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Zeitschriftenartikel zum Thema „Nematic phases“

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Veröffentlicht am 18. Mai 2024

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1

Samulski, Edward T., Denisse Reyes-Arango, Alexandros G. Vanakaras und Demetri J. Photinos. „All Structures Great and Small: Nanoscale Modulations in Nematic Liquid Crystals“. Nanomaterials 12, Nr. 1 (29.12.2021): 93. http://dx.doi.org/10.3390/nano12010093.

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The nature of the nanoscale structural organization in modulated nematic phases formed by molecules having a nonlinear molecular architecture is a central issue in contemporary liquid crystal research. Nevertheless, the elucidation of the molecular organization is incomplete and poorly understood. One attempt to explain nanoscale phenomena merely “shrinks down” established macroscopic continuum elasticity modeling. That explanation initially (and mistakenly) identified the low temperature nematic phase (NX), first observed in symmetric mesogenic dimers of the CB-n-CB series with an odd number of methylene spacers (n), as a twist–bend nematic (NTB). We show that the NX is unrelated to any of the elastic deformations (bend, splay, twist) stipulated by the continuum elasticity theory of nematics. Results from molecular theory and computer simulations are used to illuminate the local symmetry and physical origins of the nanoscale modulations in the NX phase, a spontaneously chiral and locally polar nematic. We emphasize and contrast the differences between the NX and theoretically conceivable nematics exhibiting spontaneous modulations of the elastic modes by presenting a coherent formulation of one-dimensionally modulated nematics based on the Frank–Oseen elasticity theory. The conditions for the appearance of nematic phases presenting true elastic modulations of the twist–bend, splay–bend, etc., combinations are discussed and shown to clearly exclude identifications with the nanoscale-modulated nematics observed experimentally, e.g., the NX phase. The latter modulation derives from packing constraints associated with nonlinear molecules—a chiral, locally-polar structural organization indicative of a new type of nematic phase.

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2

Dingemans, T. J., L. A. Madsen, N. A. Zafiropoulos, Wenbin Lin und E. T. Samulski. „Uniaxial and biaxial nematic liquid crystals“. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 364, Nr. 1847 (21.08.2006): 2681–96. http://dx.doi.org/10.1098/rsta.2006.1846.

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The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para -quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.

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3

Tschierske, Carsten, und Demetri J. Photinos. „Biaxial nematic phases“. Journal of Materials Chemistry 20, Nr. 21 (2010): 4263. http://dx.doi.org/10.1039/b924810b.

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4

Akpinar, Erol, und Antônio Figueiredo Neto. „Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic Mesophase“. Crystals 9, Nr. 3 (19.03.2019): 158. http://dx.doi.org/10.3390/cryst9030158.

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Nematic phases are some of the most common phases among the lyotropic liquid crystalline structures. They have been widely investigated during last decades. In early studies, two uniaxial nematic phases (discotic, ND, and calamitic, NC) were identified. After the discovery of the third one, named biaxial nematic phase (NB) in 1980, however, some controversies in the stability of biaxial nematic phases began and still continue in the literature. From the theoretical point of view, the existence of a biaxial nematic phase is well established. This review aims to bring information about the historical development of those phases considering the early studies and then summarize the recent studies on how to stabilize different nematic phases from the experimental conditions, especially, choosing the suitable constituents of lyotropic mixtures.

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5

Fradkin, Eduardo, und Steven A. Kivelson. „Electron Nematic Phases Proliferate“. Science 327, Nr. 5962 (07.01.2010): 155–56. http://dx.doi.org/10.1126/science.1183464.

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6

Shanker, Govindaswamy, Marko Prehm und Carsten Tschierske. „Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit“. Beilstein Journal of Organic Chemistry 8 (30.03.2012): 472–85. http://dx.doi.org/10.3762/bjoc.8.54.

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Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic phases (NcybA and NcybC), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and cybotactic nematic phases (SmC, NcybC), which are replaced by the nontilted CybA phases and nematic phases composed of SmA-type clusters (NcybA). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving biaxial nematic phases of the orthorhombic type. The orthogonal (NcybA) and skewed (NcybC) cybotactic nematic phases were distinguished by XRD and optical observations.

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7

Wang, Weiqiang, und Rui Zhang. „Interplay of Active Stress and Driven Flow in Self-Assembled, Tumbling Active Nematics“. Crystals 11, Nr. 9 (04.09.2021): 1071. http://dx.doi.org/10.3390/cryst11091071.

