Influence of the flow on the anchoring of nematic liquid crystals on a Langmuir-Blodgett monolayer studied by optical second-harmonic generation

摘要

The influence of capillary flow on the alignment of the nematic liquidcrystal MBBA on fatty acid Langmuir-Blodgett monolayers was studied by opticalsecond-harmonic generation. The surface dipole sensitivity of the techniqueallows probing the orientation of the first liquid crystal monolayer in thepresence of the liquid crystal bulk. It was found that capillary flow causesthe first monolayer of liquid crystal molecules in contact with the fatty acidmonolayer to be oriented in the flow direction with a large pretilt (78degrees), resulting in a quasi-planar alignment with splay-bend deformation ofthe nematic director in the bulk. itself is affected by the flow. Thequasi-planar flow-induced alignment was found to be metastable. Once the flowceases, circular domains of homeotropic orientation nucleate in the sample andexpand until the whole sample becomes homeotropic. This relaxation process fromflow-induced quasi-planar to surface-induced homeotropic alignment was alsomonitored by SHG. It was found that in the homeotropic state the first nematiclayer presents a pretilt of 38 degrees almost isotropically distributed in theplane of the cell, with a slight preference for the direction of the previousflow.