Subnanometer Interfacial Forces in Three-Dimensional Atomic Force Microscopy: Water and Octane near a Mica Surface

Three-dimensional force microscopy generates atomically resolved 3D images of solid-liquid interfaces. Here, we investigated the components of the interfacial force exerted by a tip on a mica-liquid interface by comparing density functional theory and experiments. The study involved two liquids: an aqueous solution of KCl (polar) and n-octane as a nonpolar solvent. The comparison between theory and experiments shows the presence of oscillatory and monotonically decaying terms. The periodicity of the oscillation arises from entropic effects associated with the molecular packing of the liquid, while its amplitude carries information on the solid surface (substrate and tip)-liquid interactions. The exponent of the monotonic term comes from cohesive interactions between the liquid molecules, while its amplitude reflects the interactions with the solid surfaces.