Some geometrical considerations about the influence of topography on the adhesion force as measured by AFM on curved surfaces

A simple theoretical model, which assumes a perfect pyramidal tip, has been used in order to compute the dependence of the contact area on the slope of the imaged sample. The resulting expression has been computed for an azimuthal angle of 08 (null slope) and for 458. The theoretical decrease ratio depends on the sidewall angle of the tip, which is about 108 in our case (Ultralevers, ThermoMicroscopes). For this tip geometry, the ratio between adhesion force for 0 and 458 is about 1.6. A very similar value of the decrease ratio of adhesion force has been obtained experimentally, by collecting force curves on mircrospheres of three different materials, which seems to confirm the geometrical nature of this observed dependence of the adhesion force with the surface topography.