Influence of roughness on capillary forces between hydrophilic surfaces

Capillary forces have been measured by Atomic Force Microscopy in the plate-sphere setup between gold, borosilicate glass, GeSbTe, titanium, and UV irradiated amorphous titaniumdioxide surfaces. The force measurements were performed as a function contact time and surface roughness in the range 0.2 - 15 nm rms, and relative humidity ranging between 2 and 40 %. It is found that even for the lowest attainable relative humidity 2 % very large capillary forces are still present. The latter suggests the persistence of a nanometers thick adsorbed water layer that acts as a capillary bridge between contacting surfaces. Moreover, we found a significantly different scaling behavior of the force with rms roughness for materials with different hydrophilicity as compared to gold-gold surfaces.