Origin of friction

It might seem natural to assume that friction arises simply because two touching surfaces, however smooth they look, are actually covered in microscopic bumps and grooves that mechanically interlock — like two pieces of rough sandpaper catching on each other. Under a sufficiently powerful microscope, real surfaces genuinely do show exactly this kind of irregularity and projections. But this interlocking picture cannot be the whole story: when surfaces are made extremely smooth by careful polishing (removing as many of these irregularities as possible), friction is observed not to decrease as this simple picture would predict — and in some cases it actually increases. This rules out interlocking irregularities as the true underlying cause of friction.

The modern explanation instead points to molecular attraction. When two surfaces are pressed together, the true microscopic area over which they are actually in direct contact is far smaller than their apparent (visible) area of contact would suggest, because of the very irregularities just described — the two surfaces genuinely only touch at a relatively small number of high points (Fig. 6.9). Because the real contact area is so small, the pressure at these actual points of contact is correspondingly very high, and this high local pressure produces a strong force of molecular attraction between the two surfaces exactly at those contact points. If the two surfaces are made of the same material, this attractive force is called a cohesive force; if the surfaces are of different materials, it is called an adhesive force. …