On the Adhesion Between Fine Particles and Nanocontacts: an Atomice Force Microscope Study

For adhesion forces measured between identical contacts, two kinds of variations were observed: a fast, random noise and slower fluctuations. Random noise is due to thermal vibrations and noise in the detection system. No intrinsic statistical process is required to explain it. The slower fluctuations are probably caused by plastic deformation and structural changes in the material at the nanocontact. On homogeneous, smooth surfaces, the distribution of adhesion forces measured with nanocontacts can be as narrow as 1.1%. Adhesion force versus humidity curves measured between hydrophilic surfaces can be interpreted by assuming capillary condensation of water into the contact region, which causes a meniscus force. All types of dependencies observed can be explained by appropriate and realistic tip shapes. As a possible simple model, we introduce the two-sphere model with surface roughness. The often observed changes in these dependencies when repeating experiments are interpreted as changes in tip geometry in the 1-10 nm length scale caused by tip wear.