Investigation of Capillary Forces Using Atomic Force Microscopy

An atomic force microscope tip, coated with a small amount of liquid silicone, was used to investigate the wetting and capillary bridging forces on various low- and high-energy surfaces. The low-energy surfaces were prepared by reacting alkyl and perfluoroalkyl functional silanes with a silicon wafer (Si/SiO2). Force−distance scans in air revealed that the silicone fluid forms ductile capillary bridges on the low-energy methyl and perfluoromethyl surfaces, whereas a tight bridge is formed on silica. Further studies on a silicon wafer possessing a gradient of surface energy shed more light on the relationship between surface wettability and capillary forces. These observations can be modeled in a general way using the Young−Laplace equation. The understanding of these capillary interactions at nanoscopic levels may have important applications, especially in the controlled deposition of liquid droplets on surfaces.