Frictional force microscopic detection of anisotropy at NaCl (1 0 0), (1 1 0) and (1 1 1) surfaces

Frictional coefficients were measured at atom-flat NaCl (1 0 0), (1 1 0) and (1 1 1) surfaces with frictional force microscopy. At the electrically neutral (1 0 0) and (1 1 0) surfaces in 15% relative humidity, lower friction was detected along 〈1 1 0〉 direction where the same kind of ions are aligned, than along 〈1 0 0〉 direction where different ions alternate. Low and isotropic friction was observed at the polar (1 1 1) surface. The results were explained by the gradients of electrostatic interaction potentials between a dipole at the tip and the ions at the surface. Toward higher relative humidity up to 60%, frictions at the (1 0 0) surface decreased indicating lubrication effect by water. The changes observed for the (1 1 0) surface suggested presence of adsorbed water in stripe pattern along the 〈1 1 0〉 direction. In the higher humidity range, frictional force increased at all the surfaces, probably due to larger normal force caused by the meniscus force.