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Séminaire d’Ivan Dozov

Twist-Bend Nematic Phase, the Spontaneously Bent Liquid Crystal

Ivan Dozov, on leave from LPS

Twist-Bend Nematic Phase, the Spontaneously Bent Liquid Crystal

15 years ago, I proposed that bent-shaped mesogenic molecules may present nematic phases with spontaneous bend distortion of the director [1]. In one of these phases, the twist-bend nematic (NTB), the (achiral) molecules are arranged on a conical helix with short pitch and doubly degenerated handedness ; the chiral symmetry is therefore broken. The NTB phase has no positional order but, due to the heliconical structure, it presents local polar and biaxial order, which is highly unusual for nematics ; the rotational and flip-flop symmetries are therefore also broken.
Recently, several groups reported in bent-shaped nematogenic dimers a transition to a second nematic phase, called NX. Using various experimental techniques, it was confirmed later that the NX phase is indeed a twist-bend nematic [2], with the expected doubly degenerated and short-pitched (p 10 nm) heliconical structure. Tens of NTB materials were reported in the last years and the twist-bend nematic has probably become the “hottest” topic in liquid-crystal research.
Here, I will present our recent advances in the theoretical description of the NTB phase and its experimental investigation. From symmetry reasons, we show that on large length scale, L >> p, there exists an analogy between the periodically distorted NTB phase and the layered SmA* phase. Based on this analogy and on the elastic-instability model [1], we built a coarse-grained elastic theory of the NTB phase and we calculate the macroscopic elastic constants, coherence and penetration lengths in terms of a few Frank-like nematic elastic coefficients. Furthermore, we suggest the existence of a NTB phase, analogue of the twist grain boundary phase TGBA, with penetration of the twist field in the bulk through a network of screw dislocations of the NTB pseudo-layers. Experimentally, we investigated the optic and electro-optic properties of the NTB phase, demonstrating that the ultra-fast (relaxation time < 1 µs) electro-optic effect in CB7CB [3] parallels the electroclinic effect in SmA*. Our studies enabled the estimation of the pitch and the temperature dependence of the heliconical tilt angle [4].
I will discuss also the perspectives of our future work in the frame of the starting ANR project BESTNEMATICS, dedicated to the investigation of the NTB phase and its potential practical applications in fast-switching nematic devices.

[1] I. Dozov, Europhys. Lett. 56, 247 (2001).
[2] M. Cestari et al., Phys. Rev. E 84, 031704 (2011).
[3] C. Meyer, G. R. Luckhurst and I. Dozov, Phys. Rev. Lett. 111, 067801 (2013).
[4] C. Meyer, G. R. Luckhurst and I. Dozov, J. Mater. Chem. C 3, 318 (2015).


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