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 generation
V.S.U. Fazio1, L. Komitov1, C. Radüge2, S.T. Lagerwall1 and H. Motschmann21 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
fazio@mpikg-golm.mpg.de
(Received 8 November 2000 and Received in final form 12 March 2001)
Abstract
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
