Biaxiality-driven twist-bend to splay-bend nematic phase transition induced by an electric field

Abstract

Although the existence of the twist-bend (NTB) and splay-bend (NSB) nematic phases was predicted long ago, only the former has as yet been observed experimentally, whereas the latter remains elusive. This is especially disappointing because the NSB nematic is promising for applications in electro-optic devices. By applying an electric field to a planar cell filled with the compound CB7CB, we have found an NTB-NSB phase transition using birefringence measurements. This field-induced transition to the biaxial NSB occurred, although the field was applied along the symmetry axis of the macroscopically uniaxial NTB. Therefore, this transition is a counterintuitive example of breaking of the macroscopic uniaxial symmetry. We show by theoretical modeling that the transition cannot be explained without considering explicitly the biaxiality of both phases at the microscopic scale. This strongly suggests that molecular biaxiality should be a key factor favoring the stability of the NSB phase.

Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen

“…On the contrary, cooling decreases the bend modulus of CB7CB to a very low value, about 0.6 pN, about a degree above the N/N TB phase transition before showing a slight increase. This behaviour is now well documented for twist-bend liquid crystals [29,34,59], including CB7CB [60] (reference [29] gives similar but slightly different values with a different technique). More interestingly, in the binary system studied here, we observe that the values of K 33 close to the N/N TB phase transition first decrease when 8CB is added (for φ = 42 wt %, we measured K 33 ≈ 0.3 pN) before increasing at higher fractions.…”

Methylene- and thioether-linked cyanobiphenyl-based liquid crystal dimers CBnSCB exhibiting room temperature twist-bend nematic phases and glasses

“…33 Scanning electron microscopy observations of the photo-crosslinked samples of the N TB phase revealed a helical organization of the nanostructures. [34][35][36] In addition to the N TB phase, other structurally associated mesophases, such as heliconical smectic (Sm) phases based on bent-core molecules 37,38 and bent LC dimers, 39,40 the twist grain-boundary-twistbend nematic phase, 41 and the splay-bend N (N SB ) phase electrically induced from the N TB phase [42][43][44] have been experimentally verified. Furthermore, the splay N (N S ) phase with the modulation rotated 90° with respect to the N SB phase was discovered in some rod-like mesogens.…”

The Beauty of Twist-Bend Nematic Phase: Fast Switching Domains, First Order Fréedericksz Transition and a Hierarchy of Structures

“…This was observed for the first time in [30] especially for a bent-core LC, and was theoretically predicted by Longa et al [31,32] soon after. This has recently been verified by Meyer et al [33] through birefringence measurements, which suggest that molecular biaxiality plays a key role in not only stabilising the N TB phase but also in producing a field-induced splay-bend phase. Merkel et al show through polarised IR measurements that biaxiality order parameters increase significantly in the N TB phase with a reduction in temperature [34].…”