The influence of instrumental parameters on the adhesion force in a flat-on-rough contact geometry

We have used atomic force microscopy (AFM) to measure the snap-off forces between a micron sizedflat silicon AFM tip and a rough Si(0 0 1) surface. The current paper is a natural continuation of our previous paper (Colak et al., 2014), dealing with snap-off forces between an identicalflat tip andflat Si(0 0 1). Within the applied experimental parameter windows we observed no dependence of the snap-off forces on the applied normal loads (3-18 p,N) and residence times (0.5-35 s) for the currentflat-on-rough geometry as was the case for the formerflat-on-flat geometry. The snap-off forces were found to increase with relative humidity in both geometries. As in the case of the flat-on-flat contact geometry, a strong dependence of the snap-off forces on the retraction speed of the tip was observed. Here, we find a strong decrease of the snap-off forces with increasing tip speed, especially at low velocities in the range 40-1000 nm/s for the flat-on-rough geometry. This is in contrast with the flat-on-flat geometry, where we found a strong increase of the snap-off force with increasing tip speed. These observations are explained in terms of a cross-over of the importance of capillary forces and viscous forces. We suggest that the relative importance of both forces can be checked via variation of the tip speed. (C) 2015 Elsevier B.V. All rights reserved.