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Lyotropic chromonic liquid crystals (LCLCs) are a special type of hierarchical material in which self-assembled molecular aggregates are responsible for the formation of liquid crystal phases. Thanks to its unusual material properties and bio compatibility, it has found wide applications including the formation of active nematic liquid crystals. Recent experiments have uncovered tumbling character of certain LCLCs. However, how tumbling behavior modifies structure and flow in driven and active nematics is poorly understood. Here, we rely on continuum simulation to study the interplay of extensile active stress and externally driven flow in a flow-tumbling nematic with a low twist modulus to mimic nematic LCLCs. We find that a spontaneous transverse flow can be developed in a flow-tumbling active nematic confined to a hybrid alignment cell when it is in log-rolling mode at sufficiently high activities. The orientation of the total spontaneous flow is tunable by tuning the active stress. We further show that activity can suppress pressure-driven flow of a flow-tumbling nematic in a planar-anchoring cell but can also promote a transition of the director field under a pressure gradient in a homeotropic-anchoring cell. Remarkably, we demonstrate that the frequency of unsteady director dynamics in a tumbling nematic under Couette flow is invariant against active stress when below a threshold activity but exhibits a discontinuous increase when above the threshold at which a complex, periodic spatiotemporal director pattern emerges. Taken together, our simulations reveal qualitative differences between flow-tumbling and flow-aligning active nematics and suggest potential applications of tumbling nematics in microfluidics.

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8

Ravnik, Miha, und Jun-ichi Fukuda. „Templated blue phases“. Soft Matter 11, Nr. 43 (2015): 8417–25. http://dx.doi.org/10.1039/c5sm01878a.

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We explore the templated blue phases I and II infiltrated with an achiral nematic liquid crystal using numerical modelling, demonstrating novel blue-phase like profiles and predicting a large optical Kerr effect.

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Yu, Gary, und Mark Richard Wilson. „All-atom simulations of bent liquid crystal dimers: the twist-bend nematic phase and insights into conformational chirality“. Soft Matter 18, Nr. 15 (2022): 3087–96. http://dx.doi.org/10.1039/d2sm00291d.

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Atomistic simulations of the liquid crystal dimer, CB7CB, identify the twist-bend nematic phase, show phase transitions to the nematic and isotropic phases, and probe the molecular structure and conformational chirality of molecules in these phases.

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10

Urban, S., H. Kresse und R. Dąbrowski. „Low Frequency Dielectric Relaxation Process in Liquid Crystals with Nematic and Liquid-Like Smectic Phases“. Zeitschrift für Naturforschung A 52, Nr. 5 (01.05.1997): 403–8. http://dx.doi.org/10.1515/zna-1997-0505.

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Abstract Results of dielectric studies of the low frequency relaxation process in several substances exhibiting nematic -smectic C polymorphism are presented. They are compared with the data obtained re-cently for substances with nematic-smectic A and nematic-smectic B phase sequences. It was found that the rate of molecular reorientation around the short axes does not change at the transition between the nematic and a liquid-like smectic phase (Sm A, Sm C), whereas it is considerably retarded at that of the nematic-solid-like smectic (Sm B) phase. However, the activation barrier for this motion is markedly lower in the orthogonal smectics (Sm A, Sm B) than in the nematic and Sm C phases. The analysis of numerous experimental data on the activation enthalpy in the nematic phase shows that the Arrhenius equation conforms better with results than the Diogo-Martins approach, if the nematic range exceeds 10 K.

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11

Milchev, Andrey, Sergei A. Egorov, Jiarul Midya, Kurt Binder und Arash Nikoubashman. „Blends of Semiflexible Polymers: Interplay of Nematic Order and Phase Separation“. Polymers 13, Nr. 14 (11.07.2021): 2270. http://dx.doi.org/10.3390/polym13142270.

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Mixtures of semiflexible polymers with a mismatch in either their persistence lengths or their contour lengths are studied by Density Functional Theory and Molecular Dynamics simulation. Considering lyotropic solutions under good solvent conditions, the mole fraction and pressure is systematically varied for several cases of bending stiffness κ (the normalized persistence length) and chain length N. For binary mixtures with different chain length (i.e., NA=16, NB=32 or 64) but the same stiffness, isotropic-nematic phase coexistence is studied. For mixtures with the same chain length (N=32) and large stiffness disparity (κB/κA=4.9 to 8), both isotropic-nematic and nematic-nematic unmixing occur. It is found that the phase diagrams may exhibit a triple point or a nematic-nematic critical point, and that coexisting phases differ appreciably in their monomer densities. The properties of the two types of chains (nematic order parameters, chain radii, etc.) in the various phases are studied in detail, and predictions on the (anisotropic) critical behavior near the critical point of nematic-nematic unmixing are made.

