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 0° (null slope) and for 45°. The theoretical decrease ratio depends on the sidewall angle of the tip, which is about 10° in our case (Ultralevers, ThermoMicroscopes). For this tip geometry, the ratio between adhesion force for 0 and 45° 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.