Sensitive methods for estimating the anchoring strength of nematic liquid crystals on Langmuir-Blodgett monolayers of fatty acids.

The anchoring of the nematic liquid crystal N-(p-methoxybenzylidene)-p-butylaniline (MBBA) on Langmuir-Blodgett monolayers of fatty acids (COOHC(n)H(2n+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 with 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 homeotropic alignment was studied for the four aligning monolayers. It was found that the speed of the relaxation decreases linearly with increasing length of the alkyl chain n, which suggests that the Langmuir-Blodgett film plays a role in the phenomenon. This fact was confirmed by a sensitive estimation of the anchoring strength of MBBA on the fatty acid monolayers after anchoring breaking, which takes place at the transition between two electric-field-induced turbulent states, denoted as DSM1 and DSM2 (where DSM indicates dynamic scattering mode). It was found that the threshold electric field for the anchoring breaking, which can be considered as a measure of the anchoring strength, also decreases linearly as n increases. Both methods thus possess a high sensitivity in resolving small differences in anchoring strength. In cells coated with mixed Langmuir-Blodgett monolayers of two fatty acids (n=15 and n=17) a maximum of the relaxation speed was observed when the two acids were present in equal amounts. This observation suggests an efficient method for controlling the anchoring strength in homeotropic cells by changing the ratio between the components of the surfactant film.