Binary Mass Diffusion Measurements in Nematic and Smectic Liquid Crystals by Forced Rayleigh Scattering

Abstract The binary mass diffusion constants of methyl red in 8CB were measured in the smectic A, nematic and isotropic phases by forced Rayleigh scattering (FRS). The diffusion constant parallel to the director orientation, D⊥, is always larger than the perpendicular one, D||, not only in the nematic but also in the smectic phase. The present result D|| > D⊥ in the smectic phase conflicts with that of methane diffusion, D|| & D⊥, obtained by NMR. The difference was ascribed not to the different method employed but to the different tracer molecules used. The temperature dependence shows nearly an Arrhenius type, but has sharp dip at T AN. The anomaly was attributed to the formation of cybotactic clusters of a smectic phase in a nematic medium and of a nematic phase in a smectic medium.