Solvation forces using sample-modulation atomic force microscopy

Oscillatory solvation forces in liquid were studied using off-resonance, low-amplitude, sample-modulation atomic force microscopy (AFM). Experiments were carried out with as-purchased tips and with tips modified by the attachment of 10-μm-sized glass beads on a highly oriented pyrolytic graphite (HOPG) substrate submerged in octamethylcyclotetrasiloxane (OMCTS). The sample-modulation response curves allow us to make a direct measurement of the interaction stiffness (force gradient) as a function of tip−sample distance, and we show how this technique is capable of measuring both repulsive and attractive solvation potentials in a single approach with the correct selection of cantilever stiffness. We calculate the solvation force by integrating the stiffness over the tip−sample distance, from which we see clear oscillatory behavior. Normalizing the force with the tip radius allows comparisons to be made between these results and previous surface force apparatus (SFA) data, which sometimes agree with SFA data and sometimes differ markedly. We attribute this to variations in tip roughness and geometry. Our results suggest that nanoscopic roughness can indeed significantly affect force measurements in AFM. Characterization of the bead surface revealed that the beads always contain asperities, and we conclude that these asperities give rise to significant variability in oscillatory force curves, as oscillatory forces occur only near the asperities.