(204a) On the Adhesion of Nanocontacts: an Atomic Force Microscopy Study

To optimize the handling of fine powders in industrial applications, understanding the interaction forces between single powder particles is fundamental. With the invention of the atomic force microscope (AFM), the direct measurement of the interaction between single micron-sized particles became possible. In this study we measured the adhesion forces between AFM tips or particles attached to AFM cantilevers and different solid samples. Smooth and homogeneous surface such as mica, silicon wafers or HOPG and more rough and heterogeneous surfaces such as iron particles or patterns of TiO2 nanoparticles on silicon were used. In the first part we addressed the well known issue that AFM adhesion experiments show wide distributions of adhesion forces rather than a single value. Variations in adhesion forces were found to comprise fast (i.e. form one measurement to the next) fluctuations which are in the order of the experimental error and slower fluctuations which occur over tens or hundreds of consecutive measurements. The slow fluctuations are not likely to be the result of variations in external factors such as lateral position, temperature, humidity, etc. because those were kept constant. The existence of these inevitable fluctuations in the adhesion force leads us to the conclusion that it is principally impossible to measure defined adhesion forces for a mechanical micro- or nanocontact. Even if two solid bodies are brought into contact under precisely the same conditions (same place, load, direction, etc.) the result of such a measurement will not be the same as the previous contact and it is not reproducible. The measurement itself will induce structural changes in the contact region which can change the value for the next adhesion force measurement. In the second part we studied the influence of humidity on the adhesion of nanocontacts. Humidity was adjusted relatively fast to minimize possible contamination and tip wear during one experiment. For hydrophobic surfaces, no signification change of adhesion force with humidity was observed. Adhesion force-versus-humidity curves recorded with hydrophilic surfaces either showed a maximum or continuously increased. We demonstrate that the results can be interpreted with simple continuum theory of the meniscus force. In particular, the meniscus force between an AFM tip and a surface is calculated based on model that includes surfaces roughness and takes into account different AFM tip shapes by a two-sphere-model. Experimental and theoretical results show that the precise contact geometry has a critical influence on the humidity dependence of the adhesion force and that inevitable changes of tip geometry on the sub 10 nm length scale can completely change adhesion force-versus-humidity curves. Our model can also explain the differences between earlier AFM studies, where different dependencies of the adhesion force on humidity were observed.