Predicting a Polar Analog of Chiral Blue Phases in Liquid Crystals

Abstract

In liquid crystals, if flexoelectric couplings between polar order and director gradients are strong enough, the uniform nematic phase can become unstable to the formation of a modulated polar phase. Previous theories have predicted two types of modulation: twist bend and splay bend; the twist-bend phase has been found in recent experiments. Here, we investigate other types of modulation, using lattice simulations and Landau theory. In addition to twist bend and splay bend, we also find polar blue phases, with 2D or 3D modulations of both the director and the polar order. We compare polar blue phases with chiral blue phases, and discuss opportunities for observing them experimentally.

Purely entropic self-assembly of the bicontinuous Ia 3 d gyroid phase in equilibrium hard-pear systems

“…This, though, is difficult to reconcile with the phase diagram of figure 3, in which the gyroid borders isotropic and nematic fluids for which there is no curvature elasticity. A possible explanation for this is offered by recent arguments from the Selinger group [61] that sufficiently strong bulk splay-bend coupling between polar and orientational degrees of freedom can destabilize the nematic with respect to supra-molecular modulations (i.e. periodic structures).…”

“…These arguments, in turn, hark back to the classic paper of Dozov [62] in which the central ideas of the twist-bend nematic were set out. Given that our hard pear systems clearly possess steric coupling between molecular-scale splay and bend, there appears to be a strong argument that the gyroid region observed here is indeed a realization of a modulated splay-bend phase predicted by Dozov and Selinger. In general, it is interesting to further investigate if it is possible to self-assemble other cubic phases purely entropically-like the polar blue phase, which is predicted for liquid crystals [61], or other minimal surface phases (note recent work on quenched polymeric phases [63][64][65], including simulations of bicontinuous structures [64,65] rsfs.royalsocietypublishing.org Interface Focus 7: 20160161 is advisable to be guided by biological systems again and to introduce a second component such as an oligomer or a solvent of hard spheres, which might promote a change in curvature. Alternatively, approaches based on polydisperse systems and active particles may prove effective in influencing mesoscopic structure.…”

One-dimensional theoretical analysis of coupling and confinement effects on the cybotactic clusters of bent-core nematic liquid crystals

“…Recent experimental studies show that these short-range Sm-like cybotactic clusters can exist even in the isotropic liquid phase of the bent-shaped compounds [7,9]. Studies also show that a strong coupling between the polar order and the director gradients can induce modulated polar phases destabilizing the uniform N phase of such bend-shaped molecules (e.g., twist-bend or splay-bend), and can even induce polar blue phases [12,13].…”

Comparing skyrmions and merons in chiral liquid crystals and magnets

“…Polarity is often induced by surfaces, and the phenomenon of surface-induced modulations has been studied for many years [29,30]. More recently, spontaneous bulk polarity has also been found in certain liquid crystals, and theoretical research has predicted that bulk polarity can induce blue phases [31,32]. In magnets, the analogous mechanism for broken inversion symmetry at surfaces is called Rashba spin-orbit coupling, and it has also been shown to favor the formation of skyrmions [33].…”