Topological defects and geometric memory across the nematic-smectic A liquid crystal phase transition.

We study transformations of self-organised defect arrays at the nematic-smectic A liquid crystal phase transition, and show that these defect configurations are correlated, or "remembered", across the phase transition. A thin film of thermotropic liquid crystal is subjected to hybrid anchoring by an air interface and a water substrate, and viewed under polarised optical microscopy. Upon heating from smectic-A to nematic, a packing of focal conic domains melts into a dense array of boojums-nematic surface defects-which then coarsens by pair-annihilation. With the aid of Landau-de Gennes numerical modeling, we elucidate the topological and geometrical rules underlying this transformation. In the transition from nematic to smectic-A, we show that focal conic domain packings are organised over large scales in patterns that retain a geometric memory of the nematic boojum configuration, which can be recovered with remarkable fidelity.

Directed Self-Assembly of Topological Defects of Liquid Crystals.

One of the alluring aspects of liquid crystals (LCs) is their readily controllable self-assembly behavior, leading to comprehension of complex topological structures and practical patterning applications. Here, we report on manipulating various kinds of topological defects by adopting an imprinted polymer-based soft microchannel that simultaneously imposes adjustable surface anchoring, confinement, and uniaxial alignment. Distinctive molecular orientation could be achieved by varying the surface anchoring conditions at the sidewall polymer and the rubbing directions on the bottom layer. On this pioneering platform, a common LC material, 8CB (4'-n-octyl-4-cyano-biphenyl), was placed where various topological defect domains were generated in a periodic arrangement. The experimental results showed that our platform can change the packing behavior and even the shape of topological defects by varying the rubbing condition. We believe that this facile tool to modulate surface boundary conditions combined with topographic confinement can open a way to use LC materials in potential optical and patterning applications.

Creation of liquid-crystal periodic zigzags by surface treatment and thermal annealing.

The orientation control of soft matter to create a large area single domain is one of the most exciting research topics in materials science. Recently, this effort has been extended to fabricate two- or three-dimensional structures for electro-optical applications. Here, we create periodic zigzag structures in liquid crystals (LCs) using a combination of surface treatment and thermal annealing. The LC molecules in the nematic (N) phase were initially guided by the alignment layer of rubbed polymers, which were quenched and subsequently annealed in the smectic A (SmA) phase to create periodic zigzag structures that represent modulated layer structures. Direct investigation of the zigzags was performed using microscopy and diffraction techniques, showing the alternately arranged focal conic domains (FCDs) formed. The resulting macroscopic periodic structures will be of interest in further studies of the physical properties of soft matters.

Thermal phase transition behaviours of the blue phase of bent-core nematogen and chiral dopant mixtures under different boundary conditions.

We have investigated dramatic changes in the thermal phase transition of a liquid-crystalline (LC) blue phase (BP) consisting of bent-core nematogen and chiral dopants under various boundary conditions during cooling from the isotropic phase. Blue phase III, which has arbitrary morphological characteristics, between two glass plates goes to blue phase I, which has a body-centred cubic structure, when the sample is placed on the glass as a droplet form. These two distinctive phase transition behaviours were directly observed by polarized optical microscopy as well as selective reflection spectroscopy, in which the phase transition between BPs started from the air/LC interface. We determined that this transition resulted from the ordered double-twisted cylinders near the air-boundary by confocal laser-scanning microscopy.

Morphogenesis of liquid crystal topological defects during the nematic-smectic A phase transition.

The liquid crystalline phases of matter each possess distinct types of defects that have drawn great interest in areas such as topology, self-assembly and material micropatterning. However, relatively little is known about how defects in one liquid crystalline phase arise from defects or deformations in another phase upon crossing a phase transition. Here, we directly examine defects in the in situ thermal phase transition from nematic to smectic A in hybrid-aligned liquid crystal droplets on water substrates, using experimental, theoretical and numerical analyses. The hybrid-aligned nematic droplet spontaneously generates boojum defects. During cooling, toric focal conic domains arise through a sequence of morphological transformations involving nematic stripes and locally aligned focal conic domains. This simple experiment reveals a surprisingly complex pathway by which very different types of defects may be related across the nematic-smectic A phase transition, and presents new possibilities for controlled deformation and patterning of liquid crystals.

