What are the sources of friction, and is friction
due to adhesion or not? These are questions that scientists have
been trying to answer since the early eighteenth century. As described
in the history page, Amontons, Coulomb, Newton, and Da Vinci are a few
of the first scientists to conduct work on the sources of friction.
It was not until the early twentieth century however that molecular adhesion
moved to the forefront as the main source of friction. Adhesion is
defined as the molecular attraction exerted between two bodies in contact.
Sir W. B. Hardy, a chemical physicist, published works in the 1920’s, which
stated that friction is due primarily to the molecular attraction operating
across an interface. Hardy was aware that molecular attraction operates
over short distances and that the attraction differentiates between real
area of contact and apparent area of contact. This idea was further
tested by Tomlinson, who stated that the adhesion approach is based on
the partial irreversibility of the bonding force between the atoms.
At the atomic level, two surfaces in contact appear to be very rough and have numerous hills and valleys.Even in highly polished metals, there exist series of microscopic hills and valleys on the surface. When two surfaces are brought into contact, adhesion between the molecules at the touching high points of the surfaces provides the majority of the friction. For when sliding begins, these high point bonds are continually broken and then reformed as new high points come into contact.
A highly magnified view of the contact surfaces of a book and a table showing the actual contact area, Ar , versus the apparent contact area, Ac . The black dots represent the individual points of contact that sum up to Ar that support the normal load.
Furthermore, this idea of adhesion as the source of friction is best supported by the work of F.P Bowden and David Tabor. At the University of Cambridge, Bowden and Tabor found that friction is proportional to true contact area and the shear strength of the bonds in that area rather than apparent macroscopic contact area. In this idea, the microscopic irregularities of the surfaces touch and push into one another under the pressure caused by the gravitational and normal forces. The sum of these contact points represents the true contact area, and the adhesive bonding in this area results in the apparent friction. However, Tabor found that the adhesive bonding at the so-called true contact points was so intense that the small fragments of the surface were continually being worn away. Thus, this hypothesis seemed narrow in focus because it offered no explanation to wear-free friction.
So under the supervision of Tabor, Jacob Israelachvili developed a “surface forces apparatus” which made accurate measurement of atomic-scale, wear-free friction. Using two mica surfaces, which when properly cleaved are free of atomic hills and valleys, Israelachvili’s apparatus provided atomic-scale proof that friction was proportional to the true contact area. However, it was nearly twenty years of research before Israelachvili discovered that friction did not correlate with the actual adhesive bond; rather that friction is connected to adhesive irreversibility, which is how differently the surfaces behave when they stick as compared to how they become unstuck. Thus, making it nearly seventy years before Tomlinson’s early theories received any outside support.
Finally, we can summarize the advances made in the field of friction pertaining to adhesion as follows:
1. The frictional force is proportional to the degree of ‘irreversibility’ of the force that squeezes the two the surfaces together (the degree to which
they stick relative to becoming unstuck).
2. The frictional force is proportional to the actual area of contact rather than the apparent area of contact (Furthermore, the harder the surfaces are
pressed together, the larger the actual area of contact. Thus, friction is directly proportional to the normal force.)