Nanotribology of thin liquid-crystal films studied by the shear force resonance method

We present the first resonance study on the tribology of molecularly thin liquid films. For this purpose we utilize a shear force apparatus, constructed as an attachment to the surface force apparatus, and measure the response of a film pressed between two solid surfaces and subjected to lateral oscillations with differing frequencies. There is a dramatic change in the amplitude and the phase of the output signal in the vicinity of the resonance frequency of the mechanical system. The resonance curves obtained at various normal loads are highly sensitive to the dissipation processes in the sample. The tribological parameters are extracted from the experimental data using an original theoretical model. In this way one can measure not only the friction coefficient, but also the damping coefficient, which provides information on the energy loss in the film. Attributing this loss to the work done against friction, one can calculate the energy of binding of a liquid molecule to the substrate. The samples are thin liquid crystals, 4- cyano-4-n-alkylbiphenyls (n=5, 6, 8) in various states, which can also serve as model lubricants. It is found that the films of these compounds exposed to friction are of different mechanics and energetics depending on their structure and on the intermolecular interactions.