Abstract
Thermotropic nematic materials relax strong distortions by lowering the nematic order: the uniaxial symmetry is broken and is locally replaced by biaxial domains. We investigated the dynamics of the nematic order near a boundary surface of an asymmetric π-cell submitted to an external electric field, close to the electric order reconstruction threshold. An unexpected phenomenon is observed close, but below the threshold: the biaxial order spreads on the surface inducing a consequent bulk topological behaviour equivalent to the splay-bend fast transition allowed by order reconstruction at higher voltage.
Highlights
A π-cell has a sandwich geometry, with a thin film of nematic confined between two flat glass plates
We study how the nematic distortion, induced by a strong electric field, evolves inside an asymmetric π-cell and it is lowered by the appearance of biaxial order close to a boundary surface
If the Nematic Liquid Crystal (NLC) is considered as a pure dielectric material and ions effects are neglected, the free energy F of the system includes the distortion elastic energy Fd, the bulk free energy Ft of the nematic phase and the electrostatic energy Fe due to the external electric field
Summary
A π-cell has a sandwich geometry, with a thin film of nematic confined between two flat glass plates. We study how the nematic distortion, induced by a strong electric field, evolves inside an asymmetric π-cell and it is lowered by the appearance of biaxial order close to a boundary surface. V is the amplitude of the applied voltage, which generates an electric field perpendicular to the boundary plates, d is the cell thickness and eff−1 is the effective electrical permittivity average expressed as 1 d d 0. The analysis has been made for two different amplitudes of the vertical electric field: a) 17 V/μm, above Eth, and b) 14 V/μm, below Eth. At t = 0 s, the nematic director presents a starting asymmetric splay texture with the nematic planar plane close to the lower surface with smaller pretilt. As the anchoring energy on both surfaces is strong, surface molecules hold their positions, whereas the nematic director in the bulk tends to be aligned along the electric field (Δε5CB > 0)
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