The Capillary Force Between an AFM Tip and a Surface at Different Humidity

In the present study, the meniscus profiles of water bridges formed at different relative humidity are determined using the geometric relationships including the Kelvin equation and the force equilibrium formula established for the meniscus. The pull-off forces predicted by the present model show good agreement with the experimental results reported in the literatures. When the contact angles at two solid bodies are equal, the pull-off force is slightly elevated by an increase of the relative humidity of air, and is significantly elevated by an increase of the asperity radius. Furthermore, two hydrophobic surfaces with equally large contact angles lower the pull-off force. If a difference exists between the contact angles of two solid surfaces, the asperity with a hydrophilic surface incorporating with a smooth flat plate with a hydrophobic surface reduces the pull-off force.