Adhesive force model at a rough interface in the presence of thin water films: The role of relative humidity

This paper proposes a Boundary Element Model (BEM) for the adhesive contact at the rough interface of two contacting bodies, where a thin water film is adsorbed on the surfaces due to the condensation from the humid environment. Three adhesive components contribute to the total adhesive force: solid-solid and liquid-solid van der Waals interactions and capillary force. Rather than a film with uniform thickness, the true distribution of the water film over the surfaces is considered. The capillary component of the adhesive force is first verified through the well-known capillary force model at the smooth contact of a rigid ball-on-flat configuration for different values of the Relative Humidity (RH) of the environment. Then, the adhesive contact at a rough interface with three different relative auto-correlation lengths under different normal loads is considered. It is found that the capillary force dominates the total adhesive force and it increases with RH, while the other two adhesive components are rather constant. In addition, the capillary force appears to first increase with RH and then decrease as almost the entire non-contact area of the interface is covered by a meniscus. This variation in the capillary force depends on the auto-correlation length, roughness rms of the rough interface, and the normal load. Furthermore, it is confirmed that the capillary force, while employing a water film with uniform thickness, deviates that of the true distribution of this film.