available, electrons) passing directly through contact spots, by convective heat transfer through the interface fluid and also by the radiative heat transfer. All these heat exchange mechanisms depend on the true contact area and on the opening (or gap) field between non-contacting parts of the two solids. (5) In a rather similar way, the electric current passes through the true contact area between contacting solids subjected to a difference of the electric potential. However, due to the presence of non-conducting oxides on the surface the effective conducting area is, in general, smaller than the true contact area. (6) Adhesion between solids is also considerably controlled by roughness and the effective contact area. On the other hand, the adhesion exploited by insects and small animals such as gecko [@scherge2001biological; @huber2007influence] is more related to the hierarchical structure of their limbs. (7) Fluid flow between contacting (or slightly separated) surfaces in sealing applications is controlled by their roughnesses, the out-of-contact areas, forming an opening field, and enabling the fluid to pass through the interface. Under certain critical load no possible paths exist, which corresponds to the percolation point. (8) Frictional sliding on contacting asperities is a dissipative process, the produced heat partly 