Effect of capillary force on friction force microscopy: A scanning hydrophilicity microscope

The effect of capillary force due to surface water on friction force microscopy (FFM) was examined by comparing FFM images on oxidized Si surfaces partially covered with chemically bound hydrocarbon (HC) monolayers in vacuo and in an ambient atmosphere. It was found also from force-distance curves and FFM under various relative humidities that adhesive and friction forces observed on the hydrophobic HC covered surface were almost independent of the humidity, while those on the hydrophilic bare oxidized Si surface increased with an increase in the humidity. The higher friction observed on the oxidized Si in the higher humidity was interpreted by the higher effective normal load due to the higher capillary force which originated from a surface water film formed by adsorption of water vapor in the humid air. The results suggest a novel scanning hydrophilicity microscope under a controlled humidity which can be used to map local hydrophilicities of a sample surface in a x-y plane.