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12

Du, Lingjie, Ursula Wurstbauer, Ken W. West, Loren N. Pfeiffer, Saeed Fallahi, Geoff C. Gardner, Michael J. Manfra und Aron Pinczuk. „Observation of new plasmons in the fractional quantum Hall effect: Interplay of topological and nematic orders“. Science Advances 5, Nr. 3 (März 2019): eaav3407. http://dx.doi.org/10.1126/sciadv.aav3407.

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Collective modes of exotic quantum fluids reveal underlying physical mechanisms responsible for emergent quantum states. We observe unexpected new collective modes in the fractional quantum Hall (FQH) regime: intra–Landau-level plasmons measured by resonant inelastic light scattering. The plasmons herald rotational-symmetry-breaking (nematic) phases in the second Landau level and uncover the nature of long-range translational invariance in these phases. The intricate dependence of plasmon features on filling factor provides insights on interplays between topological quantum Hall order and nematic electronic liquid crystal phases. A marked intensity minimum in the plasmon spectrum at Landau level filling factor v = 5/2 strongly suggests that this paired state, which may support non-Abelian excitations, overwhelms competing nematic phases, unveiling the robustness of the 5/2 superfluid state for small tilt angles. At v = 7/3, a sharp and strong plasmon peak that links to emerging macroscopic coherence supports the proposed model of a FQH nematic state.

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13

Bailly-Reyre, Aurélien, und Hung T. Diep. „Nematic and Smectic Phases: Dynamics and Phase Transition“. Symmetry 12, Nr. 9 (22.09.2020): 1574. http://dx.doi.org/10.3390/sym12091574.

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We study in this paper the dynamics of molecules leading to the formation of nematic and smectic phases using a mobile 6-state Potts spin model with Monte Carlo simulation. Each Potts state represents a molecular orientation. We show that, with the choice of an appropriate microscopic Hamiltonian describing the interaction between individual molecules modeled by 6-state Potts spins, we obtain the structure of the smectic phase by cooling the molecules from the isotropic phase to low temperatures: molecules are ordered in independent equidistant layers. The isotropic-smectic phase transition is found to have a first-order character. The nematic phase is also obtained with the choice of another microscopic Hamiltonian. The isotropic-nematic phase transition is a second-order one. The real-time dynamics of the molecules leading to the liquid-crystal ordering in each case is shown by a video.

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14

Aouini, Abir, Maurizio Nobili, Edouard Chauveau, Philippe Dieudonné-George, Gauthier Damême, Daniel Stoenescu, Ivan Dozov und Christophe Blanc. „Chemical-Physical Characterization of a Binary Mixture of a Twist Bend Nematic Liquid Crystal with a Smectogen“. Crystals 10, Nr. 12 (04.12.2020): 1110. http://dx.doi.org/10.3390/cryst10121110.

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Nematic twist-bend phases (NTB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature NTB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to NTB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4′,4′-(heptane-1,7-diyl)bis(([1′,1″-biphenyl]4″-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T ≈ 104 °C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4′-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable NTB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

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15

Amaral, L. Q. „Micelles forming biaxial lyotropic nematic phases“. Liquid Crystals 37, Nr. 6-7 (05.07.2010): 627–40. http://dx.doi.org/10.1080/02678292.2010.487306.

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16

Zhang, Wenlin, Enrique D. Gomez und Scott T. Milner. „Predicting Nematic Phases of Semiflexible Polymers“. Macromolecules 48, Nr. 5 (27.02.2015): 1454–62. http://dx.doi.org/10.1021/acs.macromol.5b00013.

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17

Keith, Christina, Anne Lehmann, Ute Baumeister, Marko Prehm und Carsten Tschierske. „Nematic phases of bent-core mesogens“. Soft Matter 6, Nr. 8 (2010): 1704. http://dx.doi.org/10.1039/b923262a.

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18

Kats, E. I., und V. V. Lebedev. „Landau theory for helical nematic phases“. JETP Letters 100, Nr. 2 (September 2014): 110–13. http://dx.doi.org/10.1134/s0021364014140070.

