Sliding friction From microscopic contacts to Amontons’ law

Most engineers describe sliding friction using the friction coefficient, the ratio of frictional force to normal force. While this proportionality is very simple, its origin is not trivial at all and has been subject of investigation for more than a century. The current consensus is that both frictional and normal force are proportional to the 'real contact area'. Surface roughness prevents surfaces from coming into full contact; the real contact area is simply the fraction of the apparent contact area in which the two surfaces are in molecular contact. In this thesis, we experimentally test the relation between normal force, real contact area and friction. This is not an easy task; there are no existing experimental techniques that allow measurement of the real contact area at rough interfaces with molecular-scale sensitivity. We synthesize custom-made pressure sensitive fluorescent molecules and immobilize them at the frictional interface such that they reveal the real contact area through their fluorescence intensity. This technique enables us to experimentally test contact and friction mechanics. We find that the real contact area is not proportional to the normal force due to the specific type of plastic deformations that control contact formation. We show that friction is proportional to the real contact area, however the proportionality constant that links friction to contact area evolves in time and therefore depends on the history of the contact. These results provide new insights into friction at rough interfaces and demonstrate the potential of fluorescent contact area imaging.