Experimental insights into adhesion and friction between nominally dry rough surfaces

Adhesion and friction between solids in ambient air control applications such as precision positioning and traction of tires. For all practical surfaces, the interfaces between solids are rough. Contact occurs at nano- or micro-contact junctions and the contact area of these junctions changes upon normal or shear loading and during sliding. The multiscale roughness, material parameters such as interfacial bonding, mechanical properties, sliding velocity, and normal load all contribute to adhesion and friction, resulting in diverse phenomena. Here, we highlight four recent advances relevant to materials ranging from ceramics to elastomers. They elucidate the importance of real contact area for soft and hard materials and of rate-state-friction models for explaining the transition from stick--slip to steady sliding for hard contacts. We also outline some remaining open experimental challenges related to friction and adhesion of dry multi-contact interfaces.