An investigation into the molecular orientation of sheared liquid crystalline boundary films

Abstract

AbstractThe molecular orientation of a nematic liquid crystal (4‐pentyl‐4′‐cyanobiphenyl, 5CB) in a shear flow between parallel plates was examined using infrared absorption spectroscopy; surface anchoring, film thickness, sliding velocity, and voltage between the parallel plates were independently investigated. Infrared absorbance of the C‐N stretch vibration was measured at the point where the direction of the vibration coincided with the long axis of each 5CB molecule. The molecular orientation depended on the product of the sliding velocity U and the film thickness D, i. e., UD. The effects of surface anchoring appeared to be stronger for lower UD. Calculated results for absorbance, based on a continuum theory, agreed well with experimental ones. The effective viscosity under the same conditions was also calculated, and was increased by a low voltage for lower UD with parallel orientation surfaces. This indicated the possibility of active control of the friction coefficient. With perpendicular orientation surfaces, even without any voltage, the effective viscosity increased with a decrease of UD, which indicated the possibility of a ‘smart lubricant’ the viscosity of which could be adequately controlled by varying the operating parameters of the mating surfaces.