Contribution of Capillary Adhesion to Friction at Macroscopic Solid-Solid Interfaces

Capillary adhesion is commonly present in ambient conditions. It can be measured in single-asperity contacts through atomic force microscopy using a sharp probe that is pulled off a smooth substrate. However, for macroscopic multiasperity interfaces, the measured adhesive force is always close to zero because of the elastic energy stored into the deformation of surface roughness; this is known as the adhesion paradox. Here, we experimentally show how capillary adhesion influences friction between macroscopic Si-on-Si interfaces, covered with native oxide, in two vapor environments: humid air and isopropyl alcohol (IPA) vapor. To quantify the adhesion contribution to friction, we present a boundary element method that successfully models the interplay between capillary adhesion, surface topography, and friction without adjustable parameters and show that the evolution of the surface topography during sliding dramatically increases capillary adhesion and thus friction. Replacing the water vapor with an organic (IPA) vapor, we find a lower adhesion due to the smaller surface tension.