Measuring adhesion, attraction, and repulsion between surfaces in liquids with an atomic-force microscope

Abstract

Understanding the forces such as adhesion, attraction, and repulsion between surfaces and liquids is the key not only to understanding phenomena such as lubrication and indentation but also the key to understanding how best to operate an atomic-force microscope (AFM). In this paper, we examined the cases of an insulating tip on an insulating sample (silicon nitride tip on mica) and of a conducting tip on a conducting sample (tungsten carbide tip on a gold or platinum foil). The force-versus-distance curves for these two limiting systems were very different in different liquids. In ethanol, the curve is just what one would expect theoretically: a slightly attractive force before contact, a jump into contact, then a small pull-out force, about 0.2 nN for an insulating tip on the insulating surface and about 0.5 nN for two conducting surfaces. In pure water, the behavior is complex and variable. Pull-out forces vary from 0.2 to 1.5 nN for two insulating surfaces. For two conducting surfaces the force-versus-distance curves show large pull-out forces of order of 10 nN. These large forces are probably due to adsorption contamination layers on the metal surfaces that are not removed by the solvent action of the pure water. These forces, however, can be reduced to less than one-hundredth of the original value by adding ethanol to the water. This makes ethanol a useful liquid for routine imaging of macromolecules such as DNA, proteins, and polymers, that have been adsorbed to a substrate and that must be imaged at low force. In formamide, we observed a predicted repulsive interaction in the nontouching regime for insulating surfaces as predicted by Hartmann. In different concentrations of KCl aqueous solution, we observed again a repulsive interaction in the nontouching regime due to double-layer repulsion of charged surfaces across ionic solutions. The measured Debye length agrees well with the theoretical prediction. And last, the dependence of the pull-out force on the indentation in water has been investigated. The more the tip indents the sample surface in a force-versus-distance cycle, the larger the pull-out force will be. This shows also the usefulness of the AFM for investigations of micromechanical properties.