Oily Streaks Research Articles

Recyclable Cholesteric Phase Liquid Crystal Device for Detecting Storage Temperature Failure.

The planar state of a cholesteric liquid crystal (CLC) often exhibits oily streak defects, which negatively impact the characteristics of precision optics, including transmission and selective reflection. In this paper, we introduced polymerizable monomers into liquid crystals and examined the effects of monomer concentration, polymerization light intensity, and chiral dopant concentration on oily streak defects in CLC. With the proposed method of heating cholesteric liquid crystals to the isotropic phase followed by rapid cooling, the oil streak defects presented in the liquid crystal can be successfully eliminated. Furthermore, a stable focal conic state can be obtained by a slow cooling process. Two stable states with different optical properties can be obtained based on the cholesteric liquid crystal at different cooling rates, which makes it possible to detect whether the stored procedure of temperature-sensitive material is qualified. These findings have widespread applications in devices that require a planar state without oily streak defects and temperature-sensitive detection devices.

Preparation and applications of the epoxy resin-based polymer- stabilised cholesteric liquid crystal films with microcracks at the surface

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films attracted much attention in decoration and anti-counterfeiting. Herein, the PSCLC films were prepared using the mixtures of epoxy monomers through cationic photopolymerisation. The structural colour was tunable by changing the concentration of chiral dopant in the reaction mixture and polymerisation temperature. Due to polymerisation shrinkage, microcracks were formed at the surfaces of the PSCLC films, which originated from the oily streak defects of the CLC mixtures. Colourful patterns were prepared by controlling the polymerisation temperature. In addition, since light can leak out through the microcracks, the tetraphenylethylene-doped patterns can be applied in anti-counterfeiting.

Synthesis and characterisation of novel aromatic ester swallow-tailed cholesteric liquid crystal polymers

ABSTRACT In this paper, two novel aromatic ester swallow-tailed liquid crystal monomers M1 and M2 and a straight chain cholesteric liquid crystal monomer M3 were designed and synthesised. Two series of liquid crystal polymers, P1 series and P2 series, were graft-copolymerised with M and polymethylhydrosiloxane (PHMS). All the synthesised liquid crystal monomers and polymers were characterised by IR and 1H NMR and confirmed to the predicted molecular structure. The liquid crystal and thermal properties of the synthesised compounds were tested by DSC, POM, TGA and optical rotation. The results show that the monomer M1 is a monotropic nematic liquid crystal, and the Schlieren texture appears during the cooling process. M2 is an enantiotropic nematic liquid crystal with filamentous and schlieren texture. M3 is a thermotropic enantiotropic cholesteric phase with oily streak and focal cone texture. Both polymers P1 and P2 show typical Grand-Jean texture, and with the increase of monomer M3 content, the polymer clearing point temperature Ti increases continuously, the liquid crystal mesophase range widens, and the specific rotation value increases.

Energetics of topographically designed Smectic-A oily streaks.

A thin Smectic-A liquid crystal (LC) film is deposited on a polymer vinyl alcohol-coated substrate that had been scribed with a uniform easy axis pattern over a square of side length L ≤ 85 μm. The small size of the patterned region facilitates material distribution to form either a hill (for a thin film) or divot (for a thick film) above the scribed square and having an oily streak (OS) texture. Optical profilometry measurements vs. film thickness suggest that the OS structure aims to adopt a preferred thickness z0 that depends on the nature of the molecule, the temperature, and the surface tension at the air interface. We present a phenomenological model that estimates the energy cost of the OS layer as its thickness deviates from z0.

