Evidence of the contribution of molecular orientations on the surface force friction of alkaline earth sulfate crystals

Abstract

Frictional force microscopic observation of directional CaSO₄(100), SrSO₄(001) and BaSO₄(001) surfaces showed their asymmetric nature due to the orientations of the sulfate ions. Friction levels change at monolayer steps where S–O tilt directions are reversed. The absolute tilt directions were determined by the shapes of monolayer etch pits. As intuitively expected, scanning the probe against the ‘nap’ of the surface gives higher friction, which can be explained by the larger force constant of the S–O stretching vibrations than bending modes.