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

摘要

The anchoring of the nematic liquid crystalN-(p-methoxybenzylidene)-p-butylaniline (MBBA) on Langmuir-Blodgett monolayersof fatty acids (COOHC$_{n}$H$_{2n+1}$) was studied as a function of the lengthof the fatty acid alkyl chains, $n$ ($n = 15, 17, 19, 21$). The monolayers weredeposited onto ITO-coated glass plates which were used to assemble sandwichcells of various thickness that were filled with MBBA in the nematic phase. Themechanism of relaxation from the flow-induced quasi-planar to thesurface-induced homeotropic alignment was studied for the four decreaseslinearly with increasing the length of the alkyl chains $n$ which suggests thatthe Langmuir-Blodgett film plays a role in the phenomenon. This fact wasconfirmed by a sensitive estimation of the anchoring strength of MBBA on thefatty acid monolayers after anchoring breaking which takes place at thetransition between two electric-field--induced turbulent states, denoted asDSM1 and DSM2. It was found that the threshold electric field for the anchoringbreaking, which can be considered as a measure of the anchoring strength, alsodecreases linearly as $n$ increases. Both methods thus possess a highsensitivity in resolving small differences in anchoring strength. In cellscoated 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 werepresent in equal amount. This observation homeotropic cells by changing theratio between the components of the surfactant film.