James Gleeson

Twist-bend nematic liquid crystals in high magnetic fields.

We present magneto-optic measurements on two materials that form the recently discovered twist-bend nematic (N_{tb}) phase. This intriguing state of matter represents a fluid phase that is orientationally anisotropic in three directions and also exhibits translational order with periodicity several times larger than the molecular size. N_{tb} materials may also spontaneously form a visible, macroscopic stripe texture. We show that the optical stripe texture can be persistently inhibited by a magnetic field, and a 25T external magnetic field depresses the N-N_{tb} phase transition temperature by almost 1{∘}C. We propose a quantitative mechanism to account for this shift and suggest a Helfrich-Hurault-type mechanism for the optical stripe formation.

Image analysis of shear-induced textures in liquid-crystalline polymers

Abstract Textures of several liquid-crystalline solutions of poly(benzylglutamate) and a solution of hydroxypropylcellulose were videorecorded during and after shear and the images were Fourier transformed. We find for all the solutions that bands perpendicular to the flow direction form after cessation of prolonged shearing if the Deborah number De exceeds a critical value Dec≈0·1. Here De = τ , where is the shear rate and τ is the characteristic molecular relaxation time. The bands are characterized by an initially broad spectrum of wavelengths; for the poly(benzylglutamate) solutions this spectrum narrows and shifts towards longer wavelengths as time progresses after cessation of shear, leading to an increase in the characteristic band spacing b(t) with time t. The dependences of the band spacing on the shear rate and on the solution viscosity n in poly(benzylglutamate) are very weak, but the time to form bands after shearing ceases is roughly inversely proportional to 0. Our results suggest that both ...

Viscosities of a bent-core nematic liquid crystal

Viscosity measurements are reported for a bent‐core nematic liquid crystal, 4‐ chloro‐1,3‐phenylenebis{4‐[4′‐(9‐decenyloxy)benzoyloxy]} benzoate (ClPbis10BB). The rotational viscosity was measured by analysing the dynamics of director rotation in pulsed magnetic fields, and the flow viscosities were determined by employing a new electro‐rotation technique. The results show that whereas the rotational viscosity is more than ten times larger than for calamitic liquid crystals, the flow viscosity is more than 100 times larger. Even more striking is the difference between the ratio of the flow and rotational viscosities, which for calamitic nematics is typically 0.1, whereas in this bent‐core material it is ∼50. This suggests that the large shear viscosity is primarily due not so much to the molecular size, but rather the shape. A model is discussed that may explain the observations.

Flow properties of a twist-bend nematic liquid crystal

We present the first shear alignment studies and rheological measurements in the twist-bend nematic (Ntb) liquid crystal phase of odd numbered flexible dimer molecules. It is found that the Ntb phase is strongly shear-thinning. At shear stresses below 1 Pa the apparent viscosity of Ntb is 1000 times larger than in the nematic phase. At stress above 10 Pa the Ntb viscosity drops by two orders of magnitude and the material exhibits Newtonian fluid behavior. This is consistent with the heliconic axis becoming normal to the shear plane via shear-induced alignment. From measurements of the dynamic modulus we estimate the compression modulus of the pseudo-layers to be B ∼ 2 kPa; this value is discussed within the context of a simple theoretical model based upon a coarse-grained elastic free energy.

Properties of the broad-range nematic phase of a laterally linked H-shaped liquid crystal dimer

In search for novel nematic materials, a laterally linked H-shaped liquid crystal dimer has been synthesised and characterised. The distinct feature of this material is a very broad temperature range (about 50oC) of the nematic phase, which is in contrast with other reported H-dimers that show predominantly smectic phases. The material exhibits interesting textural features at the scale of nanometers (presence of smectic clusters) and at the macroscopic scales. Namely, at a certain temperature, the flat samples of the material show occurrence of domain walls. These domain walls are caused by the surface anchoring transition and separate regions with differently tilted director. Both above and below this transition temperature, the material represents a uniaxial nematic, as confirmed by the studies of defects in flat samples and samples with colloidal inclusions, freely suspended drops, X-ray diffraction and transmission electron microscopy.

