Humidity effect on static aging of dry friction

We report an experiment about the aging properties of the static coefficien t of friction under moisture. Depending on the material used, aging propert ies may be totally driven by the presence of moisture. This may be understo od by taking into account the capillary condensation of liquid bridges betw een surfaces. A simple model describing the time-dependent construction of adhesive forces between surfaces leads to a logarithmic, humidity-dependent aging effect. The estimated magnitude for the aging rate obtained within t his description is in agreement with experimental data.