Measurements of off-Axis Friction Forces

Current measurements of frictional forces are usually done parallel to the sliding direction. However, when the distances between the moving surfaces and the dimensions of the lubricant approach the nanometer scale, the molecular structure and surface asperities can induce significant frictional forces that are not parallel to the sliding direction. We have developed a new sensor-actuator for the Surface Forces Apparatus which can measure forces and move two surfaces relative to each other in all 3 orthogonal directions with force resolution in the μN range, and distance control in nm range in the normal direction and μm range in the plane of the surfaces. I will present how “off-axis” friction forces behave when shearing two atomically smooth crystalline (lattice) surfaces separated by nanometer thick layers of hexadecane.