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19

De Matteis, Giovanni, Fulvio Bisi und Epifanio G. Virga. „Constrained stability for biaxial nematic phases“. Continuum Mechanics and Thermodynamics 19, Nr. 1-2 (21.02.2007): 1–23. http://dx.doi.org/10.1007/s00161-007-0041-1.

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20

Jadżyn, J., R. Dąbrowski, K. Glumiak und G. Czechowski. „Shear Viscosity of the Homologous Series of wCHBT (n = 0 ÷ f 12) in the Isotropic and Nematic Phases“. Zeitschrift für Naturforschung A 55, Nr. 6-7 (01.07.2000): 637–40. http://dx.doi.org/10.1515/zna-2000-6-712.

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Abstract The paper presents results of shear viscosity measurements performed on nematogenic 4-(trans-4'-n-alkylcyclohexyl)isothiocyanatobenzenes (CnH 2n + 1-CyHx-Ph-N=C=S, /nCHBT) in the isotropic (n = 0 ÷ 12) and nematic (« = 4 ÷ 12) phases. The viscosity measured in the nematic phase is, due to the flow alignment phenomenon, close to the Migsowicz T? 2 viscosity coefficient. An odd-even effect in the n dependence of the viscosity-activation energy is observed both in the nematic and isotropic phases of nCHBT.

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21

Murachver, Matthew T., Ahlam Nemati, Mirosław Salamończyk, Carson Bullock, Zachary Sabata, Haumed Rahmani, Tetiana Vorobiova et al. „Indication of a twist-grain-boundary-twist-bend phase of flexible core bent-shape chiral dimers“. Soft Matter 15, Nr. 16 (2019): 3283–90. http://dx.doi.org/10.1039/c8sm02338g.

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22

Zhang, Zhaopeng, Vitaly P. Panov, Mamatha Nagaraj, Richard J. Mandle, John W. Goodby, Geoffrey R. Luckhurst, J. Cliff Jones und Helen F. Gleeson. „Raman scattering studies of order parameters in liquid crystalline dimers exhibiting the nematic and twist-bend nematic phases“. Journal of Materials Chemistry C 3, Nr. 38 (2015): 10007–16. http://dx.doi.org/10.1039/c5tc02174j.

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23

Dunmur, David. „Anatomy of a Discovery: The Twist–Bend Nematic Phase“. Crystals 12, Nr. 3 (22.02.2022): 309. http://dx.doi.org/10.3390/cryst12030309.

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New fluid states of matter, now known as liquid crystals, were discovered at the end of the 19th century and still provide strong themes in scientific research. The applications of liquid crystals continue to attract attention, and the most successful so far has been to the technology of flat panel displays; this has diversified in recent years and LCDs no longer dominate the industry. Despite this, there is plenty more to be uncovered in the science of liquid crystals, and as well as new applications, novel types of liquid crystal phases continue to be discovered. The simplest liquid crystal phase is the nematic together with its handed or chiral equivalent, named the cholesteric phase. In the latter, the aligned molecules of the nematic twist about an axis perpendicular to their alignment axis, but in the 1970s a heliconical phase with a tilt angle of less than 90° was predicted. The discovery of this phase nearly 40 years later is described in this paper. Robert Meyer proposed that coupling between a vector order parameter in a nematic and a splay or bend elastic distortion could result in spontaneously splayed or bent structures. Later, Ivan Dozov suggested that new nematic phases with splay–bend or twist–bend structures could be stabilised if the appropriate elastic constants became negative. Theoretical speculation on new nematic phases and the experimental identification of nematic–nematic phase transitions are reviewed in the paper, and the serendipitous discovery in 2010 of the nematic twist–bend phase in 1″,7″-bis(4-cyanobiphenyl-4′-yl)heptane (CB7CB) is described.

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24

Santos, O. R., W. S. Braga, D. D. Luders, A. R. Sampaio, N. M. Kimura, M. Simões und A. J. Palangana. „Optical characterization of a biaxial nematic between uniaxial nematic lyotropic phases“. Phase Transitions 94, Nr. 6-8 (28.06.2021): 546–55. http://dx.doi.org/10.1080/01411594.2021.1945598.

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25

Pisipati, V. G. K. M., N. V. S. Rao, P. V. Datta Prasad und P. R. Alapati. „Density, Refractive Index and Ultrasonic Velocity Studies Involving N-(p-n-Pentyloxybenzylidene)-p-n-octylaniline“. Zeitschrift für Naturforschung A 40, Nr. 5 (01.05.1985): 472–75. http://dx.doi.org/10.1515/zna-1985-0509.

