Instabilities in the electric Freedericksz state of the twist-bend nematic liquid crystal CB7CB

Abstract

We report on the instabilities in the Freedericksz state of the twist-bend nematic (NTB) liquid crystal 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB). The quasi homeotropic NTB state, into which a planar (untwisted or 90°-twisted) nematic CB7CB layer transits under a strong electric field, is found to be unstable despite the material being dielectrically positive. Close to the NTB melting point, destabilization occurs through the formation of metastable toric focal conic domains (TFCDs) that, in time, transform into parabolic focal conic domains (PFCDs) with the confocal parabolae in vertical planes through the layer normal. This transformation occurs by a novel process of continued dissociation of TFCDs. We outline how the extended Volterra process could help in a general appreciation of focal conic defects in the NTB phase. At relatively lower temperatures, stripes develop competingly with TFCDs. Identifiable as oily streaks, they are both localized and polarity sensitive; they form close to the substrates; and in low frequency square wave fields, they get suppressed at the cathode and augmented at the anode at each polarity switch. The study also dwells on the N-NTB-N sandwich region, found between the N and NTB states under a small temperature gradient.

Microscale Structures Arising from Nanoscale Inhomogeneities in Nematics Made of Bent-Shaped Molecules

“…The two striped states can occur together in different sample regions. In planarly aligned CB7CB, mostly PFCDs (parabolic FCDs) and usual FCDs (with confocal hyperbolic and elliptical singular lines) are found; however, stripes of the second type, identified as oily streaks, are also formed in electrically aligned homeotropic CB7CB . From these observations, it appears that the texture in Figure a is structurally similar to that of the stripes around disclinations in the nematic phase.…”

Defects in the twist-bend nematic phase: Stabilities and instabilities of focal conic domains and related topics

“…The transition from a FCD to a PFCD has to be discontinuous, physically. It is indeed observed that the nucleation of PFCDs happens spontaneously under stress, but neither from precursory FCDs, nor directly from TFCDs, that are "destroyed" before transforming into PFCDs [10].…”

“…One may wonder whether the small grainy defects observed in the first steps of the instability process of a TFCD (Fig. 1) [10] could not be of this type.…”

“…3(b)-3(d); see Ref. [10] for the TFCD instability at the N ⇒ N TB transition under an electric field.…”

“…and C.V.Y.) [10]; the synthetic procedure followed is described in the Supplemental Material of Ref. [12].…”