Atomic force microscopy cantilever dynamics in liquid in the presence of tip sample interaction

We analyze the dynamics of an atomic force microscopy (AFM) cantilever oscillating in liquid at subnanometer amplitude in the presence of tip-sample interaction. We present AFM measurements of oscillatory solvation forces for octamethylcyclotetrasiloxane on highly oriented pyrolitic graphite and compare them to a harmonic oscillator model that incorporates the effect of the finite driving force for a typical AFM configuration with acoustic driving. In contrast to the general belief, we find—in both experiments and modeling—that the tip-sample interaction gives rise to a pronounced signature in the phase at driving frequencies well below resonance.