  I study how the contact area and the work of adhesion, between two elastic
solids with randomly rough surfaces, depend on the relative humidity. The
surfaces are assumed to be hydrophilic, and capillary bridges form at the
interface between the solids. For elastically hard solids with relative smooth
surfaces, the area of real contact and therefore also the sliding friction, are
maximal when there is just enough liquid to fill out the interfacial space
between the solids, which typically occurs for $d_{\rm K} \approx 3 h_{\rm
rms}$, where $d_{\rm K}$ is the height of the capillary bridge and $h_{\rm
rms}$ the root-mean-square roughness of the (combined) surface roughness
profile. For elastically soft solids, the area of real contact is maximal for
very low humidity (i.e., small $d_{\rm K}$), where the capillary bridges are
able to pull the solids into nearly complete contact. In both case, the work of
adhesion is maximal (and equal to $2\gamma {\rm cos}\theta$, where $\gamma$ is
the liquid surface tension and $\theta$ the liquid-solid contact angle) when
$d_{\rm K} >> h_{\rm rms}$, corresponding to high relative humidity.