Fast-relaxation, dye-doped cholesteric liquid-crystal smart window with a perfect planar state

A fast-relaxation dye-doped cholesteric liquid crystal (DDCLC) smart window without any defects is prepared by the doping of a low bend elastic constants liquid crystal dimer 1″,7″-bis-(4-cyanobiphenyl-4′-yl)heptane (CB7CB). The ratio of the bend elastic constant to the twist elastic constant of the liquid crystals (LCs) is adjusted so that it equates to unity. The transient planar (TP) state and the planar (P) state are combined, so the relaxation time from the homeotropic (H) state to the P state is greatly. Moreover, the oily streaks of the P state are removed and the light scattering is lessened. The polarisation-independent DDCLC smart window switches between three states: the haze-free dark state, the high-haze dark state and the transparent state. The smart window can be used in situations where absorbed sunlight reduces room temperature and protects privacy.

Photo‐Actuated Chiral Smectic Superstructures

AbstractChirality and hierarchy are two representative features of complex configurations inspiring exotic functions. Chiral additives are adopted to break the mirror symmetry of molecular orientation inside smectic layers, and overcome the restriction on in‐layer helical arrangement by virtue of the surface electroclinic effect. The chirality strength of liquid crystals is regulated via the photo‐isomerization of chiral molecular switch, the doping concentration control, and the mixing of different chiral additives. The dynamic evolution of chiral smectic superstructures leads to the reversible rotation of oily streaks (OSs), and even structural transformation between OSs and focal conic domains is demonstrated. This releases new freedom of chiral hierarchical configurations and may inspire fantastic applications in active optics, sensing, and micro‐manufacturing.

Experimental and DFT Studies on Well-Defined Odd-Even Effect in Organic Liquid Crystal Tetramers Incorporating Two Chiral Centers

Introduction: A new series of organic liquid crystal tetramers containing two symmetrical mesogenic units of 4-(4’-phenoxylimino)methyl)phenol joined by methylene –(CH2)mspacer wherein m varied from 5 to 10 have been synthesized. Methods: The length for each of the mesogenic units was extended further via connection with two chiral entities of (S)-2- methylbutyl-4-(4’-bromobutyloxyphenyl)benzoate situated at both terminal ends. The structures of these tetramers were elucidated using elemental analysis and spectroscopic techniques (FT-IR and 1H-NMR) whilst the texture, and the phase transition temperatures were studied using polarizing optical microscopy (POM) along with differential scanning calorimetry (DSC). Results: All the homologues in this series are enantiotropic mesogens exhibiting oily streak texture or fan-shaped texture characteristic of the chiral nematic phase. The odd-even effect can be observed upon increasing the methylene spacer length. The structure-properties connectivity has demonstrated that the phase transition temperature for tetramers with even-numbered methylene units in the spacer is relatively higher in comparison with the odd-numbered members. Conclusion: The computational study via molecular constraint dynamics performed at the DFTB level of theory has further supported that more energy is required for the even-numbered tetramer than the odd-numbered member to undergo the structural transition from folded to extended geometries.

Effect of end groups with aliphatic chain or ring structures on the thermal properties and optical textures of achiral and chiral liquid crystal compounds

Thermal properties and orientation in optical textures of liquid crystal (LC) are two crucial factors for LC application. Here, a series of achiral and chiral LC compounds (Cn) with different end groups were prepared. The chemical structures were investigated by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR). The thermal properties and optical textures were studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD). The results showed that all the compounds possessed good thermal properties and enantiotropic mesomorphism. For achiral compounds C1C4, droplets and schlieren textures of nematic phase (N) and broken fan-shape textures and dull grayish/cloudy textures of smectic phase (Sc) were seen. Oily streak and focal conic textures of chiral nematic phase (N*), fan-shape textures and dull grayish/cloudy textures of chiral smectic phase (Sc*) were observed for chiral compounds C5C7. For C1, C2, C3 and C5 with aliphatic chain in the end groups, it was found that lateral substitution in the end groups had a greater effect on the clearing point than that of the increase in the length of end groups. Moreover, due to the increasing free volume for the increase of branch chain on six-membered ring structure, the clearing point (Ti) of C4 was higher than those of C6 and C7.

Structural Landscapes in Geometrically Frustrated Smectics.

