Investigation of humidity-dependent capillary force

Due to the strong capillary condensation, the adhesion force between a Si3N 4 atomic force microscope (AFM) tip and silicon oxide was observed to first increase and then decrease with an increase of humidity. in contrast, due to weak capillary condensation, the adhesion force between the AFM tip and the N-octadecyltrimethoxysilane (OTE, CH3(CH2)(17)Si(OCH2CH3)(3)) self-asse mbled monolayer (SAM) was found to be almost independent of humidity. It wa s found that the formulation commonly used for macroscopic objects fails to explain our data. Using an accurate formulation and an assumed tip shape, we can explain the observed decrease of adhesion for SiO2 at high humidity as being due to the decreased capillary pressure force when the dimension o f the meniscus becomes comparable with the tip size. However, the observed late onset of adhesion of SiO2 cannot be understood within the framework of the classical continuum theory. We attribute this late onset to the proper ties of an ultrathin water film at molecular thickness. The new formulation predicts a vanishing water meniscus between the AFM tip and OTE over the e ntire humidity range and can also fully account for the humidity-independen t adhesion results for OTE.