Mohan Srinivasarao

Spontaneous emergence of chirality in achiral lyotropic chromonic liquid crystals confined to cylinders

The presumed ground state of a nematic fluid confined in a cylindrical geometry with planar anchoring corresponds to that of an axial configuration, wherein the director, free of deformations, is along the long axis of the cylinder. However, upon confinement of lyotropic chromonic liquid crystals in cylindrical geometries, here we uncover a surprising ground state corresponding to a doubly twisted director configuration. The stability of this ground state, which involves significant director deformations, can be rationalized by the saddle-splay contribution to the free energy. We show that sufficient anisotropy in the elastic constants drives the transition from a deformation-free ground state to a doubly twisted structure, and results in spontaneous symmetry breaking with equal probability for either handedness. Enabled by the twist angle measurements of the spontaneous twist, we determine the saddle-splay elastic constant for chromonic liquid crystals for the first time.

Biomimicry of optical microstructures of Papilio palinurus

The brilliant coloration of animals in nature is sometimes based on their structure rather than on pigments. The green colour on the wings of a butterfly Papilio palinurus originates from the hierarchical microstructure of individual wing scales that are tiled on the wing. The hierarchical structure gives rise to two coloured reflections of visible light, blue and yellow which when additively mixed, produce the perception of green colour on the wing scales. We used breath figure templated assembly as the starting point for the structure and, combining it with atomic layer deposition for the multilayers necessary for the production of interference colors, we have faithfully mimicked the structure and the optical effects found on the wing scale of the butterfly Papilio palinurus.

Chiral Nematic Fluids as Masks for Lithography

A lithographic scheme is reported that uses a cholesteric liquid crystal with its helical axis in the plane of the sample as a mask for imprinting patterns in a photoresist over a large area. On the application of an electric field the cholesteric liquid crystals produce a texture that acts as a lattice of cylindrical lenses for one polarization of a light beam. This is used in photolithography to create parallel lines over large areas.

Cylindrical posts of Ag/SiO2/Au multi-segment layer patterns for highly efficient surface enhanced Raman scattering

We fabricated a regular array of Ag/SiO₂/Au multi-segment cylindrical nanopatterns to create a highly efficient surface enhanced Raman scattering (SERS) active substrate using an advanced soft-nanoimprint lithographic technique. The SERS spectra results for Rhodamine 6G (R6G) molecules on the Ag/SiO₂/Au multi-segment nanopatterns show that the highly ordered patterns and interlayer thickness are responsible for enhancing the sensitivity and reproducibility, respectively, The multi-segment nanopattern with a silica interlayer generates significant SERS enhancement (~EF = 1.2 x 10⁶) as compared to that of the bimetallic (Ag/Au) nanopatterns without a dielectric gap (~EF = 1.0 x 10⁴). Further precise control of the interlayer distances between the two metals plays an essential role in enhancing SERS performance for detecting low concentrations of analytes such as fluorescent (Rhodamine 6G) and DNA molecules. Therefore, the highly ordered multi-segment patterns provide great sensitivity and reproducibility of SERS based detection, resulting in a high performance of the SERS substrate.

Enhanced Mobility and Effective Control of Threshold Voltage in P3HT-Based Field-Effect Transistors via Inclusion of Oligothiophenes

Improved organic field-effect transistor (OFET) performance through a polymer-oligomer semiconductor blend approach is demonstrated. Incorporation of 2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (BTTT) into poly(3-hexylthiophene) (P3HT) thin films leads to approximately a 5-fold increase in charge carrier mobility, a 10-fold increase in current on-off ratio, and concomitantly, a decreased threshold voltage to as low as 1.7 V in comparison to single component thin films. The blend approach required no pre- and/or post treatments, and processing was conducted under ambient conditions. The correlation of crystallinity, surface morphology and photophysical properties of the blend thin films was systematically investigated via X-ray diffraction, atomic force microscopy and optical absorption measurements respectively, as a function of blend composition. The dependence of thin-film morphology on the blend composition is illustrated for the P3HT:BTTT system. The blend approach provides an alternative avenue to combine the advantageous properties of conjugated polymers and oligomers for optimized semiconductor performance.

