Session D51: Liquid Crystals: Chromonics and Nematics

Lyotropic chromonic liquid crystals in the biphasic region

Lyotropic chromonic liquid crystals have a wide coexistence temperature range where the isotropic and nematic phases are in equilibrium. Negative tactoids (isotropic droplets in the nematic medium) or positive tactoids (nematic droplets in the isotropic medium) form and grow as the nuclei of the new phases in the biphasic region. We studied the growth of tactoids as a function of temperature, the prolate shape of tactoids as well as their thermal fluctuation, based on which the viscoelastic properties of chromonic liquid crystals were obtained.   

Order Parameter measurements of Chromonic Liquid Crystal Benzopurpurin 4B using polarized Raman scattering

Benzopurpurin 4B (BPP4B), a commonly used textile dye, is known to form chromonic liquid crystal phases in aqueous solutions at fairly low concentrations ($<$0.5 wt{\%}) in comparison with other chromonic liquid crystals. Also the aggregation properties and the structure of the aggregates in aqueous solutions of BPP4B are not well understood. Recently McKitterick et al. reported a study on the aggregation and phase behavior of BPP4B in water.\footnote{C. B. McKitterick, N. L. Erb-Satullo, N. D. LaRacuente, A. J. Dickson, P. J. Collings. \textit{J. Phys. Chem. B}. \textbf{2010}. 114, 1888.} Further understanding of the behavior of BPP4B in aqueous solutions can be gained by studying how the order parameter of its liquid crystalline phase varies with some relevant parameters. Planar monodomains of BPP4B were obtained using a flat rectangular capillary. Thermal evolution of the order parameter of these aligned monodomains was carried out using polarized Raman scattering measurements. Further, the concentration dependence and the effect of salt on the order parameter were studied. The variation of the order parameter with the above parameters was correlated to the structure of the aggregates using the UV-Vis absorption data.   

Textural transformations in lyotropic chromonic liquid crystals under confinement

Lyotropic chromonic liquid crystals under capillary confinement display textural transformations between planar radial and planar polar modes, in which a +1 disclination branches into two +1/2 lines. The texture transformation is characterized by the nature and kinematics of the branch point, the aperture angle, and the shape of the lines. This work presents and validates a model of these four phenomena, which yield the viscoelastic moduli of these novel mesophases.   

Molecular Aggregation in Disodium Cromoglycate

Details of molecular aggregation in the mesophases of the anti-asthmatic drug disodium cromoglycate (DSCG) have been studied using x-ray synchrotron scattering. The results show two reflections, one at wide angles corresponding to $\pi -\pi $ stacking (3.32 {\AA}) of molecules, and the other at small angles which is perpendicular to the direction of molecular stacking and corresponds to the distance between the molecular aggregates. The latter varies from 35 - 41 {\AA} in the nematic (N) phase and 27 -- 32 {\AA} in the columnar (M) phase. The temperature evolution of the stack height, positional order correlations in the lateral direction, and orientation order parameter were determined in the N, M, and biphasic regions. The structure of the N and M phases and the nature of the molecular aggregation, together with their dependence on temperature and concentration, will be presented.   

Flexopolydispersity of Nematic Liquid Crystals

Flexopolydispersity is the coupling of gradients in the director field \textbf{n}(\textbf{r}) of a nematic phase made from a polydisperse mixture of anisotropic particles in solution to the spatial change in variables describing the local mean particle shape, size, and concentration: In solutions of sufficiently polydisperse plates this coupling can lead to a nematic phase with a ``blue-phase'' like array of +1 defect lines. Such a structure has been observed in the lyotropic nematic phase of solutions of graphene oxide sheets.   

Nematic order in toroidal and higher-genus droplets

We generate toroidal and higher-genus droplets filled with nematic liquid crystal which are stabilized inside viscoelastic fluid with a non-zero yield stress. The work presented here is the preliminary observations of our experiments.   

Seeing and Sculpting Nematic Liquid Crystal Textures with the Thom construction

Nematic liquid crystals are the foundation for modern display technology and also exhibit topological defects that can readily be seen under a microscope. Recently, experimentalists have been able to create and control several new families of interesting defect textures, including reconfigurably knotted defect lines around colloids (Ljubljana) and the ``toron,'' a pair of hedgehogs bound together with a ring of double-twist between them (CU Boulder). We apply the Thom construction from algebraic topology to visualize 3 dimensional molecular orientation fields as certain colored surfaces in the sample. These surfaces turn out to be a generalization to 3 dimensions of the dark brushes seen in Schlieren textures of two-dimensional samples of nematics. Manipulations of these surfaces correspond to deformations of the nematic orientation fields, giving a hands-on way to classify liquid crystal textures which is also easily computable from data and robust to noise.   

Patterned Liquid Crystal Droplets

Geometrical confinement has a significant influence on the structure and properties of liquid crystals. Prior research in this area is mainly on polymer-dispersed liquid crystals and liquid crystals in porous media where the liquid crystal droplets are usually non-uniform in size. Here we use microfabrication techniques to pattern liquid crystals into droplets of different geometrical shapes and sizes, and study the liquid crystal ordering in these liquid crystal droplets. The experimental observations will be compared with simulation results.   

