Microscopic stick-slip in friction force microscopy

Friction force measurements were performed on 2-hydroxy stearic acid (2-HSA) and 12-hydroxy stearic acid (12-HSA) coated silica surfaces in air using an atomic force microscope. The 2-HSA displayed viscoelastic behaviour with a yield point as the static-dynamic friction transition. Steady sliding motion was replaced by microscopic stick-slip at lower velocities and higher loads. Stick-slip motion was successfully described and fitted to a phenomenological model ascribed to interfacial material melting and freezing in periodic cycles. The stick-slip periodicity is of the same order as the contact diameter. The 12-HSA did not experience a yield point and exhibited steady sliding over the entire load and velocity regime. We attribute these observations to the difference in molecular configuration, shear strength and adsorption density of the stearic acid layers.