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The density, refractive index and ultrasonic velocity dependence on temperature for N-(p-npentyloxybenzylidene)- p-n-octylaniline, 50.8, is presented. The compound 50.8 exhibits smectic-B, smectic-A and nematic phases between the solid and isotropic liquid phases. The nematic-isotropic and smectic A-smectic B phase transformations are found to be first order. The interesting smectic A - nematic transformation is confirmed from the results to be second order, although the thermal expansion coefficient and reported enthalpy data had suggested a weak first order transition. The computed adiabatic compressibility, βad, molar sound velocity, Rn, and molar compressibility Aw are presented.

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Yoshioka, Jun, Péter Salamon, Daniel A. Paterson, John M. D. Storey, Corrie T. Imrie, Antal Jákli, Fumito Araoka und Agnes Buka. „Spherical-cap droplets of a photo-responsive bent liquid crystal dimer“. Soft Matter 15, Nr. 5 (2019): 989–98. http://dx.doi.org/10.1039/c8sm01751d.

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Using a photo-responsive dimer exhibiting the transition between nematic (N) and twist-bend nematic (N_TB) phases, we prepared spherical cap-shaped droplets on solid substrates exposed to air.

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Matsuyama, Akihiko. „Thermoreversible Gel-Dispersed Liquid Crystals“. Gels 9, Nr. 12 (08.12.2023): 965. http://dx.doi.org/10.3390/gels9120965.

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A simple model is introduced to describe phase behaviours of binary mixtures of a thermoreversible gel and a low-molecular-weight liquid crystal (LC). We predict novel phase diagrams on the temperature–concentration plane, including sol–gel transition, nematic–isotropic phase transition, and phase separation. At high temperatures, the phase separation between the isotropic sol and gel phases appears. As the temperature decreases, we have the phase separation between nematic sol and isotropic gel phases, in which the nematic domains are dispersed in the isotropic gel phase. We suggest that thermoreversible gelation of reactive molecules mixed with LCs will become one of the new classes of polymer-dispersed liquid crystals.

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Emsley, J. W., M. Lelli, H. Joy, M. G. Tamba und G. H. Mehl. „Similarities and differences between molecular order in the nematic and twist-bend nematic phases of a symmetric liquid crystal dimer“. Physical Chemistry Chemical Physics 18, Nr. 14 (2016): 9419–30. http://dx.doi.org/10.1039/c5cp07304a.

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Carbon-13 chemical shift anisotropies reveal an unusual temperature dependence of the order parameters, S_zz, for the difluoroterphenyl groups in the normal nematic, N and the twist-bend nematic, N_TB, phases.

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Barbero, Giovanni, und Ioannis Lelidis. „Fourth-order nematic elasticity and modulated nematic phases: a poor man’s approach“. Liquid Crystals 46, Nr. 4 (28.08.2018): 535–42. http://dx.doi.org/10.1080/02678292.2018.1512167.

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30

Jiang, Ying, Yuehua Cong und Baoyan Zhang. „Synthesis and characterization of chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens“. New Journal of Chemistry 40, Nr. 11 (2016): 9352–60. http://dx.doi.org/10.1039/c6nj02001a.

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A novel series of siloxane-based chiral smectic side-chain liquid crystalline elastomers containing nematic and chiral mesogens were fabricated through synthesis involving a one-step hydrosilication reaction via a liquid crystalline crosslinking agent containing smectic and nematic phases.

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Brown, Stevie, Ewan Cruickshank, John M. D. Storey, Corrie T. Imrie, Damian Pociecha, Magdalena Majewska, Anna Makal und Ewa Gorecka. „Multiple Polar and Non‐polar Nematic Phases“. ChemPhysChem 22, Nr. 24 (08.11.2021): 2506–10. http://dx.doi.org/10.1002/cphc.202100644.

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32

Löwen, Hartmut. „Anisotropic self-diffusion in colloidal nematic phases“. Physical Review E 59, Nr. 2 (01.02.1999): 1989–95. http://dx.doi.org/10.1103/physreve.59.1989.

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33

Ahmed, H. A., M. Hagar, M. Alaasar und M. Naoum. „Wide nematic phases induced by hydrogen-bonding“. Liquid Crystals 46, Nr. 4 (04.09.2018): 550–59. http://dx.doi.org/10.1080/02678292.2018.1512664.

