Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

How can it be so hard to drag rubber across smooth glass if friction is caused by surface roughness?

Category: Physics      Published: December 14, 2012

rubber wiper blades
Rubber wiper blades moving across a dry windshield experience high friction. In contrast, rain on the windshield interferes with the electromagnetic attraction and reduces the friction. Public Domain Image, source: Christopher S. Baird.

Friction is not caused mainly by surface roughness. According to the book "Sliding Friction" by Bo N. J. Persson, friction is caused by weak electromagnetic forces between molecules, such as experienced in hydrogen bonds and Van der Waal bonds. If two electrically neutral, non-polar atoms are brought close enough together, they induce electric dipole moments in each other which then attract. These weak bonds cause the two objects with surfaces touching to be attracted, making it hard to slide one object along the other. Additionally, the sliding motion can generate vibrations (phonons) in the objects touching, which takes energy away from the sliding motion and therefore slows down the objects. Because friction is mainly caused by electrical attraction and not surface roughness, smooth objects can still have relatively high friction coefficients. In fact, nanoscopically smooth objects experience very high friction (as long as their chemical structure promotes significant electromagnetic forces) because they have much more of their surface in contact. Surface roughness can become significant if the roughness is very high. The best way to lower the friction of a surface is to combat both. Smooth down the surface to lower surface roughness, and then sprinkle on a lubricant which gets in the way of the chemical bonding, such as oil or graphite.

Topics: Van der Waals, electromagnetism, friction, glass, resistance, rubber, rubbing, surface roughness