Switchable Photonic Crystals Using One-Dimensional Confined Liquid Crystals for Photonic Device Application.

Photonic crystals (PCs) have recently attracted considerable attention, with much effort devoted to photonic bandgap (PBG) control for varying the reflected color. Here, fabrication of a modulated one-dimensional (1D) anodic aluminum oxide (AAO) PC with a periodic porous structure is reported. The PBG of the fabricated PC can be reversibly changed by switching the ultraviolet (UV) light on/off. The AAO nanopores contain a mixture of photoresponsive liquid crystals (LCs) with irradiation-activated cis/trans photoisomerizable azobenzene. The resultant mixture of LCs in the porous AAO film exhibits a reversible PBG, depending on the cis/trans configuration of azobenzene molecules. The PBG switching is reliable over many cycles, suggesting that the fabricated device can be used in optical and photonic applications such as light modulators, smart windows, and sensors.

Orthogonal Liquid Crystal Alignment Layer: Templating Speed-Dependent Orientation of Chromonic Liquid Crystals.

Lyotropic chromonic liquid crystals (LCLCs) have been extensively studied because of the interesting structural characteristics of the linear aggregation of their plank-shaped molecules in aqueous solvents. We report a simple method to control the orientation of LCLCs such as Sunset Yellow (SSY), disodium cromoglycate (DSCG), and DNA by varying pulling speed of the top substrate and temperatures during shear flow induced experiment. Crystallized columns of LCLCs are aligned parallel and perpendicular to the shear direction, at fast and slow pulling speeds of the top substrate, respectively. On the basis of this result, we fabricated an orthogonally patterned film that can be used as an alignment layer for guiding rodlike liquid crystals (LCs) to generate both twisted and planar alignments simultaneously. Our resulting platform can provide a facile method to form multidirectional orientation of soft materials and biomaterials in a process of simple shearing and evaporation, which gives rise to potential patterning applications using LCLCs due to their unique structural characteristics.

Orientation Control of Smectic Liquid Crystals via a Combination Method of Topographic Patterning and In-Plane Electric Field Application for a Linearly Polarized Illuminator.

We dynamically controlled the configuration of layering structures built by smectic A liquid crystal molecules using the combination method of the microchannel confinement and the in-plane electric field to realize the linearly polarized illuminator and bistable structures. Once a mild in-plane electric field (∼30 V) is applied between polymeric walls, the layer configuration was changed from the toric focal conic domains to periodic zigzag patterns of alternatively packed focal conic domains. The transformed zigzag patterns maintained their structures even after turning off the applied electric fields, revealing the ability for use in a bistable memory device. Indeed, a strong electric field (∼100 V) can make unidirectionally aligned LC molecules along with the applied electric field via zigzag patterns, and electro-optical performance of resultant textures when the sample is mixed with fluorescent dyes was characterized to show a linearly polarized light illuminator. Our electric field in and on the confined geometries will be used in the fabrication of functional structures built by polar soft materials which can broaden applications in patterning platforms and efficient electro-optical devices in the near future.

Fast Fabrication of Sub-200-nm Nanogrooves Using Liquid Crystal Material.

Self-assembly of soft materials attracts keen interest for patterning applications owing to its ease and spontaneous behavior. We report the fabrication of nanogrooves using sublimation and recondensation of liquid crystal (LC) materials. First, well-aligned smectic LC structures are obtained on the micron-scale topographic patterns of the microchannel; then, the sublimation and recondensation process directly produces nanogrooves having sub-200-nm scale. The entire process can be completed in less than 30 min. After it is replicated using an ultraviolet-curable polymer, our platform can be used as an alignment layer to control other guest LC materials.

