8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P7.4Thu 811:10-14:00Appearance of ”off-axis” friction forces in a lubricatedcontactXavier Banquy, 1 Kai Kristiansen, 1 and Jacob Israelachvili 11 ucsb, chemical engineering, 93106, santa barbara, United States of AmericaIt is widely assumed that the friction force between two bodies in contact and in relative motion isparallel to the direction of motion ans for this reason, measurements of frictional forces are usuallydone parallel to the driving direction. However, recent atomistic simulations have shown thatwhen the distances between the moving surfaces approach the nanometer scale, the atomic scalesurface asperities can induce significant frictional forces that are not parallel (i. e. , “off-axis”) tothe direction of the driving force. Not experimental evidence of off-axis frictional force has beenreported so far. Using a recently developed 3D sensor-actuator for the Surface Forces Apparatus,we measured interfacial forces between two crystalline and atomically smooth surfaces of micaseparated by a nanometers thick layer of hexadecane relative to each other. Our results show thatoff-axis frictional forces exist even in presence of a molecular thin film of lubricant. Appearance ofoff-axis friction forces appears to be intimately related to the underlying structure of the surfacesbut also to the ordering of lubricant molecules in the confined film.4