Liquid crystal alignment at macroscopically isotropic polymer surfaces: Effect of an isotropic-nematic phase transition

Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

The devices of the portable computing revolution are being made possible by nematic liquid crystal display (LCD) technology. The optical changes viewed in a dynamic LCD image are based on reorienting molecules by coupling electronically generated electric fields to molecular dielectric anisotropy. This takes place in appropriate fluid electro-optic structures stabilized by nonpolar orientational coupling of molecules to surfaces. The recent observation of ferroelectric nematics having spontaneous macroscopic electric polarization density has introduced a much stronger polar coupling of electric field to molecular reorientation in nematics. This development opens opportunities for advanced electro-optics, but these will require polar control of molecular orientation by surfaces. The generation of polar-structured surfaces and their coupling to nematic polarity is demonstrated in this paper.

We show that surface interactions can vectorially structure the three-dimensional polarization field of a ferroelectric fluid. The contact between a ferroelectric nematic liquid crystal and a surface with in-plane polarity generates a preferred in-plane orientation of the polarization field at that interface. This is a route to the formation of fluid or glassy monodomains of high polarization without the need for electric field poling. For example, unidirectional buffing of polyimide films on planar surfaces to give quadrupolar in-plane anisotropy also induces macroscopic in-plane polar order at the surfaces, enabling the formation of a variety of azimuthal polar director structures in the cell interior, including uniform and twisted states. In a π-twist cell, obtained with antiparallel, unidirectional buffing on opposing surfaces, we demonstrate three distinct modes of ferroelectric nematic electro-optic response: intrinsic, viscosity-limited, field-induced molecular reorientation; field-induced motion of domain walls separating twisted states of opposite chirality; and propagation of polarization reorientation solitons from the cell plates to the cell center upon field reversal. Chirally doped ferroelectric nematics in antiparallel-rubbed cells produce Grandjean textures of helical twist that can be unwound via field-induced polar surface reorientation transitions. Fields required are in the 3-V/mm range, indicating an in-plane polar anchoring energy of w_P ∼3 × 10⁻³ J/m².