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 heterogeneo us films of polyvinyl alcohol (PVA). The phase was measured as a function o f distance and drive frequency over regions of the film with different diss ipative properties. When driving below the free resonance frequency at mode rate amplitudes, the tip-sample interaction jumps between non-contact and i ntermittent contact regimes, giving rise to large, region-specific changes in phase within a single image. Amplitude damping largely determines the im aging regime. Resistance to intermittent contact can be overcome by selecti ng larger drive amplitudes at drive frequencies below the free resonance. P hase contrast then is related primarily to differences in viscoelastic loss . Upon nearing quasistatic contact, viscoelastic loss can produce a transit ion from intermittent contact to non-contact as the amplitude is heavily da mped. (C) 1999 Elsevier Science B.V. All rights reserved.