Nanorheology of thin liquid crystal films studied by shear force resonance method

The structure and mechanics of very thin liquid crystal films depend on the intermolecular interactions in confined dimensions. The rheology of such films has been investigated by a shear force apparatus constructed as an attachment to the surface forces apparatus. The novelty of this method is that the rheological parameters are extracted from the amplitude and the phase of the output signal as a function of the resonance frequency. The apparent viscosity of the liquid crystal film is calculated from the damping coefficient by using a simple theoretical model. The viscosity of nanometer thin films of 4-cyano-4-hexylbiphenyl was found to be larger than the bulk value due to increased interactions in the molecular layers adjacent to the solid surfaces transferring shear. Further decreasing of the film thickness (increasing of the normal load) results in an increase elf the resonance frequency due to the transition from viscous to contact friction. The latter observation opens the door to gaining tribological information at the nanodimension.