaces, iirc

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Yes. Cold welding is literally welding. It forms a metallic bond between the parts, at least at a few points. This requires not just very close contact (sub nm), but the surface structure rearranges a bit for the crystal lattices to mesh. It would become essentially impossible to separate without damaging the surface as it wouldn’t necessarily break exactly where the original boundary was.

van der Waals is a weaker type of bonding than the covalent / ionic / metallic bonds that hold many solids together but it applies to literally everything regardless of the electronic structure. It is also the force that holds many non-polar molecular liquids such as liquid nitrogen, methane, etc. together.


If you don’t have any gauge blocks or silicon wafers handy, you can try this for yourself by tearing apart an old 3.5" hard drive.

The platters will stick together if you put them together right after you take them out (no fingerprints!)

For whatever reason they don’t stick well once they’ve been lying around a while. Dust? Micro-corrosion? Don’t know.

(from my spousal unit, the old computer guy)

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