Local measurement of the zenithal anchoring strength

We present an electro-optic method for measuring the zenithal anchoring str ength of nematic liquid crystals, based on the determination of the distort ion produced by a small electric field. This method yields the zenithal anc horing strength at small applied torques, and remarkably, only needs local measurements (optical path difference versus applied voltage, sample thickn ess), in contrast to the classical methods that use measurements integrated over the entire sample. We determine the zenithal anchoring strength for t wo nematic liquid crystals (5 CB and 5 OCB) with positive dielectric anisot ropy, onto poly(tetrafluoroethylene) (PTFE) treated surfaces, that yield pl anar liquid crystal cells. We find that the anchoring at the PTFE-5 CB inte rface is strong, with an extrapolation length similar to 30 nm, and indepen dent of temperature far enough from the isotropic transition. We observe a pretransitional weakening of the anchoring strength near the nematic-isotro pic transition, due to the reduction of the orientational order parameter a t the interface. With 5 OCB, we measure a stronger anchoring, with an extra polation length similar to 15 nm. This result may be explained by the incre ase of the van der Waals interactions between the liquid crystal molecules and the surface, due to the presence of the oxygen atom.