Atomic force microscopy at solid-liquid interfaces

An atomic force microscope (AFM) operating in force modulation mode is used to study solvation forces at the interface between a graphite (HOPG) surface and the liquids octamethylcyclotetrasiloxane (OMCTS) and 12-dodecanol. Simple analytical models are used that adequately describe the response of the cantilever as the modulation frequency and tip-sample interaction change. The analysis of AFM force curves yields the tip-sample interaction stiffness and damping. Hydrodynamic damping is significant for all the levers used and at present this limits the sensitivity of detecting weak tip-surface damping effects. The main results are: (i) Confinement of liquid between two surfaces can lead to oscillatory structural forces even when one of the surfaces has very high curvature. This could influence topographic images at the atomic level in liquids. In these experiments the typical radius for a sharp AFM tip is measured as R-tip approximate to 14 nm. (ii) The effective viscosity increases by similar to 4 orders of magnitude for a sharp tip interacting with the OMCTS solvation layers nearest the surface. (iii) The AFM data are compared to published results obtained using the surface force apparatus (SFA). The AFM data for the interaction stiffness and damping are qualitatively similar to the SFA results, but the magnitude of the effects is smaller. This most likely arises from the limited interaction area over which the confined molecules can exhibit cooperative behavior (similar to 100 nm(2)). (iv) Oscillatory solvation forces can also be observed with very blunt tips (R-tip approximate to 350 nm), and the data suggest that in this case tip microasperities dominate the tip-sample interaction.