ATOMIC-FORCE MICROSCOPIC IMAGING IN LIQUIDS - EFFECTS OF THE FILM COMPRESSED BETWEEN THE SUBSTRATE AND THE TIP

Measurements are made of the forces acting on the tip of an atomic for ce microscope when the sample and cantilever are in air and also immer sed in polar solvents like water and DMSO. For large tip/substrate sep arations (>1000 nm) the liquid drag force can be modeled using a class ical hydrodynamic drag force expression. For separations between the t ip and substrate smaller than around 100 nm in a water medium, the for ce due to the effective viscosity increase of the compressed films for high tip/substrate relative velocity is comparable to the contributio n of the double-layer repulsion and the van der Waals attraction. Afte r tip/substrate contact these compressed films produce an attractive f orce that is a function of the liquid medium. In the DMSO medium the a ttractive force between tip and substrate shows an adhesive force that has at least two components indicating a multilayered structure betwe en the tip and substrate during the tip/substrate separation. The scan ning of a surface immersed in water and DMSO with a tip ''in contact'' produces distinct AFM images which depend on the liquid. These images show diverse symmetries and spacings between features which presumabl y correspond to the solvated atomic structure and are only obtained wi th atomic resolution for a scanning speed of similar to 2 nm/s. It is possible to estimate the viscous relaxation time of the compressed lay er value to be similar to 250 ms from the observed resolution of subst rate structures as a function of scanning speed.