R. Barberi

Fast bistable nematic display with grey scale

We present a novel principle for a fast bistable nematic display with intrinsic grey scale. The geometry of a single pixel is the usual hybrid texture in between two conductive flat plates. The device is written by creating surface walls when an electric field is applied, above the thresholds to achieve both the planar anchoring breaking and an electro-hydrodynamic flow. Erasing is obtained by simply breaking the anchoring in absence of vortices. The distorted regions around surface walls depolarize the incident light. As the surface defect density can be modulated, a grey scale is achieved.

Control of transient biaxial order in calamitic nematics

The biaxial order reconstruction in nematics (BORN) is a dynamical effect which can change the topology of nematic textures. The BORN can be induced in a nematic material submitted to strong external constraints, as in the case of bulk textural distortions due to an electric field in a π cell. In this work, we show how suitable dopants for a thermotropic calamitic nematic can influence its transient biaxial order and, hence, the electric BORN threshold.

Nematic anchoring transitions on bistable SiO films driven by temperature and impurities

The influence of temperature and impurities on the surface orientation of nematic 5CB on obliquely SiO evaporated glass plates has been studied. We observe on cooling a continuous anchoring transition from the planar alignment to the tilted one. The experimental data have been interpreted by using elastic models developed by Durand and coworkers in which the surface order parameter can vary in space, close to the surface.

Surface order reconstruction in nematics

Investigations on π-cell, a sandwich cell with the director rotating by 180°, demonstrate the possibility to obtain nematic transitions between two textures with different topologies, for instance between an untwisted state and a π-twisted one. These fast textural changes can be obtained by bulk order reconstruction, which allows the director reorientation between two perpendicular directions without macroscopic rotations of the director itself, or by anchoring breaking, which transforms a weak planar anchoring in a homeotropic surface state. Now, we demonstrate that order reconstruction close to a boundary surface with strong or infinite anchoring conditions provides transitions equivalent to anchoring breaking.

Metallomesogens as Biaxial Dopants in a Calamitic Nematic Liquid Crystal

The electric field induced nematic order reconstruction is a way to achieve controlled transitions between uniaxial nematic textures with different topologies, hence allowing intrinsic textural bistability. The order reconstruction connects perpendicular nematic director orientations by exchanging two eigenvalues of the nematic order tensor Q , implying intermediate transient biaxial order. This phenomenon avoids the full nematic melting; in fact the nematic scalar order parameter does not vanish. The Biaxial Order Reconstruction in a calamitic Nematic (BORN) is mainly governed by the biaxial coherence length ξ b . Therefore, by varying ξ b , one can favour or inhibit the transient biaxial order. Recently, it has been demonstrated that ξ b can be controlled by suitable dopants and in this work we study the BORN transition threshold for liquid crystal mixtures made of the commercial nematic liquid crystal E7 and three different metallomesogens with board-like shape.

Electrically controlled defects at a liquid crystal–polyimide interface

We report results on the effect of introducing a small quantity of oligomer particles inside a liquid crystal (LC) display. We observe a dramatic change in the properties of the LC–substrate interface. Upon applying an electric field, a π‐wall defect network appears that is stable in the absence of further electric perturbation but can be erased by a suitable pulse in a well‐defined range of temperature and frequency. The appearance of the defects and their dimensions depend on the cell age, i.e. on the time required by the guest to deposit on the surface. Hypotheses on the phenomenon are reported.

A Tool to Control the Nematic Surface Alignment: Anchoring Competition

Abstract The problem to obtain a suitable surface texture for a nematic liquid crystal is dealt by means of the fine control of the anchoring properties of independent surface aligning attractors. This approch allows to obtain monostable oblique and planar surface directors and even bistable anchoring conditions. Both cases of azimuthal and zenithal anchorings are considered. Experimental observations are made using composite surfaces with thin superposed different aligning layers. Phenomenological models are used to describe the experimental results and to suggest new applications of this point of view, that can be easily extended to all mesomorphic materials.