Highly Polarized Fluorescent Illumination Using Liquid Crystal Phase.

Liquid crystal (LC) materials are currently the dominant electronic materials in display technology because of the ease of control of molecular orientation using an electric field. However, this technology requires the fabrication of two polarizers to create operational displays, reducing light transmission efficiency below 10%. It is therefore desirable to develop new technologies to enhance the light efficiency while maintaining or improving other properties such as the modulation speed of the molecular orientation. Here we report a uniaxial-oriented B7 smectic liquid crystalline film, using fluorescent bent-core LC molecules, a chemically modified substrate, and an in-plane electric field. A LC droplet under homeotropic boundary conditions of air/LC as well as LC/substrate exhibits large focal conic like optical textures. The in-plane electric field induced uniaxial orientation of the LC molecules, in which molecular polar directors are aligned in the direction of the electric field. This highly oriented LC film exhibits linearly polarized luminescence and microsecond time-scale modulation characteristics. The resultant device is both cheap and easy to fabricate and thus has great potential for electro-optic applications, including LC displays, bioimaging systems, and optical communications.

Airflow-aligned helical nanofilament (B4) phase in topographic confinement.

We investigated a controlled helical nanofilament (HNF: B4) phase under topographic confinement with airflow that can induce a shear force and temperature gradient on the sample. The resulting orientation and ordering of the B4 phase in this combinational effort was directly investigated using microscopy. The structural freedom of the complex B7 phase, which is a higher temperature phase than the B4 phase, can result in relatively complex microscopic arrangements of HNFs compared with the B4 phase generated from the simple layer structure of the B2 phase. This interesting chiral/polar nanofilament behaviour offers new opportunities for further exploration of the exotic physical properties of the B4 phase.

In-Plane Switching Mode for Liquid Crystal Displays Using a DNA Alignment Layer.

We successfully fabricated the in-plane switching mode (IPS) LC display (LCD) based on a double stranded DNA (dsDNA) alignment layer. As widely known, the DNA has the right-handed double helical structure that has naturally grown grooves with a very regular period, which can be used as an alignment layer to control the orientation of liquid crystal (LC) molecules. The LC molecules on this topographical layer of DNA material align obliquely at a specific angle with respect to the direction of DNA chains, providing an instant and convenient tool for the fabrication of the IPS display compared to the conventional ways such as rubbing and mechanical shearing methods. The electro-optical performance and response time of this device were also investigated. Our result will be of great use in further exploration of the electro-optical properties of the other biomaterials.

Liquid-crystalline blue phase II system comprising a bent-core molecule with a wide stable temperature range.

A thermodynamically stable blue phase II (BPII) has been prepared, and its electrooptical (EO) performance has been evaluated in a host system of a conventional rodlike nematogen mixed with a bent-core molecule. For the mixed system presented, the widest temperature range of BPII stability, during cooling/heating, was >6 °C. This range is much wider than those of conventional nematogens blended with chiral dopants. EO observations show that the BPII produced exhibited stable EO performance based on the EO Kerr effect. The temperature dependence of the Kerr effect was found to be in approximate agreement with the Landau-de Gennes theory. Furthermore, this material demonstrated very fast, sub-millisecond-scale, response times, thus showing potential for use in high-speed EO devices.

Liquid-crystalline blue phase laser with widely tunable wavelength.

A lasing peak shift of more than 100 nm is realized due to the large shift of a photonic bandgap of a liquid-crystalline blue phase.

Photoisomerization-induced stable liquid crystalline cubic blue phase.

We have succeeded in obtaining a photoinduced liquid crystalline cubic BP in a mixture of photoinactive and photoresponsive bent-core mesogens. The UV stimulus can convert N* to cubic BP through photoisomerization of the azobenzene linkages included in the photoresponsive bent-core molecule.