Influence of surface acoustic waves on lateral forces in scanning force microscopies

We present a detailed study of the influence of ultrasonic surface acoustic waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and multimode scanning acoustic force microscopy (SAFM) were used to measure a nd to distinguish between the influence of in-plane and vertical surface os cillation components on the cantilever's torsion and bending. The experimen ts show that friction can locally be suppressed by Rayleigh-type SAWs. Thro ugh the mapping of crossed standing wave fields, the wave amplitude depende nce of the friction is visualized within microscopic areas without changing other experimental conditions. Above a certain wave amplitude threshold, f riction vanishes completely. We found that the friction reduction effect is caused by the vertical oscillation components of the SAW. Purely in-plane polarized Love waves do not give rise to a significant friction reduction e ffect. Thus, we conclude that the mechanical diode effect, i.e., the effect ive shift of the cantilever off of the oscillating surface, is responsible for the SAW-induced lubrication. This explanation is supported by vertical and lateral SAFM measurements: in areas with completely vanishing friction, low frequency vertical cantilever oscillations are still observable, where as lateral (torsional) cantilever oscillations are no longer excited. Addit ionally, at very high Rayleigh wave amplitudes an effect of lateral force r ectification was observed. It results in a scan direction-independent appea rance of the LFM traces. (C) 2001 American Institute of Physics.