Zenithal bistability in a nematic liquid-crystal device with a monostable surface condition

Abstract

The ground-state director configurations in a grating-aligned, zenithally bistable nematic device are calculated in two dimensions using a Q tensor approach. The director profiles generated are well described by a one-dimensional variation of the director across the width of the device, with the distorted region near the grating replaced by an effective surface anchoring energy. This work shows that device bistability can in fact be achieved by using a monostable surface term in the one-dimensional model. This implies that is should be possible to construct a device showing zenithal bistability without the need for a micropatterned surface.

Phase behavior of a nematic liquid crystal in contact with a chemically and geometrically structured substrate

“…The calculated value of the average tilt of the nematic director just above the grating surface in the HAN texture is comparable to the one obtained in the case of the deeper grooves [11,13].…”

“…Finally, we emphasize that the groove depths for which zenithal bistability is found within the present model are considerably smaller than those considered theoretically (A = 0.8 p) [11,13] and used experimentally (A = 0.35 p) [12] for a NLC confined between a pure geometrically patterned surface and a flat substrate [11,13]. The calculated value of the average tilt of the nematic director just above the grating surface in the HAN texture is comparable to the one obtained in the case of the deeper grooves [11,13].…”

“…Adjacent regions of planar and homeotropic anchoring within a single substrate have been created, for example, by microcontact printed polar and apolar thiols, respectively, on an obliquely evaporated ultra-thin gold layer. Whereas many experimental and theoretical studies have focussed on the understanding of the interactions and phase behavior of NLCs in contact with either geometrically structured substrates (see e.g., [1,2,3,4,5,6,7,8,9,10,11,12,13]) or chemically patterned substrates (see e.g., [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29]), NLCs near geometrically structured and chemically patterned substrates have not been investigated yet. In view of the rich behavior of NLCs even on geometrically structured or chemically patterned substrates, a combination of both surface treatments may open new possibilities for an improved performance of NLC cells.…”

“…In bistable nematic devices there are two stable nematic director orientations which have substantially different tilt angles. The zenithally bistable nematic devices that have been studied recently consist of a NLC confined between a grating surface and a flat surface [11,12,13,33]. These surfaces are coated with a homeotropic agent.…”

Cooperative liquid-crystal alignment generated by overlaid topography

“…Current LC devices such as displays require specific and predictable LC alignment characteristics [1][2][3][4][5] and future LC applications, such as in organic photovoltaics (OPV), will depend on effective alignment of novel materials systems [6]. In the latter case, for example the discotic and polymeric composite systems under development can be difficult to orient in electrically efficient geometries, a problem complicated by the nanoheterogeneity required for efficient PV performance.…”