A phenomenological free energy model is proposed to describe the behavior of smectic liquid crystals, an intermediate phase that exhibits orientational order and layering at the molecular scale. Advantageous properties render the functional amenable to numerical simulation. The model is applied to a number of scenarios involving geometric frustration, leading to emergent structures such as focal conic domains and oily streaks and enabling detailed elucidation of the very rich energy landscapes that arise in these problems.

Thermal and optical properties of amphitropic liquid crystals derived from cholesterol and cinnamic acid

ABSTRACT In this paper, we firstly demonstrate the synthesis of chiral amphitropic liquid crystals derived from cholesterol as well as their thermal and optical properties. Chiral monomeric cholesteryl-4-(6-acryloyloxyhexyloxy) cinnamate (CCM) derived from cholesterol and cinnamic acid has been synthesised showing amphitropic liquid crystal phases. The synthesised CCM exhibits oily streak textures under a polarised optical microscope (POM), corresponding to cholesteric liquid crystal phase formed after heating over 120°C and after being treated with organic solvents such as dichloromethane, chloroform, tetrahydrofuran and toluene. Both the thermotropic and lyotropic cholesteric liquid crystals exhibit selective light reflection, and the wavelength of the reflected light falls in the visible light region showing colourful appearances. The results suggest that CCM could be used for the development of sensors for both temperature and solvents. Homopolymer (Poly-CCM) was synthesised via free radical polymerisation. The synthesised polymer did not exhibit any mesomorphic behaviour either by heating or being treated with solvents. These results indicate that the long chain hydrocarbons and sidechains of CCM were disturbed and restricted after polymerisation leading to the loss of liquid crystalline properties. However, cholesteric construction of monomeric CCM was fixed via UV-polymerisation. The synthesised Poly-CCM film shows Bragg reflection significantly.

Smectic Defect Engineering Enabled by Programmable Photoalignment

AbstractTopological defects are vital for tailoring soft matter properties and inspiring remarkable applications. Arbitrary guiding and dynamic tuning of director distributions are highly pursued in defect engineering of liquid crystals. Till now, the orientation control of smectic defect walls remains a challenge. Here, photoalignment is adopted to preset the surface anchoring in order to guide smectic oily streaks. Flexible defect engineering such as deflecting, bending, and splaying is demonstrated. Based on their combination, more complicated defect arrays are realized in a predictable manner. After electric stimuli involved, new functions of tunability and rotatability are unlocked. This work brings new insights to soft matter architectures, and will upgrade the existing micromachines, nanoparticle manipulations and advanced photonic applications.

Novel cholesterol-based liquid crystalline dimers: the α-cyanostilbene group served as a photofluorophore and mesogenic moiety

ABSTRACT Cyanostilbene units are vigorous candidates for chromophores and mesogens. Therefore, a series of dimer liquid crystals based on cyanostilbene and cholesterol were synthesised and characterised. The structure conforms to the molecular design and its physicochemical properties are stable. With the growth of the inner spacer flexible carbon chain, the mesomorphic properties are greatly different: not only the odd–even effect of the clearing points (/clearing enthalpies) is caused but also the samples exhibit increasingly complex enantiotropic mesophases, including SmC*, SmA and N* phases. Corresponding textures of these phases were observed, such as polygonal textures, focal-conic textures with dechiralisation lines and oily streaks textures, etc., and phase structures were further verified using variable-temperature XRD. Through the combined use of UV and fluorescence emission spectrometers, it was found that the increase of water content caused the enhancement of fluorescence intensities and the redshift of UV absorptions, which confirmed the fascinating aggregation-induced emission enhancement (AIEE) effect of the ChM-CN molecules.

