Detection mechanism of S-O tilt directions on CaSO4(100) by frictional force microscopy

Tilted S-O bonds on a CaSO4(100) surface exhibit asymmetry under frictional force microscopic observation. The detection mechanism of the directional bonds was explained as being an interaction between the probe surface and an array of tilted mass-spring systems at the sample surface. When the scan speed of the probe was changed in the range of 5-91 mu m/s under 40% relative humidity, the asymmetric component of friction increased linearly with the speed, while the total friction signal remained constant at speeds higher than 25 mu m/s. A higher scan speed is more suitable fur the detection of the tilted bonds. At lower scan speeds, the total friction signal decreased in intensity and the asymmetry signal became unstable. This is due to the condensation of water beneath the tip, A similar experiment under 60-65% relative humidity yielded much smaller signals for both the total friction and the asymmetry component.