TABLE OF CONTENTS Lateral force maps of surfaces that are not atomically smooth are often dominated by the topography. We distinguish two cases:
The local slope of the topography, ? s/ ? x causes a lateral force F topo ? ? topo F n cos ? ? ? topo F N, where F N = F L + F A is the total normal force, F L the externally applied loading force, F A the attractive force, ? topo = tan ? = ? s/ ? x and ? is indicated in Fig. 4.1. The approximation is valid for small slopes with ? << 1. In principle, this kind of lateral force would even occur for frictionless sliding, originating simply from a component of the normal force. In contrast to frictional forces, the lateral force F topo does not depend on scan direction and can be distinguished by examining the forward and backward scan. This effect is observed on homogeneous, rather smooth surfaces, where the radius of curvature of the probing tip is small compared with the surface roughness. Small hillocks of polycarbonate on a compact disc are shown in Fig. 4.1, where lateral forces independent of scan direction are found. The derivative image of the topography is in good agreement with the back- and forward scan. Similar examples are described in the literature [1]
