Cholesteric-nematic transition induced by a magnetic field in the strong-anchoring model

Abstract

We investigate the cholesteric-nematic transition induced by an external bulk field in a sample of finite thickness ℓ. The analysis is performed by considering a tilted magnetic field with respect to the easy direction imposed by rigid boundary conditions inducing planar orientation. In the case of parallel orientation between the magnetic field and of the easy direction, in the limit of ℓ→∞ we reobtain the results of de Gennes where the effective pitch of the cholesteric is a continuous function of the magnetic field diverging at the critical field related to the cholesteric-nematic transition. For finite ℓ we obtain a cascade of transitions, where the bulk expels a half-pitch at a time to avoid divergences in the elastic energy, in a similar manner as solids expel defects in the presence of strong deformation. In the case of oblique orientation between the magnetic field and the easy direction, only the completely untwisted state depends on the tilt angle. Therefore, only the cholesteric-nematic transition depends on the tilt angle while all the other magnetic transition values are unchanged.

Unwinding of a cholesteric liquid crystal and bidirectional surface anchoring

“…The discrete pitchjumps coincide with a change in nature of the twist profile that provides the global energy minimizer. More recently, Scarfone et al [20] generalize the problem of [19] to consider an in-plane magnetic field tilted at some angle with respect to fixed parallel twist directions on the substrates. The analysis of Lelidis et al [21] allows for an incomplete number of half twists in the liquid crystal layer by imposing strong homogeneous anchoring with non-parallel director twists on the two confining plates.…”

Nematic twist–bend phase in an external field

“…In case of modulated nematics their response to an external field can become highly nontrivial [11,47]. In cholesterics, for example, it is possible to unwind the orientational spiral through an intermediate heliconical structure [48,49], both for bulk sample [50] and in confined geometry [51,52]. A more comprehensible, field-induced modification of cholesterics involves reorientation of helical axis [11], or changing the pitch [53].…”

Phase Separations in Mixtures of a Flexible Polymer and a Cholesteric Liquid Crystal in the Presence of an External Field

“…Untwisting of a cholesteric phase by an external magnetic or electric field has been studied theoretically [1][2][3][4][5][6][7][8][9][10][11][12][13] and experimentally [14][15][16][17][18][19][20][21], since it has been first observed by de Gennes [1] and Meyer [14][15][16]. Applying the external field perpendicular to a helical axis of the cholesteric phase, untwisting of the helix takes place with positive dielectric or diamagnetic anisotropy.…”