Synthesis, mesomorphic and photo-switching behaviours of novel azobenzene chiral liquid crystals containing (-)-menthyl

ABSTRACT A new series of chiral liquid crystal, Mt4AZOnAB (n = 2–18), containing a multi-chiral centre menthyl and an azobenzene moiety connected with a flexible butanedioyloxy and a varying length of alkanoyloxy in a terminal chain were designed and synthesised. The thermal properties and optical textures were investigated by differential scanning calorimetry and polarising optical microscopy. Selective reflection of the CLCs was measured by ultraviolet/visible spectrometer. Their photo-switching properties were well-demonstrated by UV irradiation. All CLCs show characteristic of selective reflection in heating stage and BPI/II texture in cooling course. Before and after irradiation, CLCs undergoes a transition from oily streak texture to focal conic texture; however, BPI/II texture in cooling stage shows a reversible phase transition.

From Chains to Monolayers: Nanoparticle Assembly Driven by Smectic Topological Defects.

In this Letter, we show how advanced hierarchical structures of topological defects in the so-called smectic oily streaks can be used to sequentially transfer their geometrical features to gold nanospheres. We use two kinds of topological defects, 1D dislocations and 2D ribbon-like topological defects. The large trapping efficiency of the smectic dislocation cores not only surpasses that of the elastically distorted zones around the cores but also surpasses the one of the 2D ribbon-like topological defect. This enables the formation of a large number of aligned NP chains within the dislocation cores that can be quasi-fully filled without any significant aggregation outside of the cores. When the NP concentration is large enough to entirely fill the dislocation cores, the LC confinement varies from 1D to 2D. We demonstrate that the 2D topological defect cores induce a confinement that leads to planar hexagonal networks of NPs. We then draw the phase diagram driven by NP concentration, associated with the sequential confinements induced by these two kinds of topological defects. Owing to the excellent large-scale order of these defect cores, not only the NP chains but also the NP hexagonal networks can be oriented along the desired direction, suggesting a possible new route for the creation of either 1D or 2D highly anisotropic NP networks. In addition, these results open rich perspectives based on the possible creation of coexisting NP assemblies of different kinds, localized in different confining areas of a same smectic film that would thus interact thanks to their proximity but also would interact via the surrounding soft matter matrix.

Defect formation in N* LCs via photovoltaic fields: impact of surface treatment

ABSTRACTField-induced defect creation was investigated in chiral nematic liquid crystal test cells, where a Fe-doped lithium niobate thin-cut was applied as field generating substrate. These photo responsive test cells were exposed to focused continuous wave laser light, which resulted in the formation of localised defects. The covering indium tin oxide coated glass of the test cells was treated with a polyvinyl alcohol alignment layer. This alignment layer was either applied with a doctor blade technique or by spin casting of a polyvinyl solution, which was thermally annealed, subsequently. The surface roughness of the alignment layers was investigated with atomic force microscopy, which was interesting to understand how defects and locally lying helices were supported or suppressed in various samples. Well-confined circular defects were formed in test cells with spin-casted alignment layers and the appearance and unwanted interaction with oily streaks could be efficiently reduced in these samples. The results are promising. Samples with an oily streak-free homogenous standing helix orientation of the chiral nematic liquid crystal are an ideal canvas for rewritable pattern formation in a chiral nematic LC: Such samples have high potential as reconfigurable diffractive elements such as rewritable diffraction gratings.

The effect of various functional groups on mesophase behavior and optical property of blue phase liquid crystal compounds based on (−)‑menthol

In order to explore the effect of various functional groups on the mesomorphic properties, a series of menthol-based blue phase liquid crystal compounds (A, B1, B2, C1, C2, D1, D2) with various core structures in the molecules were synthesized. Their chemical structures and phase behavior were characterized with FT-IR, 1H NMR, differential scanning calorimeter (DSC) and polarizing optical microscopy (POM). The selective reflection and photoisomerization were investigated with ultraviolet/visible (UV/VIS) spectrometer. In our case, all BPLCs showed oily streak texture with selectively reflection and blue phase (BP), irrespective of the structure of mesogenic cores. This demonstrated that (−)‑menthol had high chirality, and via succinyloxy spacer group could drive molecules to arrangement in double twist cylinders of BP. The length-to-diameter ratio, the bridge bond in the aryl rings and the conjugate action in the mesogenic core had a significant effect on Tm, Ti and temperature range of BP. BPLC A without conjugated structure in three-benzene mesogenic core revealed a larger temperature range of BP than other BPLCs with conjugated structure.

