Emergent properties from contact between rough interfaces

Interface phenomena at the micro- and nanoscales are of paramount importance in nature and technology. Real surfaces present roughness over multiple scales, and understanding the role of roughness in surface physics (heat and electric transfer, hydrophobic properties), surface chemistry (chemical reactions) and tribology (stress transfer, adhesion, lubrication) is a very active research topic. This chapter focuses on the key research question of how nonlinear interactions between contact patches induced by roughness across different length scales influence the emergent physico-mechanical properties of an interface. Special attention is given to the scaling of the real area of contact with the applied normal load, the dependency of the thermal and electric contact conductance on the normal pressure, the evolution of the free volume network between rough surfaces in contact, the role of adhesion and also the evolution of partial slip in frictional contacts.