抄録
This article critically discusses van der Waals and capillary forces as a function of relative humidity (RH) measured by atomic force microscope (AFM), which, as recently reported, show a discontinuous behavior at a low and high RH. We measure adhesion forces systematically on hydrophilic aluminum and copper oxide sample surfaces in a controlled humid environment using three different AFM probes, an AFM cantilever with a standard probe tip, super sharp probe tip, and an AFM tipless cantilever. In addition, we estimate the van der Walls and capillary forces by a prolate spheroid-plane, oblate spheroid-plane, or tilted plane-plane geometry in view of the actual AFM probe shapes, and compare with the experimental data in adhesion force measurements. Our studies indicate that a discontinuous behavior of capillary force on the hydrophilic surface at low RH is due to the influence of the sample surface roughness, and that at high RH is due to the effect of shape of the AFM probe. The modified formulations predict adhesion force better than conventional ones between the AFM probe and sample surface in the RH range of 10-85%.
