A study of the dielectric relaxation in nematic liquid crystals using the Freedericksz transition technique

Abstract

We have used the Freedericksz transition technique to study the low frequency dielectric relaxation of nematic liquid crystals arising from the orientational relaxation of the long axis component of thte molecular dipoles. The experiments have been done in two geometries: (i) the splay deformation produced by an AC electric field in a homogeneously aligned sample and (ii)the effect of a crossed AC electric field on the bend deformation produced by a magnetic field in a homeotropically aligned sample. The experiments have ben done on (a)4'-n-hexyl-4-cyanobiphenyl (6CB) and (b) trans-p-nnonyloxy-a-methyl-p'-cyanophenyl cinnamate (90MCPC). In 6CB the relaxation occurs in the neighbourhood of 5 MHz (at room temperature). In 90MCPC the dielectric anisotropy changes sign in the neighbourhood of 1 MHz (at 54[ddot]C) to become negative at higher frequencies. We have also observed electrohydrodynamic instabilities in 90MCPC, with the domain sizes comparable to sample thickness at frequencies in the neighbourhood of dielectric isotropy.