LANGMUIR-BLODGETT-FILM AND NEMATIC INTERFACE - INFLUENCE OF THE FILM DENSITY ON THE NEMATIC ORIENTATION

The orientation induced by a Langmuir-Blodgett film on a Nematic Liqui d Crystal (NLC) is theoretically analyzed. We show that the effective surface energy is due to different contributions connected with steric and van der Waals interactions between the nematic and the solid subs trate. The analysis shows that the Langmuir-Blodgett film orientation depends on the surface density of the molecules. The initial homeotrop ic orientation may become unstable giving rise to a tilted film. The a verage orientation of the nematic molecules is also analyzed. We show that, in the event in which the steric interaction Nematic-Langmuir-Bl odgett film is very large with respect to the dispersion interaction N ematic-Substrate, the nematic orientation coincides with the one of th e him. On the contrary, when the two interactions are comparable, the orientation of the two media may differ. In particular, we analyze how the stable orientation depends on the surface molecular density of th e film.