Bonding of perfectly flat surfaces

[Chronos]

Nor do van der Waals forces actually require a perfectly smooth surface: A rough surface will work, too, provided that you can match the roughness of the surface precisely enough. That’s how geckos can walk on walls: Their feet have oodles of tiny hair-like structures that match the shape of the wall precisely, and stick to it via van der Waals forces.

[Mangetout]

@Claude_Remains

Thank you. So the optics DO become one? My friend is laughing at me and tying to disprove me by using pieces of broken glass here around the garage as proof that I am full of it. I keep telling him that it is on the molecular level and that common glass will not do the trick.

@CalMeacham

I’ve heard of this, too. The problem with using ordinary glass is that it’s not flat enugh – nowhere near. You need optically flat glass which is perfect enough that you can see interference fringes. And both surface have to be scrupulously clean, as well.

OK, but what if you try to reunite a piece of glass broken in two? Should not those two surfaces be a perfect fit for one another? (I imagine the answer is no, due perhaps to difficulty of alignment, or minute loss of material during the break)

[randompattern]

They’re not permanently bonded but, can’t be lifted straight away from each other without twisting or sliding them apart. They would separate by doing this but would be damaged (scratched) in the process, which is bad for optics.

[CalMeacham]

@Mangetout

OK, but what if you try to reunite a piece of glass broken in two? Should not those two surfaces be a perfect fit for one another? (I imagine the answer is no, due perhaps to difficulty of alignment, or minute loss of material during the break)

You’d think so, but I’ve never heard of this happening. I suspect bits of material falling off the broken sides and getting between the two halves would be enough to spoil it, along with random dirt that easily gets in there.

Incidentally, I’ve polished a LOT of flat surfaces to 0.3 micron grit, and never seen this effect of van der Waals bonding. I know I’ve heard of it, but never seen it myself.
I HAVE had a problem getting microscope slides apart. When you try to separate the ones boxed together, they frequently stick, and you can see interference fringes between adjacent ones. But they aren’t van der waals sticking – they can be separated with enough effort. And I’m pretty certain microscope slides aren’t ground and polished to perfect flatness. I believe they used to be float glass, but I don’t know if it’s still done that way.