Electric Field Induced Birefringence in Nematic Liquid Crystal Films: Evidence for Wall Quenching of Director Fluctuations

Abstract

The effect of an electric field on the measured order parameter of a liquid crystal is considered. It is pointed out that an external field may influence the apparent degree of order by changing the microscopic order parameter, and by quenching director fluctuations. In nematic liquid crystal the latter dominates, but for smectic phases the effects of field-induced molecular order may be observable.

Measurements are reported of the electric field-induced birefringence in nematic 44′-n-pentylcyanobiphenyl (5CB) and the nematic and smectic phases of 44′-n-octylcynobiphenyl (8CB). The modular dependence of induced order on electric field strength predeicted for nematics is experimentally confirmed by Fourier analysis of the electro-optic response. At low field strength, and in the smectic phase, the induced order varies as the square of the field strength, and the dependece on cell thickness is not as predicted by continuum theory. By introducing two scaling parameters into the theory it is possible to fit quantitatively the experimental results for the nematic phase, and it is suggested that these parameters may describe the effect of the cell walls on the fluctuation mode spectrum in the liquid crystal. The birefringence induced in the smectic phase by an electric field is measured to be 100 times smaller than in the nematic phase, and this can be explained in terms of field-induced microscopic order.