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

The influence of capillary flow on the alignment of the nematic liquid crys tal 5CB on fatty acid Langmuir-Blodgett monolayers was studied by optical s econd-harmonic generation (SHG). The surface dipole sensitivity of the tech nique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flo w causes the first monolayer of liquid crystal molecules in contact with th e fatty acid monolayer to be oriented in the flow direction with a large pr etilt (78 degrees), resulting in a quasi-planar alignment with splay-bend d eformation of the nematic director in the bulk. The large pretilt angle als o suggests that the Langmuir-Blodgett film itself is affected by the how. T he quasi-planar flow-induced alignment was found to be metastable. Once the how ceases: circular domains of homeotropic orientation nucleate in the sa mple and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alig nment was also monitored by SHG. It was found that ill the homeotropic stat e the first nematic layer presents a pretilt of 38 degrees almost isotropic ally distributed in tile plane of the cell, with a slight preference for th e direction of the previous flow.