Magnetic and Electric Field Induced Periodic Deformations in Nematics

Abstract

This work reports theoretical studies on the occurence of a static periodic distortion (PD) in a nematic sample under the action of crossed electric (E) and magnetic (H) fields applied on a uniformly aligned initial director configuration (no ). Linear stability analysis with rigid anchoring hypothesis is employed. For homeotropic alignment of a nematic such as M1 which has negative dielectric anisotropy (ϵA<0) and positive diamagnetic anisotropy (χA>0), PD occurs when E and H are both in the sample plane. The direction of periodicity depends on the angle between E and H; PD is unfavorable when H is tilted sufficiently away from E. In a material (such as CCH-7) with ϵA>0 and χA<0, the direction of periodicity changes continuously in the H, E plane as a function of the strength and tilt of the stabilizing H relative to E. Possible effects of flexoelectricity, weak anchoring and director pretilt are discussed briefly

Application of field-cycling NMR relaxometry to the study of ultrasound-induced effects in the molecular dynamics and order of mesomorphic materials

“…Processes like those taking place in Fréedericksz transitions of surface-oriented nematics under the action of a magnetic field may lead to the formation of spatially periodic structures or "inversion walls" as a consequence of spontaneous symmetry breaking [59,60]. Wrinkled nematic states of this sort may also be created by electric fields due to electric conduction [61], or combining the action of electric and magnetic fields [62]. In general, these states have a metastable character (memory capacity) [59,63].…”

Magnetic and Electric Field Induced Periodic Deformations in Nematics-Effect of Director Pretilt

“…The linear perturbation model of earlier work [18,21] has been extended to study the effect of director pretilt on the occurrence of static periodic deformations produced by the application of E and H fields in two materials (M1 and 5CB) having positive diamagnetic anisotropy and dielectric anisotropies of opposite sign. The rigid anchoring hypothesis is used.…”

“…The technique of ref. [18] along with rigid anchoring hypothesis can be employed to show that PD may occur in these materials under the action of crossed fields [21]. The periodicity wavevector in the sample plane may change continuously or discontinuously depending upon material parameters; in some cases, PD may be suppressed over a range of magnetic tilts.…”

“…In ref. [21], only the homeotropic initial alignment is considered. It should be interesting to study the effect of director pretilt on the phase diagrams of different PD modes.…”

“…In section 2, the governing equations of ref. [21] are briefly restated along with the boundary conditions for the rigid anchoring hypothesis and the H D thresholds are worked out for different materials. In section 3 the PD thresholds and periodicities are obtained numerically as functions of different parameters for M1; section 4 describes similar studies on 5CB.…”