Molecular level friction as revealed with a novel scanning probe

Friction at the molecular level is examined with a novel scanning probe mic roscope that observes the onset of energy dissipation and adhesive forces s imultaneously. Friction is monitored by measuring the damped:vibrational am plitude of an oscillating probe tip, analogous to shear-force feedback comm only used in near-field scanning optical microscopes. A mechanically stable interfacial force sensor is used to measure normal forces at the tip indep endently and decoupled from the lateral forces, allowing one to observe fri ction from the early stages of the purely adhesive tensile regime to the la tter stage of repulsive compression. Measurements on model lubricant silane and alkanethiol self-assembled monolayers indicate that friction dramatica lly increases with the strength of adhesive interactions between the tip an d the monolayers. These adhesive interactions occur over an appreciable (7 +/- 1 Angstrom) displacement range, suggesting tensile reorientation of the lubricant chains and subsequent energy losses due to collective chain moti on as well as dissipative hydrogen-bond breaking.