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The general features of tapping mode operation of a scanning force microscope are presented. Relevant factors of tapping mode such as forces, deformation, and contact times can be calculated as functions of tapping frequency, amplitude damping, and sample elastic and viscoelastic properties. Typical contact times per oscillation are about 10 -7 s for hard samples and 6 × 10 -7 s for soft materials, i.e., between one and two orders of magnitude smaller than their equivalents in contact mode force microscopy. The model proposed allows the determination of the phase lag between excitation signal and cantilever response. Major factors to phase contrast are viscoelastic properties and adhesion forces with little participation from elastic properties. Experiments performed on droplets of glycerin deposited on graphite illustrate the ability to image them by recording phase changes.
Tamayo et al. (Mon,) studied this question.