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34

Fontana, M. P., und G. Gallone. „Analysis of QENS in aligned nematic phases“. Physica Scripta T57 (01.01.1995): 168–74. http://dx.doi.org/10.1088/0031-8949/1995/t57/029.

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35

Simões, M., M. Pazetti, N. M. Kimura und A. J. Palangana. „Micellar shape anisotropy on lyotropic nematic phases“. Physica A: Statistical Mechanics and its Applications 389, Nr. 19 (Oktober 2010): 4000–4008. http://dx.doi.org/10.1016/j.physa.2010.05.056.

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Kee, Hae-Young, Christoph M. Puetter und David Stroud. „Transport signatures of electronic-nematic stripe phases“. Journal of Physics: Condensed Matter 25, Nr. 20 (19.04.2013): 202201. http://dx.doi.org/10.1088/0953-8984/25/20/202201.

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37

Ballauff, Matthias. „Polymer Liquid Crystals Forming Biaxial Nematic Phases“. Angewandte Chemie 100, Nr. 5 (Mai 1988): 775. http://dx.doi.org/10.1002/ange.19881000546.

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38

Ballauff, Matthias. „Polymer Liquid Crystals Forming Biaxial Nematic Phases“. Angewandte Chemie International Edition in English 27, Nr. 5 (Mai 1988): 753. http://dx.doi.org/10.1002/anie.198807531.

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39

Shrivastav, Gaurav P. „Self-Assembly of an Equimolar Mixture of Liquid Crystals and Magnetic Nanoparticles“. Crystals 11, Nr. 7 (19.07.2021): 834. http://dx.doi.org/10.3390/cryst11070834.

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We studied the equilibrium self-assembly of an equimolar mixture of uniaxial liquid crystals (LCs) and magnetic nanoparticles (MNPs) using molecular dynamics simulations. The LCs are modeled by ellipsoids interacting via Gay–Berne potential, and MNPs are represented by dipolar soft spheres (DSS). We found that the LCs show isotropic, nematic, and smectic phases when the mixture is compressed at a fixed temperature. The DSS form chain-like structures, which remain randomly oriented at low densities where the LCs are in the isotropic phase. At intermediate and high densities, the DSS chains align along the nematic and smectic directors of LCs. We found that the DSS inside a chain follow a ferromagnetic ordering. However, the mixture does not show a significant macroscopic magnetization. The extent of nematic order in the DSS remains very similar to the LCs in intermediate densities. At high densities, the DSS have a lower extent of nematic order than the LCs. The structure of the LC–DSS mixture was further analyzed via projected pair correlation functions for distances parallel and perpendicular to directors in the nematic and smectic phases.

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40

Nandi, B., M. Saha und Prabir K. Mukherjee. „Landau Theory of Nematic to Smectic-A Phase Transition“. International Journal of Modern Physics B 12, Nr. 02 (20.01.1998): 207–12. http://dx.doi.org/10.1142/s0217979298000156.

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A phenomenological Landau-like theory is presented here, which describes the nematic and smectic-A phase. The problem of the first or second order nature of the nematic to smectic-A phase transition is explored and the stability of the different phases are also calculated. On the basis of this work it is argued that the nematic to smectic-A transition is of first order nature.

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Yadav, Neelam, Yuri P. Panarin, Wanhe Jiang, Georg H. Mehl und Jagdish K. Vij. „Collective Relaxation Processes in Nonchiral Nematics“. Crystals 13, Nr. 6 (16.06.2023): 962. http://dx.doi.org/10.3390/cryst13060962.

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Nematic–nematic transitions in a highly polar nematic compound are studied, in thick cells in which the molecules are aligned parallel to the substrates but perpendicular to the applied electric field, using dielectric spectroscopy in the frequency range 1 Hz to 10 MHz over a wide temperature range. The studied compound displays three nematic phases under cooling from the isotropic phase: ubiquitous nematic N; high polarizability NX; and ferroelectric nematic NF. Two collective processes were observed. The dielectric strength and relaxation frequency of one of the processes P2 showed a dependence on the thickness of the cell. The process P1 is the amplitude mode, while the process P2 is the phason mode.

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Heinemann, S., H. Kresse, S. Urban und R. Dabrowski. „Dielectric Investigations in Smectic B-Phases“. Zeitschrift für Naturforschung A 51, Nr. 12 (01.12.1996): 1203–8. http://dx.doi.org/10.1515/zna-1996-1210.

