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 force microscope when the sample and cantilever are in air and also immersed in polar solvents like water and DMSO. For large tip/substrate separations (>1000 nm) the liquid drag force can be modeled using a classical hydrodynamic drag force expression. For separations between the tip and substrate smaller than around 100 nm in a water medium, the force due to the effective viscosity increase of the compressed films for high tip/substrate relative velocity is comparable to the contribution of the double-layer repulsion and the van der Waals attraction. After tip/substrate contact these compressed films produce an attractive force that is a function of the liquid medium. In the DMSO medium the attractive force between tip and substrate shows an adhesive force that has at least two components indicating a multilayered structure between the tip and substrate during the tip/substrate separation. The scanning 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 presumably correspond to the solvated atomic structure and are only obtained with atomic resolution for a scanning speed of ∼2 nm/s. It is possible to estimate the viscous relaxation time of the compressed layer value to be ∼250 ms from the observed resolution of substrate structures as a function of scanning speed.