Smectic phase in suspensions of gapped DNA duplexes

Smectic ordering in aqueous solutions of monodisperse stiff double-stranded DNA fragments is known not to occur, despite the fact that these systems exhibit both chiral nematic and columnar mesophases. Here, we show, unambiguously, that a smectic-A type of phase is formed by increasing the DNAs flexibility through the introduction of an unpaired single-stranded DNA spacer in the middle of each duplex. This is unusual for a lyotropic system, where flexibility typically destabilizes the smectic phase. We also report on simulations suggesting that the gapped duplexes (resembling chain-sticks) attain a folded conformation in the smectic layers, and argue that this layer structure, which we designate as smectic-fA phase, is thermodynamically stabilized by both entropic and energetic contributions to the systems free energy. Our results demonstrate that DNA as a building block offers an exquisitely tunable means to engineer a potentially rich assortment of lyotropic liquid crystals.

Aggregation, Pretransitional Behavior, And Optical Properties In The Isotropic Phase Of Lyotropic Chromonic Liquid Crystals Studied In High Magnetic Fields

We report results on the high-field magneto-optical response of four aqueous, lyotropic, chromonic liquid crystal formulations in the isotropic phase. Measurements of the field-induced birefringence at temperatures above the isotropic-nematic coexistence region at high magnetic fields reveal qualitative differences in different materials; these differences can be attributed to the nature of aggregation and are discussed within the context of competing aggregation models. Extending these measurements to very high fields and large optical phase differences reveals the presence of an unexpected optical phenomenon: magnetic field-induced circular birefringence, measured in the Voigt geometry, in a system containing no molecularly chiral species. Possible origins of this effect are discussed.

Nanostructures of Nematic Materials of Laterally Branched Molecules

The synthesis and small-angle X-ray scattering (SAXS) characterization is reported for 20 laterally branched mesogenic molecules, which are derived from the common rod-shaped 2,5-bis([4-(octyloxy)phenyl]carbonyloxy) benzoic acid unit. These compounds have a varying degree of flexibility, in that their lateral branch is formed upon conversion of the acid to either an ester or an amide, and most laterally branched molecules exhibit relatively wide nematic liquid-crystal phases with a direct nematic-to-crystal transition at lower temperatures. SAXS studies reveal the presence of smectic-like nanostructures (clusters) with short-range order in the nematic phase, with characteristic correlation lengths from 3 to over 10 nm. The smectic layers that are contained in these clusters are tilted with respect to the nematic director by angles ranging from 0° (i.e. untilted) to 55°. In some compounds, the intensity of the SAXS peak corresponding to the smectic layer spacing depends strongly on temperature. The main features of the nanostructures can be understood based on the molecular structure; therefore, guiding future synthetic work towards more precisely controlled and technologically useful nanostructures in nematics.

Spontaneously modulated chiral nematic structures of flexible bent-core liquid crystal dimers

ABSTRACT We report the induction of spontaneously undulated chiral nematic structures of liquid crystal (LC) dimers with rigid aromatic molecular arms linked by flexible chains with an odd number of carbons. When a small amount of chiral dopants (CD) are added to the dimers, we find the formation of different stripe textures on cooling 4–10 μm films in the nematic phase. The temperature where the stripes form depends on the film thickness and the direction of the stripes depends on the CD concentrations. We show that the experimentally observed stripes are due to undulation instabilities that spontaneously form as a result of the anomalously small bend elastic constant that prefers director bend instead of twist deformation, the opposite of the situation in usual cholesteric LCs. GRAPHICAL ABSTRACT

Extraordinary properties of nematic phases of bent-core liquid crystals

We briefly review systematic and comprehensive studies on several chlorine-substituted bent-core liquid crystal materials in their nematic phases. The results, in comparison to rod-shaped molecules, are both extraordinary and technologically significant. Specifically: a) Electrohydrodynamic instabilities provide unique patterns including well defined, periodic stripes and optically isotropic structures. b) Rheological measurements using different probe techniques (dynamic light scattering, pulsed magnetic field, electrorotation) reveal that the ratio of the flow and rotational viscosities are over two orders of magnitudes larger in bentcore than in calamitic materials which proves that the molecule shape and not its size is responsible for this behaviour. c) Giant flexoelectric response, as measured by dynamic light scattering and by directly probing the induced current when the material is subject to oscillatory bend deformation, turns out to be more than three orders of magnitude larger than in calamitics and 50 times larger than molecular shape considerations alone would predict. The magnitude of this effect renders these materials as promising candidates for efficient conversion between mechanical and electrical energy. d) The converse of this effect when the bent-core material sandwiched between plastic substrates 4 times thicker than the liquid crystal material provided displacements in the range of 100nm that is sensitive to the polarity of the applied field thus suggesting applications as beam steering and precision motion controls.