Imaging of droplets of aqueous solutions by tapping-mode scanning force microscopy

Droplets of aqueous solutions of CaCl2 and P2O5 have been imaged with a scanning force microscope operated in tapping mode. We found two distinctly different modes of imaging, as revealed by both the obtained topography and the resonance curves of the cantilever under imaging conditions. One mode seems to comprise the formation of a liquid bridge between the tip and the sample and thus leads to distortions of the imaged droplets. The other mode seems to circumvent the formation of a bridge and yields droplet-profiles which are in perfect agreement with theoretical predictions. The phase lag between the cantilever motion and the piezo voltage was monitored simultaneously with the topography signal, providing a strong material contrast between the liquid and the substrate.