Synthesis and characterization of new chiral liquid crystal monomers containing steroid unit

To reduce the bulky steric hindrance, improve the reactivity of diosgenin and cholesterol, and obtain mesophase of their derivatives, the commercially available cholesterol and diosgenin were allowed to be structurally modified. Four new chiral LC intermediate compounds (c∼f) and the corresponding monomers (M1−M4) with different longer spacer were synthesized. The chemical structures, optical texture, thermal behavior and mesophase structure of all the mesogenic compounds obtained in this study were characterized by FT-IR, 1H-NMR, polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction measurements. The experimental results showed that the intermediate compounds containing diosgenyl groups c and d only showed a mesophase and exhibited fan-shaped texture of a smectic A (SA) phase, while the compounds containing terminal cholesteryl groups e and f showed two mesophases and exhibited fan-shaped texture of a SA phase, and oily streak texture and focal conic texture of cholesteric phase, respectively. The four chiral monomers M1−M4 all revealed cholesteric oily streak texture and focal conic texture. In addition, the melting temperature (Tm) and isotropic temperature (Ti) of the mesogenic compounds decreased with increasing the flexible spacer length. Compared with the intermediate compounds or monomers based on cholesterol, the compounds or monomers based on diosgenin showed higher Tm and Ti.

FORMATION OF THE DISCLINATION DEFECTS IN THE DYE-DOPED CHOLESTERIC LIQUID CRYSTALLINE STRUCTURE UNDER THE INFLUENCE OF THE LASER BEAM

In the work the influence of the laser beam on the optical properties of highly absorbing dye-doped cholesteric liquid crystalline (CLC) structure is investigated. By means of polarizing microscope studies step-like variation of the CLC helical pitch and formation of the disclination defect lines (oily streaks) is observed.

Observations of a streak texture in the hybrid-aligned smectic-C phase.

A novel structure was observed below the smectic-A-smectic-C phase transition in a very thin open cell having an air interface above and enforced planar anchoring at the substrate below. The structure appears as periodic dark and light streaks running perpendicular to the oily streaks, which are present in the smectic-A phase [D. Coursault et al., Soft Matter, 2016, 12, 678]. These new streaks, which we call "soapy streaks", form by extending from one oily streak to the next in discrete steps, eliminating optical evidence at visible wavelengths of the oily streaks. At lower temperatures the streaks can undulate and exhibit a sawtooth-like structure; such a structure is chiral in two dimensions. A possible scenario for the origin of these streaks is presented.

Instabilities in the electric Freedericksz state of the twist-bend nematic liquid crystal CB7CB.

We report on the instabilities in the Freedericksz state of the twist-bend nematic (NTB) liquid crystal 1'',7''-bis(4-cyanobiphenyl-4'-yl)heptane (CB7CB). The quasi homeotropic NTB state, into which a planar (untwisted or 90°-twisted) nematic CB7CB layer transits under a strong electric field, is found to be unstable despite the material being dielectrically positive. Close to the NTB melting point, destabilization occurs through the formation of metastable toric focal conic domains (TFCDs) that, in time, transform into parabolic focal conic domains (PFCDs) with the confocal parabolae in vertical planes through the layer normal. This transformation occurs by a novel process of continued dissociation of TFCDs. We outline how the extended Volterra process could help in a general appreciation of focal conic defects in the NTB phase. At relatively lower temperatures, stripes develop competingly with TFCDs. Identifiable as oily streaks, they are both localized and polarity sensitive; they form close to the substrates; and in low frequency square wave fields, they get suppressed at the cathode and augmented at the anode at each polarity switch. The study also dwells on the N-NTB-N sandwich region, found between the N and NTB states under a small temperature gradient.