Focal Conic Domains Research Articles

Thermally responsive microlens arrays fabricated with the use of defect arrays in a smectic liquid crystal

Defects in a smectic liquid crystal, toric focal conic domains (TFCDs), were developed for the fabrication of thermally responsive microlens arrays. In this system, each TFCD acts as a separate focusing unit via the intrinsic molecular orientation of the TFCDs, and the focusing ability of the microlens varies with local environmental temperature.

Towards tunable defect arrangements in smectic liquid crystal shells utilizing the nematic–smectic transition in hybrid-aligned geometries

We produce and investigate liquid crystal shells with hybrid alignment—planar at one boundary, homeotropic at the other—undergoing a transition between the nematic (N) and smectic-A (SmA) phases. The shells display a dynamic sequence of patterns, the details depending on the alignment agents and on the diameter and thickness of the shell. In shells of sufficient diameter we typically find a transient striped texture near the N–SmA transition, stabilising into a pattern of tiled, more or less regularly spaced focal conic domains in the SmA phase. The domain size and spacing decrease with reduced shell thickness. In case of strong homeotropic anchoring at one boundary and small shell size, however, the increased curvature favors homeotropic against planar alignment in the smectic phase, and the shell then tends to adapt to complete homeotropic alignment at the final stage of the transition. This is the first study of hybrid-aligned smectic shells and the results constitute a beautiful demonstration of the capacity for dynamic structure formation and reformation via self-assembly in soft matter. The new patterns extend the range of arrays of topological defects that can be realised with liquid crystals in spherical morphology and the correlation between the feature arrangements and the variable parameters of the shell and its environment opens a route towards tunability. However, the observed strong impact from increasing curvature, even for these rather large shells, indicates that the choice of alignment agents inducing planar or homeotropic alignment with varying strength will become critical when targeting the most attractive colloidal size scale of about a micron or smaller.

Pillar‐Assisted Epitaxial Assembly of Toric Focal Conic Domains of Smectic‐A Liquid Crystals

SU-8 pillar-assisted epitaxial assembly of toric focal conic domains (TFCDs) arrays of smectic-A liquid crystals is studied. The 3D nature of the pillar array is crucial to confine and direct the formation of TFCDs on the top of each pillar and between neighboring pillars, leading to highly ordered square and hexagonal array TFCDs. Excellent agreement between the experimentally obtained critical pillar diameter and elasticity calculation is found.

Elasticity of smectic liquid crystals with focal conic domains

We study the elastic properties of thermotropic smectic liquid crystals withfocal conic domains (FCDs). After the application of the controlled preshearat different temperatures, we independently measure the shear modulusG′ and theFCD size L. We find out that these quantities are related by the scaling relationG′≈γeff/L, whereγeff is the effective surface tension of the FCDs. The experimentally obtained value ofγeff shows the same scaling as the effective surface tension of the layered systems , where K and B are the bending modulus and the layer compression modulus, respectively. The similarity ofthis scaling relation to that of the surfactant onion phase suggests an universal rheologicalbehavior of the layered systems with defects.

Smectic Liquid Crystal Defects for Self‐Assembling of Building Blocks and Their Lithographic Applications

AbstractRecently, it has been reported that liquid crystal (LC) defects can be used to create highly periodic templates by controlling the surface anchoring and the elastic properties of LC molecules. The self‐assembled defect ordering of the LC materials takes advantage of the ability to achieve fast stabilization of molecular ordering and structure due to the reversible and non‐covalent interactions of the LC molecules. In this Featre Article, the defect structures of liquid crystalline materials will be demonstrated by the surface anchoring and elastic properties. A particular focus are the focal conic domains (FCDs) that are commonly observed in SmA liquid crystals and their lamellar lyotropic counterparts, which form periodic defect ordered structures. In addition, methodologies for creating lithographic templates from the defect order will be described. Finally, the review closes with a discussion of toric focal conic domain arrays that have been fabricated in this manner and used for various applications.

Influence of mesoscopic structures on single molecule dynamics in thin smectic liquid crystal films

Mesoscopic structures in liquids have an impact on the diffusion dynamics of the constituting molecules. Smectic 8CB liquid crystals on silicon wafers show the formation of mesoscopic structures on the μm scale at a film thickness of 200 nm. Depending on the kind of substrate (thermally grown or native SiOx), we observed the formation of focal conic domains (FCDs) and a new type of terraced holes, respectively. Dynamics are described via single perylene diimide tracer molecule tracking of translational diffusion and in the case of FCDs by a combination of translation and rotation detected viafluorescence correlation spectroscopy. Tailoring perylene diimide molecules such that the optical transition dipole moment follows the liquid crystal director allows mapping out FCDs and investigating the dynamics within a single FCD.