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Abstract Several liquid crystalline substances exhibiting SB -phases have been investigated by dielectric and calorimetric measurements. Special attention is taken to the low frequency relaxation process at the phase transition nematic-SB . From the large change of relaxation frequencies, crystallinity of the SB -phases has been concluded. Nevertheless, motion is still present and not frozen.

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43

Subala, S. Sandy, B. Syama Sundar und S. Sreehari Sastry. „Synthesis and Characterization of Nonsymmetric Liquid Crystal Dimer Containing Biphenyl and Azobenzene Moiety“. Journal of Chemistry 2013 (2013): 1–6. http://dx.doi.org/10.1155/2013/939406.

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Calamitic liquid crystalline dimer containing azobenzene moiety and a decyloxy biphenyl linked by flexible spacers {4-[7-(4′-decyloxy-biphenyl-4-yloxy)-alkyloxy]-phenyl}-(4-decyl-phenyl)-diazene has been synthesized and characterized by spectroscopic methods. The transition temperatures and phase behaviours were studied by Differential Scanning Calorimeter (DSC) and Polarizing Optical Microscope (POM). The synthesized compounds exhibited enantiotropic liquid crystal phase with higher spacer display nematic and smectic C phases while lower spacer shows nematic and smectic A phases.

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Jadżyn, J., C. Legrand, P. Kędziora, B. Żywucki, G. Czechowski und D. Bauman. „Dielectric Behaviour of 1-(4-Isothiocyanatophenyl)-2-(4-hexyl- bicyclo[2,2,2]octane-1)ethane in Nematic and Isotropic Phases“. Zeitschrift für Naturforschung A 51, Nr. 8 (01.08.1996): 933–38. http://dx.doi.org/10.1515/zna-1996-0808.

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Abstract Static and dynamic investigations of the dielectric properties of the liquid crystal 1-(4-isothio-cyanatophenyl)-2-(4-hexyl-bicyclo[2,2,2]octane-1)ethane in the nematic and isotropic phases have been carried out in the frequency region from 1 kHz to 1 GHz. Two relaxation processes, described by the Debye functions, have been observed not only in the isotropic but also in the nematic phase, when the measurements of the electric permittivity vs. frequency have been made parallel to the orientation axis of liquid crystal. These processes are related to the rotation of the permanent dipole moment around two main molecular axes. The height of the potential barrier which hinders the rotation of the liquid crystal molecule around the short axis in the ordered nematic phase and the order parameter of the liquid crystal investigated have been estimated on the basis of the relaxation time values in the nematic and isotropic phases.

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Mirzaei, Javad, Martin Urbanski, Heinz-S. Kitzerow und Torsten Hegmann. „Hydrophobic gold nanoparticles via silane conjugation: chemically and thermally robust nanoparticles as dopants for nematic liquid crystals“. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 371, Nr. 1988 (13.04.2013): 20120256. http://dx.doi.org/10.1098/rsta.2012.0256.

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We examine for the first time how chemically and thermally stable gold nanoparticles (NPs), prepared by a silane conjugation approach, affect both the thermal and the electro-optical properties of a nematic liquid crystal (LC), when doped at concentrations ranging from 0.25 to 7.5 wt%. We find that the octadecylsilane-conjugated gold NPs stabilize both the enantiotropic nematic and the monotropic smectic-A phases of the LC host with a maximum stabilization of 2 ° C for the nematic and 3.5 ° C for the smectic-A phases for the mixture containing 1 wt% of the silanized particles. The same mixture shows the lowest values for the Fréedericksz transition threshold voltage and the highest value for the dielectric anisotropy. Generally, all NP-containing mixtures, except mixtures with NP concentrations exceeding 5 wt%, reduce the threshold voltage, increase the dielectric anisotropy and reduce both rise and decay time; the latter particularly at temperatures at least 10 ° C below the isotropic–nematic phase transition on cooling.

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DONG, RONALD Y. „RECENT DEVELOPMENTS IN BIAXIAL LIQUID CRYSTALS: AN NMR PERSPECTIVE“. International Journal of Modern Physics B 24, Nr. 24 (30.09.2010): 4641–82. http://dx.doi.org/10.1142/s0217979210056487.

