Sensitive methods for estimating the anchoring strength of nematic liquid crystals on Langmuir-Blodgett monolayers of fatty acids - art. no. 061712

The anchoring of the nematic liquid crystal N-(p-methoxybenzylidene)-p-buty laniline (MBBA) on Langmuir-Blodgett monolayers of fatty acids (COOHCnH2n+1 ) was studied as a function of the length of the fatty acid alkyl chain n ( n = 15,17,19,21). The monolayers were deposited onto glass plates coated wi th indium tin oxide, which were used to assemble sandwich cells of various thicknesses that were filled with MBBA in the nematic phase. The mechanism of relaxation from the flow-induced quasiplanar to the surface-induced home otropic alignment was studied for the four aligning monolayers. It was foun d that the speed of the relaxation decreases linearly with increasing lengt h of the alkyl chain n, which suggests that the Langmuir-Blodgett film play s a role in the phenomenon. This fact was confirmed by a sensitive estimati on of the anchoring strength of MBBA on the fatty acid monolayers after anc horing breaking, which takes place at the transition between two electric-f ield-induced turbulent states, denoted as DSM1 and DSM2 (where DSM indicate s dynamic scattering mode). It was found that the threshold electric held f or the anchoring breaking, which can be considered as a measure of the anch oring strength, also decreases linearly as n increases. Both methods thus p ossess a high sensitivity in resolving small differences in anchoring stren gth. In cells coated with mixed Langmuir-Blodgett monolayers of two fatty a cids (n = 15 and n = 17) a maximum of the relaxation speed was observed whe n the two acids were present in equal amounts. This observation suggests an efficient method for controlling the anchoring strength in homeotropic cel ls by changing the ratio between the components of the surfactant film.