ABSTRACT
When confined between two solid surfaces or in pores, gases naturally condense into a liquid at a pressure lower than the bulk saturation value. This so-called capillary condensation [1] plays a crucial role in imaging and patterning utilizing an
atomic force microscope (AFM) tip. Due to spatial confinement provided by the tip in close proximity to the surface, water molecules in the air condense to form a meniscus connecting the surface and tip (as illustrated in Fig. 1). This meniscus gives rise to a large capillary force on the AFM tip which governs the force (image) measured by the tip [2, 3]. The meniscus also serves as a transport channel for molecules that are deposited on the surface in dip-pen nanolithography (DPN) [4]. It has been speculated that the resolution of DPN is dependent on the width of the meniscus. Despite their importance in imaging and lithography using an AFM tip, the molecular properties of the meniscus and the capillary force are poorly understood. It would be interesting to know how the meniscus structure and the capillary force are controlled by the change in the tip-surface distance (mechanical control) or by change in humidity (thermodynamic control). It would be equally interesting to know how the meniscus and the force depend on the physicochemical properties (hydrophilicity) or geometrical shapes (roughness and curvature) of the tip and surface. Numerous experiments have been conducted which have reported that the capillary force sensitively depends on the humidity and the hydrophilicity of the tip [5-9]. A transparent interpretation of these experiments, however, is hampered by various unknown factors, such as tip geometry, surface corrugation, and contamination. For example, there has been controversy as to whether the capillary force for a mica surface should be a monotonically increasing function of humidity or not [6].
