What causes friction between perfectly flat surfaces?

[ARC123]

I know that most friction is caused from rough uneven surfaces rubbing each other, but what is the cause of friction between 2 flat surfaces?

 

[Hornbein]

I know that most friction is caused from rough uneven surfaces rubbing each other, but what is the cause of friction between 2 flat surfaces?

I know that at least some of friction is due to the Van der Waals force between nearby atoms. That is the force that geckos use to climb walls.

 

[x0rnot]

It's from the atoms repelling each other due to the forces present in their atomic structure. The reason we have friction is the same reason we can't just walk through walls. No matter how thin or perfectly flat a material is, there will be friction due to those repulsive forces acting on each surface.