Highly ordered defect arrays of 8CB (4′-n-octyl-4-cyano-biphenyl) liquid crystal via template-assisted self-assembly

A synthetic rod-like smectic liquid crystal containing a rigid aromatic group and semi-fluorinated chains generates well-ordered toroidal structures called toric focal conic domains (TFCDs) as self-assembling building blocks when confined within rectangular micro-channels. However, in the case of common smectic liquid crystals with alkyl-terminated chains, the ordered arrangement of TFCDs is challenging work because of their high mobility and low elasticity. Here, we present a simple method to fabricate highly mono-dispersed periodic patterns using the most commonly used smectic liquid crystal, 8CB (4′-n-octyl-4-cyano-biphenyl), by changing the geometry of confined microchannel with a controlled surface polarity. This is attributed to the combining effects between antagonistic surface anchoring conditions and precisely directed molecular orientation onto inclined side walls of a trapezoidal channel. Furthermore, size and ordering of arrays of 8CB can be simply controlled by varying the feature dimension of bottom width (l) and depth (h) in the trapezoid-shaped channel. The use of 8CB will generalize fabrication of the spatial arrangement of TFCD arrays for future nanotechnology and liquid crystal research.

Controlling Liquid Crystal Alignment Using Photocleavable Cyanobiphenyl Self-Assembled Monolayers

We report on the development of novel cyano-biphenyl-based thiolate self-assembled monolayers designed to promote homeotropic alignment of calamitic liquid crystals. The molecules developed contain an ortho-nitrobenzyl protected carboxylic acid group that on irradiation by soft UV (365 nm) is cleaved to yield carboxylic acid groups exposed at the surface that promote planar alignment. Using a combination of wetting, X-ray photoelectron spectroscopy, Fourier transform-infrared reflection absorption spectroscopy, and ellipsometry we show that high photolysis yields (>90%) can be achieved and that the patterned SAMs are suitable for the controlled alignment of calamitic liquid crystals. This study further shows that such photo-patterned SAMs can be used to control the formation of focal conic domains (FCDs) in the smectic-A phase in terms of positioning and size confinement on surfaces.

Power of the Poincaré Group: Elucidating the Hidden Symmetries in Focal Conic Domains

Focal conic domains are typically the "smoking gun" by which smectic liquid crystalline phases are identified. The geometry of the equally spaced smectic layers is highly generic but, at the same time, difficult to work with. In this Letter we develop an approach to the study of focal sets in smectics which exploits a hidden Poincaré symmetry revealed only by viewing the smectic layers as projections from one-higher dimension. We use this perspective to shed light upon several classic focal conic textures, including the concentric cyclides of Dupin, polygonal textures, and tilt-grain boundaries.

Optically Selective Microlens Photomasks Using Self‐Assembled Smectic Liquid Crystal Defect Arrays

A photomask that combines two imaging elements (microlens arrays and clear windows) in one structure has been developed by using toric focal conic domain (TFCD) micro arrays of smectic liquid crystals. Their application as optically selective photomasks by simply adjusting (i) the illumination dose, (ii) the size of TFCD array mask, and (iii) the tone of photoresist (PR) is demonstrated (see figure).

20.4: The Scattering Profile of Polymer Stabilized Cholesteric Texture Displays

AbstractWe measured the scattering profile of the normal mode and reverse mode polymer stabilized cholesteric texture displays PSCT. We studied the effect of pitch and focal conic domain size on the scattering efficiency by varying the concentration of chiral dopant. We also calculate the contrast ratio as a function of collection angle. The data is useful for designing flexible reflective displays and switchable windows.

Fabrication of two-dimensional dimple and conical microlens arrays from a highly periodic toroidal-shaped liquid crystal defect array

A self-assembly fabrication method was developed for the preparation of microlens arrays (MLAs). The procedure used the focal conic structures of semi-fluorinated smectic liquid crystals (LCs), the periodic toric focal conic domains (TFCDs), which were prepared on a surface-modified substrate. This LC-based MLA system focuses light via the intrinsic molecular orientations of the TFCDs, leading to a highly efficient MLA with good optical properties. The thickness of the smectic LC film could be used to control both the microlens feature size, over the range 5–15 μm, and the microlens focal length, over the range 1–3 μm. In addition, we prepared two-dimensional hexagonally packed polymer MLAs with conical or dimple shapes by successive replica molding, using a UV-curable photopolymer (NOA63) and polydimethylsiloxane (PDMS), from a TFCD array template containing a dimple structure. The LC-based TFCD MLAs and the secondary replicated polymer MLAs, using NOA63 and PDMS molds, showed good lens performances. We anticipate that this LC self-assembly method will be applicable to the large-scale fabrication of MLAs. The method allows fabrication of dynamic MLAs that are responsive to external fields, such as electric or magnetic fields, or to thermal variations.

Self-assembled periodic liquid crystal defects array for soft lithographic template

We have developed highly periodic patterns with sub-micrometer features over large-areas using toric focal conic domains (TFCDs) originated from smectic liquid crystal (LC) as a new self-assembling building block. TFCDs are accomplished by precisely controlling the surface and interfacial properties of smectic LC. In order to apply the smectic liquid crystal defect arrays in soft lithography, the hexagonal arrays of domain patterns are used as molds for ultraviolet (UV) curable polymers, thereby providing LC defect stamps with high spatial resolution over large areas. Our method was further utilized to transfer patterns with sub-micrometer features from the polymer stamp surface to a secondary surface by microcontact printing (μ-CP). Accordingly, we show that such LC materials can be very strong candidates for the periodic templates, compared to other soft-building blocks such as block copolymers, colloids and surfactants. We anticipate our finding be a starting point for more sophisticated lithographic applications based on liquid crystalline materials.

