Surface-field-induced heliconical instability in the cholesteric phase of a mixture of a flexible dimer (CB7CB) and a rodlike molecule (8CB)

Abstract

In 1968, P.G. de Gennes and R.B. Meyer independently predicted that a cholesteric phase can form a stable oblique helicoidal (or heliconical) structure provided that K3 < K2 where K3 (K2) is the bend (twist) constant. This structure usually develops under electric field when the material is of positive dielectric anisotropy and was observed for the first time in 2014 by J. Xiang et al. in a cholesleric phase made of a liquid crystal dimer material (CB7CB) in which K3 is anomalously small. Following a recent theoretical prediction by Poy and Žumer, I show that confining a similar cholesteric phase between two glass plates treated for unidirectional anchoring can lead to a similar heliconical instability. In that case, the confinement induces a surface-field that acts as an effective electric field E with E ≡ 1/d where d is the sample thickness. The experiment was conducted in a mixture of CB7CB +50 wt% 8CB doped with a small amount of the chiral molecule R811. In addition, I show that this mixture presents an unexpected compensation point near the transition to the NTB phase.

Heliconical-fluctuation-induced compensation point in the diluted cholesteric phase of mixtures containing the flexible dimer CB7CB

“…In practice, the classical Cano wedge technique [1] cannot be used to measure the pitch close to a N TB phase transition. The main reason is due to the existence of a band instability of the texture [17] that shifts the χ-lines from their usual equilibrium position in d = P/4; 3P/4; 5P/4... (with d the sample thickness), making impossible a precise measurement of the pitch. Another limitation comes from the large viscosity of the mixtures containing CB7CB leading to equilibration times of the texture that can exceed one day.…”

“…The details of this measurement are given in Ref. [17]. In short, the sample is placed under the microscope and is illuminated with a polarized light beam whose incident polarization is parallel to the director on the bottom plate treated for planar unidirectional anchoring.…”

“…For the mixtures M1, we used the birefringence data given in Ref. [17]. For the other mixtures, the birefringence was measured using a Berek rotating compensator following the protocole described in Ref.…”

“…For the other mixtures, the birefringence was measured using a Berek rotating compensator following the protocole described in Ref. [17]. The major advantage of this technique is that the measurements are very fast because the samples equilibrate within a few minutes after each temperature change.…”

“…[15] where the LC used is the CCN-37 (4α,4'α-propylheptyl-1α,1'α-bicychlohexyl-4β-carbonitrile) and the chiral dopant is the cholesteryl chloride (CC), a molecule known to give a compensation point when it is mixed at high concentration with several usual LC such as 8CB (4-n-octyl-4'-cyanobiphenyl), 8OCB (4-n-octyloxy-4'-cyanobiphenyl) or MBBA (N-(4methoxybenzylidene)-4-butylaniline) [16]. Another example was recently found by one of us (PO) in a mixture of a flexible dimer CB7CB (see below) and a rodlike molecule (8CB) doped with a very small amount of the chiral molecule R811 [17]. In this mixture, a surprising compensation point appeared in the vicinity of the twist-bend nematic phase (N TB ), contrasting with previous studies only mentioning a divergence of the pitch at the transition [18].…”