Casimir forces are probably involved- gage blocks work with flatness/separation (tens of nanometers) that is too high for intermolecular attraction (hundreds of

Casimir forces are probably involved- gage blocks work with flatness/separation (tens of nanometers) that is too high for intermolecular attraction (hundreds of picometers). Casimir attraction works at longer distances, but it's far from well understood. It's most likely a combination of all the common guesses, although the air pressure one is pretty weak.