Discontinuous orientational changes in nematics Effects of electric and magnetic fields

Abstract

Theoretical studies are presented on discontinuous changes produced in homogeneous deformations under the competing influences of an electric field E applied parallel to the sample planes and a magnetic field H applied at different angles in the plane containing E and n ₀ the initial director orientation. Orientational bistability can be affected by the tilt of n ₀ at the boundaries, the relative magnitudes of H and E, the tilt angle of H and even the order in which E and H are applied to the sample. In particular, the bend transition can be made continuous by applying H parallel to E in materials with positive diamagnetic susceptibility anisotropy (X _a > 0). Depending upon the tilt of H and the relative strengths of H and E it may be possible to change (continuously or otherwise) the nature of distortion in domains. Ab initio calculations show that the E induced twist transition should be continuous in samples having lateral dimensions large compared with thickness. In such a sample one can envisage the possibility of removing the discontinuity at the Freedericksz transition by tilting n ₀ sufficiently away from the homeotropic in a plane normal to E. A more complete theoretical analysis may be necessary to understand the discontinuous twist Freedericksz transition observed experimentally when the electrode gap is not large compared to the sample thickness. The possibility that the hysteresis width of a discontinuous Freedericksz transition might depend on domain formation above threshold cannot be ruled out. In the bend geometry non-rigid anchoring of the director at the boundaries is briefly dealt with to indicate that flexoelectricity might play a role in the initial build up of low level deformation prior to the transition when E is static. The flexoelectric contribution may become negligible in the presence of a sufficiently strong stabilizing H when the anchoring is strong enough and E is time varying. On the basis of the present results and previous work it may be concluded that variation of tilt of H may have considerable influence on the formation of the modulated phase above the bend transition.