Controllable alignment of nematic liquid crystals around microscopic posts: Stabilization of multiple states

Abstract

Nematic liquid crystal materials are rod-like molecules that align in a locally common direction called the director. This gives rise to anisotropic properties that are used in electro-optical devices such as displays, which usually consist of a sandwich of material between two substrates. The behavior of the director at the substrate surfaces is a critical design consideration, and a wide variety of surface treatments has been reported. Most are dominated by molecular interactions, e.g., high surface energy materials that give homogeneous alignment (the director parallel to substrate). The elastic properties of nematics can also be used to influence alignment by shaping the surface on the micron scale. The potential that this offers to engineer device properties is relatively unexplored; to date, the majority of results reported concentrate on essentially two-dimensional effects. Here we show that the three-dimensional configuration of nematics around microscopic posts results in multiple stable director orientations, and can be used in particular to implement bistable displays with a broad range of control over the optical and switching properties.

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.…”

“…Experiments with LCs in cells made of a topographically patterned plate and a nonaligning plate show a variety of alignments (i) parallel channels align the director n, the local mean molecular long axis direction, along the channels [10]; ii) checkerboard patterns give in-plane bistable [11], tilted [12], or homeotropic alignment [12], depending on the pattern depth; (iii) patterns of alternating channel orientation produce spatially varying director field and controllable pretilt [13]; (iv) patterns of square posts induce 3 bistable, tilted alignment [1]; and (v) noncentro-symmetric patterns induce a surface electroclinic effect [14]. Uncovered LC-filled channels exhibit periodic organization of smectic defects [15].…”

Sidewall control of static azimuthal bistable nematic alignment states

“…Some of the morphologies that have been fabricated include microscale periodic arrays of posts or wells [1][2][3][4][5], surface steps [6], periodic gratings [1,[7][8][9] and bigratings [10], and surface gratings created by nanoscale rubbing [11,12]. The different energetically stable states that are produced by the interaction between the nematic liquid crystal and the surface structures can occur at distinct zenithal out-of-plane tilt angles [1,[6][7][8], at distinct azimuthal in-plane twist angles [1,3,9,11,12], or at some combination of the two [2].…”

Multistable alignment states in nematic liquid crystal filled wells

“…Azimuthal bistability, where the two stable states occur at different inplane rotation angles in the liquid crystal layer, has been achieved using surface gratings, surface bigratings, and periodic arrays of post structures. [1][2][3] Zenithal bistability, where the two stable director configurations differ in their net outof-plane tilt, has been achieved in devices using surface grating structures. 1,4,5 A silicon oxide film can also give rise to two equivalent easy directions of a nematic liquid crystal at different tilt and azimuthal angles.…”