Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry

Abstract

Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

Mechanical ordering of pigment crystallites in oil binder: Can EPR reveal the gesture of an artist?

“…The anisotropic interactions can also be helpful to analyze the orientation distributions of paramagnetic species in disordered materials, thus providing information on the texturation of the sample at a macroscopic scale (Hentschel, et al, 1978;. For instance, EPR has already been used to investigate ordering in liquid crystals (Meirovitch, et al, 1982;Imrie, et al, 1997;Yankova, et al, 2013;Bogdanov and Vorobiev, 2022), in polymer films (Vorobiev and Chumakova, 2005), in muscle fibers (Fajer, 1994), in oriented bacteria (Franck et al, 1979) or of graphene oxide membranes (Chumakova, et al, 2022). This approach should also be fruitful in the field of cultural heritage where complex composite magnetic materials are often to be found in cultural artifacts.…”

Mechanical ordering of pigment crystallites in oil binder: can electron paramagnetic resonance reveal the gesture of an artist?

“…The anisotropic interactions can also be helpful for analyzing the orientation distributions of paramagnetic species in disordered materials, thus providing information on the texturation of the sample at a macroscopic scale (Hentschel et al, 1978;Friesner et al, 1979). For instance, EPR has already been used to investigate ordering in liquid crystals (Meirovitch et al, 1982;Imrie et al, 1997;Yankova et al, 2013;Bogdanov and Vorobiev, 2022), in polymer films (Vorobiev and Chumakova, 2005), in muscle fibers (Fajer, 1994), in oriented bacteria (Frank et al, 1979) or in graphene oxide membranes (Chumakova et al, 2022). This approach should also be fruitful in the field of cultural heritage, where complex composite magnetic materials are often to be found in cultural artifacts.…”