Mechanisms of dynamic force microscopy on polyvinyl alcohol: Region- specific non-contact and intermittent contact regimes

Dynamic force microscopy (DFM) phase signals were studied using heterogeneous films of polyvinyl alcohol (PVA). The phase was measured as a function of distance and drive frequency over regions of the film with different dissipative properties. When driving below the free resonance frequency at moderate amplitudes, the tip-sample interaction jumps between non-contact and intermittent contact regimes, giving rise to large, region- specific changes in phase within a single image. Amplitude damping largely determines the imaging regime. Resistance to intermittent contact can be overcome by selecting larger drive amplitudes at drive frequencies below the free resonance. Phase contrast then is related primarily to differences in viscoelastic loss. Upon nearing quasistatic contact, viscoelastic loss can produce a transition from intermittent contact to non-contact as the amplitude is heavily damped.