Weak anchoring energy and pretilt of a nematic liquid crystal

We extend the wedge technique of Rivière et al. to measure the pretilt angle F and the (weak) anchoring energy of a nematic liquid crystal oriented on a suitably treated glass surface. The liquid crystal is placed in a wedge shaped space formed by the plate to be studied and a « standard » plate giving a strong homeotropic anchoring. Instead of using a total reflection technique, we measure the optical path difference (Δl) between the ordinary and extraordinary waves, as a function of the sample thickness (d). From an analysis of this variation, we obtain the pretilt angle from the region of large thickness, and the anchoring energy from the region of small thickness, a domain which could not be studied with the previous technique. The transition thickness d is comparable with the extrapolation length L associated with the surface energy. Typical values of F and L for rubbed silane coated plates are ˜ 0.8 radian and ˜ 1 µm respectively