The interplay between grain boundaries and disclinations in condensed matter physics

Abstract The concept of grain boundary, much studied for its relationship with dislocation motion in plastic deformation, is here developed and associated with that of disclination. After defining a disclination in terms of a Volterra process, we investigate the nature of the interplay between both, with application to two cases: i) in polycrystals the triple junctions result from the stress free merging of disclinations bordering the incident grain boundaries; the plastic properties of nanocrystals, which cannot be attributed directly to dislocations, are related to these disclinations; ii) in lamellar liquid crystalline phases grain boundaries are often the place not only of dislocations, but also of focal conic domains, a stunning disclination system described by G. Friedel one hundred years ago.

Fabrication of a Superhydrophobic Surface from a Smectic Liquid‐Crystal Defect Array

AbstractA novel fabrication method is developed for the preparation of superhydrophobic surfaces. The procedure uses focal conic structures of semi‐fluorinated smectic liquid crystals (LCs) whose periodic toric focal conic domains (TFCDs) are prepared on a surface modified substrate. Reactive ion etching (RIE) on the periodic TFCD surface leads to a superhydrophobic surface with a water contact angle of ∼160° and a sliding angle of ∼2° for a 10 µL water droplet. The results show that this phenomenon is due to the development of a dual‐scale surface roughness arising from the nanoscale protuberance caused by applying the RIE process to the top of the microscale TFCD arrays. The unique surface behavior is further verified by demonstrating that RIE on a flat lamellar liquid crystal film, in which the director is aligned parallel with surface, results in a relatively low hydrophobicity as compared to when periodic TFCDs are subjected to REI. The observations made in this publication suggest that a new approach exists for selecting potential candidates of superhydrophic surface formation based on spontaneous self‐assembly in smectic liquid‐crystalline materials.

Liquids with conics

A red thread marks the history of liquid crystals from the start; it is the concept of focal conic domains in smectic phases. Discovered by G. Friedel and F. Grandjean early in the beginning of the 20th century, focal conic domains still attract the attention of researchers in the field. The presence of focal conic domains is a touchstone of layered media. They rightly excited the interest of P.-G. de Gennes in a seminal paper about the Apollonius tiling of toric focal conic domains. In this paper we shall review the development of this notion over the last one hundred years, firstly emphasising its importance in the appearance and broadening of the concept of order and of its singularities in liquid crystals and beyond in condensed matter physics, through the renewed notions of dislocations, disclinations, grain boundaries, kinks and focal conic domain imperfections, then describing the empirical evidence of focal conic domains in various liquid crystalline materials, and finally how they have inspired research in visualisation methods and molecular architectures.

Structural Origin of Circularly Polarized Iridescence in Jeweled Beetles

The iridescent metallic green beetle, Chrysina gloriosa, which selectively reflects left circularly polarized light, possesses an exoskeleton decorated by hexagonal cells (approximately 10 microm) 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.

Reinvestigation of liquid crystalline property of dimesogenic compound consisting of cholesterol and cyanobiphenyl

The dimesogenic compound consisting of cholesterol and cyanobiphenyl mesogens interlinked by ω-oxyalkanoyl spacer was synthesized. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). The experimental results indicated that this compound exhibited mesophase over a much wider temperature range and a new mesophase blue phase (BP). Focal conic domains (FCDs) and droplets texture to the smectic A phase ( S A), oily streaks typical to the chiral nematic ( N*) phase were found in the liquid crystalline state, respectively. Besides, it exhibited a smectic layer spacing about 1.0 time to the length of the molecule, which indicated a monolayer type of S A ordering.

Influence of phase sequence on focal conic domains in smectic films

We present a study of smectic focal conic domains in films of various liquid-crystal compounds on a solid substrate. Focal conic domains are generated as a result of the antagonistic anchoring conditions of the film surfaces, random planar at the substrate interface and homeotropic at the air interface. The curved arrangement of the smectic layers in focal conic domains leads to a depression in the film/air interface above each domain. Using atomic force microscopy, we determine the temperature dependence of the depth of the depressions in different smectic phases and at different phase transitions. In most cases, our results are not in accordance with the assumption of strictly incompressible smectic layers, and we discuss the observed behavior with respect to the known results on the layer compressibility modulus B .

Focal Conic Stacking in Smectic A Liquid Crystals: Smectic Flower and Apollonius Tiling

We investigate two different textures of smectic A liquid crystals. These textures are particularly symmetric when they are observed at crossed polars optical microscopy. For both textures, a model has been made in order to examine the link between the defective macroscopic texture and the microscopic disposition of the layers. We present in particular in the case of some hexagonal tiling of circles (similar to the Apollonius tiling) some numeric simulation in order to visualize the smectic layers. We discuss of the nature of the smectic layers, which permit to assure their continuity from one focal conic domain to another adjacent one.