Molecular Level Friction As Revealed with a Novel Scanning Probe

Friction at the molecular level is examined with a novel scanning probe microscope that observes the onset of energy dissipation and adhesive forces simultaneously. Friction is monitored by measuring the damped vibrational amplitude of an oscillating probe tip, analogous to shear-force feedback commonly used in near-field scanning optical microscopes. A mechanically stable interfacial force sensor is used to measure normal forces at the tip independently and decoupled from the lateral forces, allowing one to observe friction from the early stages of the purely adhesive tensile regime to the latter stage of repulsive compression. Measurements on model lubricant silane and alkanethiol self-assembled monolayers indicate that friction dramatically increases with the strength of adhesive interactions between the tip and the monolayers. These adhesive interactions occur over an appreciable (7 ( 1 Å) displacement range, suggesting tensile reorientation of the lubricant chains and subsequent energy losses due to collective chain motion as well as dissipative hydrogen-bond breaking.