A New Device for Measuring 3D Friction Forces and Applying Electrochemical Potential

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. In addition, we can add an electrochemical attachment in order to control the surface potential of one 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, and how these friction forces are affected by parameters like relative orientation of the surfaces, temperature, and sliding history and velocity. In the end I will discuss how an applied electrochemical potential may change the friction forces in an aqueous system.