Frictional force microscopy (FFM) was applied to the detection of frictional asymmetry due to molecular tilt, and anisotropy at various atom-flat surfaces of ionic crystals. Less tilted S=O bonds at CaSO4(100) face and more tilted C=O bonds at (10–14) face of calcite gave contrasting results of asymmetry, which are explained by describing the bonds as mass-spring systems. Frictional anisotropy arises from different periodicity of atoms depending upon the scan directions of the probe. Alkali halide surfaces showed anisotropy due to arrangement of electric charges. Surface geometry also causes anisotropy at corrugated CaSO4(001), etc. Adsorbed water can reduce or enhance friction depending upon the relative humidity and the nature of the surfaces.

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