Mapping local electrostatic forces with the atomic force microscope

We have used an atomic force microscope (AFM) to measure locally electrostatic interaction between the AFM's tip and a charged surface. We investigated small amphiphilic positively charged bilayer patches adsorbed to the negatively charged mica surface in various electrolytes at different concentrations. Thus we could determine the lateral resolution of the AFM in detecting repulsive and attractive electrostatic forces to be better than 25 nm. We have investigated the electrostatic interactions between the tip and the sample in a variety of mono- and bivalent electrolytes. In addition, when imaging these structured surfaces with the AFM, we could observe contrast reversal at low forces, which can be explained by the electrostatic forces between tip and sample.