Eur. Phys. J. E 5, 309-315
Influence of the flow on the anchoring of nematic liquid crystals on Langmuir-Blodgett monolayers studied by optical second-harmonic generationV.S.U. Fazio1, L. Komitov1, C. Radüge2, S.T. Lagerwall1 and H. Motschmann2
1 Department of Microelectronics and Nanoscience, Liquid Crystal Physics, Chalmers University of Technology and Göteborg University, SE-41296 Göteborg, Sweden
2 Max-Plank-Institute of Colloids and Interfaces, D-14476 Golm/Potsdam, Germany
(Received 8 November 2000 and Received in final form 12 March 2001)
The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a slight preference for the direction of the previous flow.
61.30.-v - Liquid crystals.
61.30.Hn - Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering surface-induced ordering, wetting, prewetting transitions, and wetting transitions.
42.65.Ky - Harmonic generation, frequency conversion.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2001