Scaling law on molecular orientation and effective viscosity of liquid-crystalline boundary films

The molecular orientation of a nematic liquid crystal (4-pentyl-4'-cyanobiphenyl : 5CB) in a shear flow between parallel plates was investigated using infrared absorption spectroscopy; surface anchoring, film thickness, sliding velocity and voltage between parallel plates were independently selected. Infrared absorbance of the C-N stretch vibration was measured, where the direction of the vibration was coincidental to the long axis of each 5CB molecule. Molecular orientation depended on the product of the sliding velocity U and the film thickness D, i.e. UD. The effects of the surface anchoring appeared more strongly for lower UDs. Calculated results on the absorbance based on a continuum theory agreed well with experimental ones. The effective viscosity under the same conditions was also calculated. The effective viscosity was increased by low voltage for lower UDs with parallel orientation surfaces, which indicated the possibility of an active control of friction coefficient. With perpendicular orientation surfaces, even without voltage, the effective viscosity increased with the decrease of UD, which indicated the possibility of a smart lubricant which could modify its viscosity to compensate for the variation of the operating parameters of mating surfaces.