The mechanism of controlling liquid crystal surface pretilt angle on plasma beam sputtered films

In liquid crystal (LC) devices, the surface alignment is essential. The polyimide (PI) film is commonly used to make LC molecules parallel to the surface. A rubbing process is usually applied to choose a particular direction on the surface. A pretilt angle is also induced, which is useful but usually very small. In previous works, we have found out that the sputtered ion-oxide films can give a homeotropic alignment to LC, i,e, the LC molecules are perpendicular to the surface. In this work, we combine these two effects by sputtering the ion-oxide particles onto the PI coated glasses. By adjusting the sputtering conditions, the LC alignment are controlled. A wide range of pretilt angles have been achieved, while the rubbing process is no longer required. A thorough study by varying the sputtering conditions, such as voltage, current, and time duration, and observing the pretilt angles is carried out. The sputtered surfaces are examined with scanning electron microscope to see the coverage. By considering the charge distribution and electric field within the sputter, a quantitative model is then developed, which explains how the sputtering conditions affect the pretilt angles almost perfectly.   

Transient Splitting of Conoscopic Isogyres of a Uniaxial Nematic

The phase identification is often based on conoscopic observations of homeotropic cells: A uniaxial nematic produces a pattern with crossed isogyres, while the biaxial nematic shows a split of isogyres. We demonstrate that the splitting of isogyres occurs even when the material remains in the uniaxial nematic phase. In particular, in the bent core material J35, splitting of isogyres is caused by change of the temperature. The effect is transient and the isogyres return to a uniaxial (crossed) configuration after a certain time that depends on sample thickness, temperature, and rate of temperature change; the time varies from a few seconds to tens of hours. The transient splitting is caused by the temperature-induced material flow that triggers a (uniaxial) director tilt in the cell. The flows and the director tilt are demonstrated by the CARS microscopy and fluorescent confocal polarizing microscopy (FCPM). This transient effect is general and can be observed even in E7 and 5CB. The effect should be considered in textural identifications of potential biaxial nematic materials.   

A comparison between ``cybotactic'' groups in bent-core and rod-like nematic liquid crystals

It is becoming increasingly clear that short-range smectic-CP order is the basis for some of the unusual macroscopic properties of bent-core nematic (BCN) liquid crystals. By analyzing small angle X-ray diffraction patterns taken on a bent-core and a chemically related calamitic (rod-like) nematic, we have attempted to clarify the nature of the ``cybotactic groups'' (or molecular clusters) contributing to this short-range order in BCNs, and to distinguish their signature from the scattering due to smectic fluctuations normally observed above a nematic to smectic transition. We find that persistent, finite-sized, tilted smectic clusters, with short, temperature-independent correlation lengths, account for the scattering observed from the BCN, while the calamitic material provides a remarkably clear example of temperature-dependent fluctuations in smectic order observed even far above a smectic-C phase. Supported by NSF DMR-0964765   

Optimizing liquid crystalline properties for bio-sensing at aqueous interfaces

Recent studies show that surfactant or phospholipid assemblies can be monitored at interfaces between aqueous solutions and thermotropic liquid crystals. The capability of these liquid crystals to change birefringence with the reordering induced by the decorated surface allows to study and characterize dynamical phenomena happening at the interfaces. In this work we tune the surface anchoring and the viscoelastic properties of the liquid crystal mesogens in order to increase sensitivity and optimize the response to events at the surface.   

Competing Ordering Processes at Liquid Crystal Surfaces Laden with Semifluorinated Alkane Molecules

Ellipsometric measurements elucidate the interplay between the surface order at the isotropic liquid crystal/air interface and a structural phase transition in a Gibbs film on the same interface. Gibbs films formed by the semifluorinated alkane C18H37--C12F25 exhibit a sharp transition from a dilute state at higher temperatures to a dense state at lower temperatures. The transition temperature can be tuned by controlling the C18H37--C12F25 concentration in the bulk liquid crystal phase. When the transition takes place in the temperature range in which a molecular thin smectic or nematic film exists at the isotropic liquid crystal/air interface, the smectic surface order is destroyed while the nematic surface order is affected by a change of the orientation of the liquid crystal molecules. The ellipsometric data indicate that both behaviors result from a change of the anchoring condition of the liquid crystal molecules in contact with the Gibbs film.   

Revealing the inner arrangement of cholesteric liquid crystals confined in polymeric electrospun fibers

Cholesteric liquid crystals, like other types of LCs, can be confined inside polymeric fibers by coaxial electrospinning. In this way the interesting optical properties of cholesterics could be transferred to very long fibers that can form flexible or rigid mats according to the outer sheath material. Selective reflection was easily detected from polymeric fibers with cholesteric LC core. Despite the uniformity of the external morphology of the fibers, evaluated by SEM, defects in the optical texture could be observed in some locations as well as differences in the wavelength of the reflected light. The reason for such differences needs to be clarified in order to achieve a uniform, controlled optical texture. The understanding was achieved by direct observation of the cross section of the LC-filled fibers by cutting and sectioning the fibers by Focused Ion Beam (FIB). This revealed differences in dimensions of the inner cavity correlating them to the observed wavelengths of the selectively reflected light, but also changes in shape, in some parts with strongly varying width that accounts for the defect lines observed. We could also visualize the effect of flow instability of the jet during spinning, inducing the formation of chains of LC droplets.   

Macroscopic torsional strain and induced molecular conformational deracemization

A macroscopic helical twist is imposed on an achiral nematic liquid crystal by controlling the azimuthal alignment directions at the two substrates. On application of an electric field the director rotates in the substrate plane. This electroclinic effect, which requires the presence of chirality, is strongest at the two substrates and increases with increasing imposed twist distortion. We present a simple model involving a tradeoff among bulk elastic energy, surface anchoring energy, and deracemization entropy that suggests the large equilibrium director rotation at the surfaces induces a deracemization of chiral conformations in the molecules, quantitatively consistent with experiment.