The area of contact between stationary and moving surfaces

Abstract When two plane surfaces are placed together the area of intimate contact must be very much less than the apparent area. Even if the surfaces are very carefully polished and are made as flat as possible, hills and valleys will still be present on the surface. The upper surface will be supported on these irregularities, and large areas will be separated by a distance which is great compared with the dimensions of a molecule. We do not know very much about the size of these irregularities nor the degree of flatness of the surfaces. Optical methods cannot reveal irregularities much smaller than half a wave-length of light. The scattering of protons (Knauer and Stern 1929) or the diffraction of electrons (Thomson 1934; Finch, Quarrell and Wilman 1935) can give information about the structure of a small portion of the surface, but these methods would not reveal the presence of isolated peaks nor show how flat are the surfaces over a large area. Since it is difficult, even with the most refined technique, to prepare surfaces which are flat to within one or two thousand angstroms, we may expect that the area of intimate contact, that is, the area over which the surfaces are within the range of the molecular attraction, will, for most surfaces, be very small. Some knowledge of the real area of contact is essential for any complete understanding of the mechanism of friction, and this paper describes an attem pt to estimate this area for both stationary and for moving surfaces.