Alignment of a Ferroelectric Nematic Liquid Crystal on Surfaces Treated with Obliquely Evaporated SiO2

Abstract

A nematic liquid crystal is an orientationally ordered liquid where molecules align along a local director but with no polar order. The material RM734 is one of very few discovered substances that can assume the ferroelectric nematic phase, a liquid crystal phase first practically realized in 2017 where the director has not only an orientation, but also a direction. This thesis qualitatively describes the alignment of RM734 in the nematic and ferroelectric nematic phase on a surface treated with oblique evaporation of SiO2. The alignment is studied by means of a thin cell where one surface has been treated with oblique evaporation of SiO2 and the opposite surface is covered by a polymer which has been rubbed to achieve circular boundary conditions. The alignment is revealed by the formation and behavior of domains, depending on the local mutual alignment directions on the two surfaces. The domains created in the higher symmetry nematic phase has a large impact on the alignment in the ferroelectric nematic phase. It is found that the material aligns at an azimuthal angle 0◦ < ϕ < 90◦ from the evaporation plane in either of two symmetrical directions approximately in the substrate plane, potentially opening up for the design of devices based on bistable alignment. The angle ϕ depends on both temperature and evaporation angle, and turns out to fixate under certain conditions. This adds a new layer of complexity to the alignment, as time becomes an important factor in its behavior. RM734 is also studied in a cell with parallel non-polar boundary conditions. Using this cell, the behavior of the liquid crystal at the transitions between the nematic and ferroelectric nematic phases is mapped, and a spontaneous twist without a preferred handedness is demonstrated in the ferroelectric nematic phase.