What Causes Friction?

As an engineering undergrad, I've always wondered, in the back of my mind--what actually causes friction? Curiously, unless my memory is utterly failing me, this topic was not covered, even cursorily, in any of my classes. Friction is introduced as a given, like gravity. Anyway, for some reason--which may have to do with greater free time, as my brood enters Young Adulthood--I finally bothered to research the topic.

It is surprisingly esoteric and unsettled.

What seems like the most obvious idea--microscopic surface irregularities--seems to have been largely discarded (except that surface irregularities seem to be cited as contributing to the adhesion theory). Some of the different theories I came across:

• Molecular adhesions (van der Waals' forces). But then why is it not proportional to area?
• Collisions and entanglements--seems like an atomic level version of roughness.
• "Gunk in the middle"
• Sound waves
• Then there is grand tour, which I frankly don't have the energy, nor, quite likely, the knowledge and brainpower, to digest.

Someday, maybe in the long winter of American Siberia, I will muster the energy and ambition for a Part 2 on this.