Local measurement of the zenithal anchoring strength

We present an electro-optic method for measuring the zenithal anchoring strength of nematic liquid crystals, based on the determination of the distortion produced by a small electric field. This method yields the zenithal anchoring strength at small applied torques, and remarkably, only needs local measurements (optical path difference versus applied voltage, sample thickness), in contrast to the classical methods that use measurements integrated over the entire sample. We determine the zenithal anchoring strength for two nematic liquid crystals (5 CB and 5 OCB) with positive dielectric anisotropy, onto poly(tetrafluoroethylene) (PTFE) treated surfaces, that yield planar liquid crystal cells. We find that the anchoring at the PTFE-5 CB interface is strong, with an extrapolation length similar to 30 nm, and independent of temperature far enough from the isotropic transition. We observe a pretransitional weakening of the anchoring strength near the nematic-isotropic transition, due to the reduction of the orientational order parameter at the interface. With 5 OCB, we measure a stronger anchoring, with an extrapolation length similar to 15 nm. This result may be explained by the increase of the van der Waals interactions between the liquid crystal molecules and the surface, due to the presence of the oxygen atom.