2000-06-27
11:15 at NO G33

Molecular level processes responsible for energy dissipation in friction

Miquel Salmeron

Lawrence Berkeley National Laboratory, Berkeley, California, USA

Using Atomic Force Microscopy, Surface Forces Apparatus and Sum frequency Generation, my group investigates the structural and frictional properties of monolayers of molecules with long alkane chains. These include alcohols, thiols and silanes. We found that depending on the pressure applied to the film by the sliding surface, several conformation changes take place in the alkane based molecules. At pressures of several 10's of MPa, terminal gauche distortions of the methyl groups take place that, however, do not alter the close packing and ordering of the near vertical chains. At higher loads, between 0.1 and 1 GPa, rigid molecular tilts occur that result in series of metastable tilted configurations due to interlocking of the zig-zag methylene units in the chains. Specific values of the pressure are needed to change one tilted configuration to the next. The energy spent in this process give rise to stepwise increases in the friction force.