Blue phases are types of liquid crystal phases that appear in a temperature range between a chiral nematic phase and an isotropic liquid phase. Because blue phases have a three-dimensional cubic structure with lattice periods of several hundred nanometres, they exhibit selective Bragg reflections in the range of visible light corresponding to the cubic lattice. From the viewpoint of applications, although blue phases are of interest for fast light modulators or tunable photonic crystals, the very narrow temperature range, usually less than a few kelvin, within which blue phases exist has always been a problem. Here we show the stabilization of blue phases over a temperature range of more than 60 K including room temperature (260–326 K). Furthermore, we demonstrate an electro-optical switching with a response time of the order of 10−4 s for the stabilized blue phases at room temperature.

Polymer-stabilized liquid crystal blue phases

A smectic liquid-crystal phase made from achiral molecules with bent cores was found to have fluid layers that exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. These instabilities combine to form a chiral layer structure with a handedness that depends on the sign of the tilt. The bulk states are either antiferroelectric-racemic, with the layer polar direction and handedness alternating in sign from layer to layer, or antiferroelectric-chiral, which is of uniform layer handedness. Both states exhibit an electric field-induced transition from antiferroelectric to ferroelectric.

https://science.sciencemag.org/content/sci/278/5345/1924.full.pdf

Spontaneous formation of macroscopic chiral domains in a fluid smectic phase of achiral molecules

Chirality and liquid crystals are both widely expressed in nature and biology Helical assembly of mesophasic molecules and colloids may produce intriguing chiral liquid crystals To date, chiral liquid crystals of 2D colloids have not been explored As a typical 2D colloid, graphene is now receiving unprecedented attention However, making macroscopic graphene fibres is hindered by the poor dispersibility of graphene and by the lack of an assembly method Here we report that soluble, chemically oxidized graphene or graphene oxide sheets can form chiral liquid crystals in a twist-grain-boundary phase-like model with simultaneous lamellar ordering and long-range helical frustrations Aqueous graphene oxide liquid crystals were continuously spun into metres of macroscopic graphene oxide fibres; subsequent chemical reduction gave the first macroscopic neat graphene fibres with high conductivity and good mechanical performance The flexible, strong graphene fibres were knitted into designed patterns and into directionally conductive textiles

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Graphene chiral liquid crystals and macroscopic assembled fibres

Physics of colloidal dispersions in nematic liquid crystals

The blue phases of cholesteric liquid crystals are liquids that exhibit orientational order characterized by crystallographic space-group symmetries. We present here a pedagogical introduction to the current understanding of the equilibrium structure of these phases accompanied by a general overview of major experimental results. Using the Ginzburg-Landau free energy appropriate to the system, we first discuss in detail the character and stability of the usual helical phase of cholesterics, showing that for certain parameter ranges the helical phase is unstable to the appearance of one or more blue phases. The two principal models for the blue phases are two limiting cases of the Ginzburg-Landau theory. We explore each limit and conclude with some general considerations of defects in both models and an exact minimization of the free energy in a curved three-dimensional space.

Crystalline liquids: the blue phases

The nematic--to--smectic-A transition in liquid crystals is analogous to the normal to superconducting transition in metals with the Frank director n in liquid crystals playing the role of the vector potential A in metals. The liquid-crystal analog of an external magnetic field is a field, arising, for example, from molecular chirality, leading to a nonzero \ensuremath{\nabla}\ifmmode\times\else\texttimes\fi{}n in the equilibrium nematic phase. The cholesteric (twisted nematic) phase is the analog of a normal metal in an external magnetic field. In type-II superconductors in an external magnetic field, the Abrikosov flux lattice phase with partial flux penetration intervenes between the low-temperature Meissner phase and the high-temperature normal-metal phase. In this paper we study the analog in liquid crystals containing chiral molecules of the Abrikosov phase in superconductors. Using a covariant form of the de Gennes free energy, we find that in mean-field theory a state, which we call the twist-grain-boundary (TGB) state, with regularly spaced grain boundaries consisting of parallel screw dislocations, intervenes between the smectic and cholesteric phases. We calculate the liquid-crystal analogs of the upper and lower critical fields ${H}_{c2}$ and ${H}_{c1}$. The properties of the TGB phase depend on the angle 2\ensuremath{\pi}\ensuremath{\alpha} between axes of dislocations in adjacent grain boundaries. \ensuremath{\alpha} can be rational or irrational. When \ensuremath{\alpha}=p/q for mutually prime integers p and q, the TGB state has a q-fold screw axis and quasicrystalline symmetry for crystallographically forbidden q. Our calculations ignore exponentially small terms favoring lock in at rational \ensuremath{\alpha}. We calculate the x-ray scattering intensities in the cholesteric phase near the TGB phase boundary and in the TGB phase for rational and irrational \ensuremath{\alpha}. We also discuss experimental difficulties in observing the TGB state and the possible effects fluctuations not included in mean-field theory might have on its existence.

Abrikosov dislocation lattice in a model of the cholesteric-to-smectic-A transition.

We generalize the Chen-Lubensky model for the point where the nematic, smectic-A, and smectic-C liquid-crystalline phases meet (the NAC point) to include the effects of molecular chirality. We calculate the phase diagram for this model when the possibility of a twist-grain-boundary (TGB) phase is included and when it is not. In the latter case, we find that the NAC multicritical point becomes a critical end point where first-order cholesteric to smectic-A and cholesteric to chiral-smectic-C phase boundaries meet a second-order smectic-A to chiral-smectic-C phase boundary. In the former case, we find that the TGB phase always intervenes between the cholesteric and smectic-A phases near the NAC multicritical point of the nonchiral system. The connection between these results and the recent observations of a chiral smectic phase near the NAC point by Goodby and co-workers [Nature 337, 449 (1988); J. Am. Chem. Soc. 111, 8119 (1989)] is discussed briefly.

S. R. Renn

Papers

Abrikosov dislocation lattice in a model of the cholesteric-to-smectic-A transition.

Twist-grain-boundary phases near the nematic-smectic-A-smectic-C point in liquid crystals