Jung Ok Park

Structural Origin of Circularly Polarized Iridescence in Jeweled Beetles

Bright Shiny Beetles The beautiful iridescent colors found on the wings of butterflies and on the bodies of beetles have attracted the attention of brilliant minds over the past centuries, starting with Newton, who understood that these colors must involve “thin film structures.” In 1911 Michelson described the metallic appearance of these beetles, and in the late 1960s Neville and Caveney discussed the optical properties in the context of cholesteric liquid crystals. Sharma et al. (p. 449; see the Perspective by Vukusic) examined the metallic green beetle Chrysina gloriosa, which selectively reflects left circularly polarized light when illuminated with unpolarized light. The underlying cellular structure of the beetle exoskeleton is organized primarily in a hexagonal pattern, with variations in the pentagonal and heptagonal arrangements depending on the local curvature. Thus, the ordering of the cells in concentric, nested arcs is indeed analogous to the ordering of the molecules in a cholesteric (or chiral nematic) liquid crystal. The cellular ordering in the exoskeleton of a beetle is analogous to the molecular ordering in cholesteric liquid crystals. The iridescent metallic green beetle, Chrysina gloriosa, which selectively reflects left circularly polarized light, possesses an exoskeleton decorated by hexagonal cells (~10 μm) that coexist with pentagons and heptagons. The fraction of hexagons decreases with an increase in curvature. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in dark field. With use of confocal microscopy, we observe that these cells consist of nearly concentric nested arcs that lie on the surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. These textures provide the basis for the morphogenesis as well as key insights for emulating the intricate optical response of the exoskeleton of scarab beetles.

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.

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.

Characterization of ordered array of micropores in a polymer film

Microporous polymer films prepared by templating “breath figures” tend to have closely packed, hexagonal arrays of pores. We characterize the hexagonal order of the structured films using Fraunhofer diffraction patterns in reciprocal space, as well as with the use of Voronoi diagrams and bond-orientational correlation functions using real space images. The average spacing of the pores over a wider area can be calculated by two-dimensional Bragg equation, while the analysis of microscope images provides a direct measurement of the average open pore size and its distribution. The use of bond-orientational correlation function is particularly useful in quantifying the order of various films formed under various conditions. Further, we show how the sequential ordering of water drops formed during breath-figure-templated assembly process can also be characterized using Voronoi analysis and bond orientational correlation function.

Using chiral tactoids as optical probes to study the aggregation behavior of chromonics

Significance Confined liquid crystals occupy a sweet spot in their continued relevance to a host of fundamental studies as well as being exploited for many technological applications. We report on interesting phenomenology in a particularly exciting class of liquid crystals called chromonics by observing the director configurations in tactoids as the phase boundary is traversed. Unique chiral structures in chromonic tactoids are rationalized by appealing to the variation of the aggregate lengths as the concentration and temperature change. We arrive at an interesting conclusion that higher concentrations have shorter aggregates at the nematic–biphasic transition temperature. Our study opens up pathways to exploit this unique class of water-soluble liquid crystals for a host of potential applications while tuning their concentration and temperature. Tactoids are nuclei of an orientationally ordered nematic phase that emerge upon cooling the isotropic phase. In addition to providing a natural setting for exploring chromonics under confinement, we show that tactoids can also serve as optical probes to delineate the role of temperature and concentration in the aggregation behavior of chromonics. For high concentrations, we observe the commonly reported elongated bipolar tactoids. As the concentration is lowered, breaking of achiral symmetry in the director configuration is observed with a predominance of twisted bipolar tactoids. On further reduction of concentration, a remarkable transformation of the director configuration occurs, wherein it conforms to a unique splay-minimizing configuration. Based on a simple model, we arrive at an interesting result that lower concentrations have longer aggregates at the same reduced temperature. Hence, the splay deformation that scales linearly with the aggregate length becomes prohibitive for lower concentrations and is relieved via twist and bend deformations in this unique configuration. Raman scattering measurements of the order parameters independently verify the trend in aggregate lengths and provide a physical picture of the nematic–biphasic transition.

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.

Magnetic field induced instabilities in nematic solutions of polyhexylisocyanates

Abstract Magnetic field induced Freedericksz transition in the twist geometry of an initially well-aligned nematic solution of a rod-like polymer, polyhexylisocyanate (PHIC), is studied. For the angle α between the magnetic field and the unperturbed director n 0 , one finds a homogeneous distortion if this angle is less than 45°. For angles greater than 45°, the reorientation is inhomogeneous, and is coupled to secondary flow, giving rise to spatially periodic structures. The director deformation involves an out-of-plane tilt at short times, which manifests itself as a phase grating for transmitted polarized light. It is found that the wavelength of the instability is independent of the magnetic field strength in the experimental range, contrary to prior results for nematic solutions of polybenzylglutamate. We postulate that the insensitivity of the wavelength of the periodic structure is due to the nonflow-aligning character of PHIC solutions.

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.