Destabilizing effect of a surface electric field generated by ionic adsorption on the molecular orientation of nematic liquid crystals
Departamento de Fısica, Universidade Estadual de Maringá, Avenida Colombo 5790, 87020-900, Maringá, Paraná, Brazil
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Accepted: 22 December 2004
Published online: 1 February 2005
The destabilizing effect of a surface electric field, produced by selective ionic adsorption, on the molecular orientation of a nematic-liquid-crystal sample is analyzed for a cell in the shape of a slab of thickness d. The electric-field distribution considered in the analysis is the one obtained in the limit in which essentially all the positive ions are adsorbed. Because of the coupling of this surface field with the nematic director, the surface anchoring energy depends on the thickness of the sample as well as on the adsorption energy characterizing the surfaces. A relation connecting the threshold field for the destabilization of the homeotropic pattern to the adsorption energy and to the thickness of the sample is established in closed form, after solving a set of two coupled non-linear equations determining the electric-field distribution across the sample. It is shown that the values of surface electric field generated by adsorbed ions that can lead to a destabilization of the homeotropic alignment can be attained by real samples.
PACS: 61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order – / 61.30.Hn Surface phenomena: alignment, anchoring, anchoring transitions, surface-induced layering, surface-induced ordering, wetting, prewetting transitions, and wetting transitions – / 41.20.Cv Electrostatics; Poisson and Laplace equations, boundary-value problems –
© EDP Sciences, Società Italiana di Fisica and Springer-Verlag, 2005