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A survey of recent studies of biaxial liquid crystals (LCs), whose nematic and/or smectic-A phases do not possess optical uniaxiality (viz., more than one optical axis exists), is given in this review. In particular, we emphasize on how Nuclear Magnetic Resonance (NMR) spectroscopy can help to advance the understanding of phase biaxiality in general, and to examine recent debates on the existence of biaxial nematic phase reported in low molecular mass bent-core or V-shaped mesogens. A general discussion of orientational order parameters is detailed, particularly in smectic-C (SmC) and biaxial nematic phases. How these orientational order parameters can be determined by various techniques such as NMR, IR absorbance and Raman scattering studies, will be mentioned. Recent X-ray observations of SmC clusters in the nematic phase of V-shaped mesogens are highlighted and contrasted with probable theory. Moreover, deuterium and carbon-13 NMR techniques are briefly reviewed, and their possible utilization to identify phase biaxiality in these biaxial LC systems is explored.

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Lee, Jae-Jin, Sangsub Kim, Hiroya Nishikawa, Yoichi Takanishi, Hiroshi Iwayama, Changsoon Kim, Suk-Won Choi und Fumito Araoka. „Chiroptical Performances in Self-Assembled Hierarchical Nanosegregated Chiral Intermediate Phases Composed of Two Different Achiral Bent-Core Molecules“. International Journal of Molecular Sciences 23, Nr. 23 (23.11.2022): 14629. http://dx.doi.org/10.3390/ijms232314629.

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In this paper, chiral intermediate phases composed of two achiral molecules are fabricated by utilizing nanophase separation and molecular hierarchical self-organization. An achiral bent-core guest molecule, exhibiting a calamitic nematic and a dark conglomerate phase according to the temperature, is mixed with another achiral bent-core host molecule possessing a helical nanofilament to separate the phases between them. Two nanosegregated phases are identified, and considerable chiroptical changes, such as circular dichroism and circularly polarized luminescence, are detected at the transition temperatures between the different nanophase-separated states. The nanosegregated chiral phase—wherein the helical nanofilament and dark conglomerate phases are phase-separated—exhibits the highest chiroptical intensities. The luminescence dissymmetry factor, |glum|, in this phase is amplified by an order of magnitude compared with that of another nanosegregated phase, wherein the helical nanofilament and nematic phases are phase-separated.

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Aliev, Mikhail A., Elena A. Ugolkova und Nickolay Yu Kuzminyh. „The helicoidal modulated nematic phases in a model system of V-shaped molecules“. International Journal of Modern Physics B 33, Nr. 10 (20.04.2019): 1950079. http://dx.doi.org/10.1142/s0217979219500796.

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The phase behavior of the melt of the symmetric V-shaped molecules has been inspected within the Landau–de Gennes approach. The arms of V-shaped molecule are modeled by rod-like segments connected at an external angle [Formula: see text]; these segments are assumed to be composed of monomer units. The phase diagram of the system contains regions of stability of the isotropic, homogeneous nematic, and modulated nematic phases: nematic twist bend (N[Formula: see text]) phase that is characterized by the three directors which move on a helix and one of these directors forms a fixed nonzero cone angle with the helix axis, and the N0 modulated phase in which one director is parallel to the helix axis while two others are orthogonal to the helix axis. The characteristic periods of modulated structures were found to be of order a few molecular lengths.

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CLIMENT-EZQUERRA, BLANCA, und FRANCISCO GUILLÉN-GONZÁLEZ. „A review of mathematical analysis of nematic and smectic-A liquid crystal models“. European Journal of Applied Mathematics 25, Nr. 1 (07.10.2013): 133–53. http://dx.doi.org/10.1017/s0956792513000338.

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We review the mathematical analysis of some uniaxial, liquid crystal phases. Firstly, we state the models for the two different studied phases: nematic and smectic-A liquid crystals. The spatial and temporal profiles of the liquid crystal configurations will be described by means of strongly nonlinear parabolic partial differential systems, which are presented at the same time. Then we will state some results about existence, regularity, time-periodicity and stability of solutions at infinite time for both models. It is our aim to show that, although nematic and smectic-A phases have different physical properties and are modelled by different nonlinear parabolic problems, there exists a common mathematical machinery to rewrite the models and obtain analytical results.

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To, T. B. T., T. J. Sluckin und G. R. Luckhurst. „Molecular field theory for biaxial nematics formed from liquid crystal dimers and inhibited by the twist-bend nematic“. Physical Chemistry Chemical Physics 19, Nr. 43 (2017): 29321–32. http://dx.doi.org/10.1039/c7cp04350c.

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