Control of periodic defect arrays of 8CB (4 '-n-octyl-4-cyano-biphenyl) liquid crystals by multi-directional rubbing

In this study, we introduce a simple multi-directional rubbing method to generate various periodic patterns of toric focal conic domains (TFCDs), a typical smectic (Sm) LC defect by using the commonly used Sm LC materials, 4′-n-octyl-4-cyano-biphenyl (8CB). We have successfully obtained a variety of spatially arranged TFCD arrays such as parallelogrammic, hexagonal and square arrays by changing the rubbing angle (Ω) between two different rubbing directions. Our experimental results demonstrate that frustration and distortion of nematic LC molecules at the intersections act as effective seeds to help positioning of TFCDs. Furthermore, the existence of nematic–Smectic A (SmA) phase transition is quite important in controlling the arrangement of TFCDs as the alignment of the nematic director strongly influences the arrangement of TFCDs formed in the SmA phase.

Theory and modeling of nematic disclination branching under capillary confinement

Defects in nematic liquid crystals under controlled confinement serve as a useful tool to characterize material properties as well as to reveal texture stability and pattern formation mechanisms in anisotropic soft matter. In particular, nematics inside micro-capillaries can exhibit a large variety of textures with point defects, line defects and loop defects, whose stability is dictated by factors such as geometry and temperature. In this paper we present a theoretical model, scaling, and simulation of a texture transition between two commonly observed patterns (planar polar and radial), through the uniform translation of a shape-invariant disclination branch point in generic calamitic nematic liquid crystals under capillary confinement and strong radial anchoring. Using the “nematic elastica” model derived from the Q-tensor Landau–de Gennes and n-vector Frank–Oseen equations, the geometry, stability, and energetics of disclination branching of a straight high order +1 disclination into a pair of curved +1/2 disclinations under capillary confinement are fully characterized, including the branch angle, the curvature and final separation of the +1/2 lines, and the scaling of these quantities with the capillary radius (R). It is found that the branching and disclination shapes adjust to the capillary confinement by regulating their tension-to-bending stiffness ratio in such a way that the resulting branch angle is close to π/3, the branch curvature is 3/R and the final disclination distance is . These new findings based on the “nematic elastica” are also useful to predict other novel structures that arise in conical and toroidal geometries of current interest and can be used to assess the Frank elasticity of nematic liquid crystals.

Orientational Order of a Lyotropic Chromonic Liquid Crystal Measured by Polarized Raman Spectroscopy

Lyotropic chromonic liquid crystals are distinct from thermotropic nematics from a fundamental standpoint as the structure of the aggregating columns is a function of both the temperature and concentration. We report on the thermal evolution of orientational order parameters, both the second (=scalar) (⟨P200⟩ (=S)) and fourth (⟨P400⟩) order, of sunset yellow FCF aqueous solutions, measured using polarized Raman spectroscopy for different concentrations. The order parameter increases with the concentration, and their values are high in comparison with those of thermotropic liquid crystals. On the basis of Raman spectroscopy, we provide the strongest evidence yet that the hydrozone tautomer of SSY is the predominant form in aqueous solutions in the isotropic, nematic, and columnar phases, as well as what we believe to be the first measurements of (⟨P400⟩) for this system.

Theory and simulation of ovoidal disclination loops in nematic liquid crystals under conical confinement

We present analysis, scaling and modelling based on a previously presented nonlinear nonlocal nematic elastica equation of disclination loop growth in nematic liquid crystals confined to conical geometries with homeotropic anchoring conditions. The +1/2 disclination loops arise during the well-known planar radial to planar polar texture transformation and are attached to +1 singular core disclination at two branch points. The shape of the +1/2 loops is controlled by the axial speed of the branch points and the bending stiffness of the disclination both of which being affected by the confinement gradients (reduction in cross-sectional area) of a conical geometry. Motion towards the cone apex results in faster branch point motions and weaker curvature changes, but motion away from the apex results in slower branch point motion and stronger curvature changes. The simultaneous action of these effects results in novel ovoidal disclination loops. The numerical results are condensed into useful power laws and integrated into a shape/energy analysis that reveals the effects of confinement and its gradient on ovoidal disclination loops. These new findings are useful to characterise the Frank elasticity of new nematic mesophases and to predict novel defect structures under complex confinement.

Homeotropically aligning phase separated columnar structures for fabrication of flexible electrooptical devices

A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photo-curable pre-polymer is reported. Vertical polymer columns and micro-fibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LC-substrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electro-optical devices with high contrast coupled with fast response times. This simple single-step technique eliminates the need for the traditional polymer alignment layer pre-deposited on substrates.