Using a magnetic Frederiks transition technique, we measure the temperature and concentration dependences of splay K1, twist K2, and bend K3 elastic constants for the lyotropic chromonic liquid ...crystal sunset yellow formed through noncovalent reversible aggregation of organic molecules in water. K1 and K3 are comparable to each other and are an order of magnitude higher than K2. At higher concentrations and lower temperatures, K1 and the ratios K1/K3 and K1/K2 increase, which is attributed to elongation of self-assembled lyotropic chromonic liquid crystal aggregates, a feature not found in conventional thermotropic and lyotropic liquid crystals formed by covalently bound units of a fixed length.
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We present freeze-fracture transmission electron microscopy (FF-TEM), dielectric spectroscopy and electro-optic measurements on a dimeric liquid crystal mixture, which previously was proposed to form ...the twist-bend nematic (N
tb
) phase. Our FF-TEM studies provide a direct image of a 10.5 nm periodic structure, consistent with the expected nanoscale, heliconical twist-bend modulation of the molecular orientation. Dielectric measurements in the 100 Hz to 10 MHz range reveal three nearly Debye-type dispersion processes in the nematic and the twist-bend phase. Low frequency 8 V/µm electric fields applied on planar cells cause the optical-scale stripe texture (another characteristic feature of the N
tb
phase) to disappear. Higher (>16 V/µm) fields gradually realign the heliconical axis along the electric field; it relaxes back after the field removal.
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We have determined the nematic-isotropic transition temperature as a function of an applied magnetic field in three different thermotropic liquid crystalline dimers. These molecules are comprised of ...two rigid calamitic moieties joined end to end by flexible spacers with odd numbers of methylene groups. They show an unprecedented magnetic field enhancement of nematic order in that the transition temperature is increased by up to 15 K when subjected to a 22 T magnetic field. The increase is conjectured to be caused by a magnetic-field-induced decrease of the average bend angle in the aliphatic spacers connecting the rigid mesogenic units of the dimers.
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We report on measurements of magnetic-field induced nematic order in the bent-core liquid crystal 4-chlororesorcinol bis4-(4-n-dodecyloxybenzoyloxy) benzoate. Using the 31 T solenoid at the National ...High Magnetic Field Laboratory, we have observed, at temperatures less than 1 degrees above the clearing point, a first-order transition to the nematic phase. The critical magnetic field at which this occurs increases with temperature. We discuss these results within the context of both Maier-Saupe and Landau-de Gennes mean-field models for the nematic-isotropic transition. The implications of possible tetrahedratic order are also discussed. To our knowledge, this is the first observation of such a magnetic-field induced transition in a thermotropic liquid crystal; the reasons for which this behavior is now attainable are discussed.
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We report the induction of spontaneously undulated chiral nematic structures of liquid crystal (LC) dimers with rigid aromatic molecular arms linked by flexible chains with an odd number of carbons. ...When a small amount of chiral dopants (CD) are added to the dimers, we find the formation of different stripe textures on cooling 4-10 μm films in the nematic phase. The temperature where the stripes form depends on the film thickness and the direction of the stripes depends on the CD concentrations. We show that the experimentally observed stripes are due to undulation instabilities that spontaneously form as a result of the anomalously small bend elastic constant that prefers director bend instead of twist deformation, the opposite of the situation in usual cholesteric LCs.
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We present studies of chiral nematic liquid crystals composed of flexible dimer molecules subject to large dc magnetic fields between 0 and 31 T. We observe that these fields lead to selective ...reflection of light depending on temperature and magnetic field. The band of reflected wavelengths can be tuned from ultraviolet to beyond the IR-C band. A similar effect induced by electric fields has been presented previously, and was explained by a field-induced oblique-heliconical director deformation in accordance with early theoretical predictions. The use of magnetic field here instead of electric field allows precise measurements of some material constants and holds promise for wireless tuning of selective reflection.
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We present magneto-optic measurements on two materials that form the recently discovered twist-bend nematic (N_{tb}) phase. This intriguing state of matter represents a fluid phase that is ...orientationally anisotropic in three directions and also exhibits translational order with periodicity several times larger than the molecular size. N_{tb} materials may also spontaneously form a visible, macroscopic stripe texture. We show that the optical stripe texture can be persistently inhibited by a magnetic field, and a 25T external magnetic field depresses the N-N_{tb} phase transition temperature by almost 1{∘}C. We propose a quantitative mechanism to account for this shift and suggest a Helfrich-Hurault-type mechanism for the optical stripe formation.
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We report results from a comprehensive suite of measurements, including small angle X-ray scattering (SAXS), polarising optical microscopy, dynamic light scattering, elastic properties and dielectric ...and diamagnetic susceptibilities on macroscopic liquid crystalline properties of branched compound, 2-(octyloxycarbonyl)-1, 4-phenylene bis(4-(octyloxy)benzoate). SAXS studies reveal very distinct, although short-range smectic-C-type clusters, leading to increased viscosities, very similar to that been reported in many bent-core compounds. The measured elastic constants and their relative values are similar to calamitic liquid crystals, presumably because the clusters are not polar, unlike in bent-core liquid crystals, which (together with the director tilt) result in a layer chirality.
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The existence of the elusive biaxial phase has been the subject of much discussion since it was predicted by Freiser in 1970. More recently, there have been numerous attempts to find a thermotropic ...liquid crystal that exhibits a biaxial phase and with this, conflicting reports about whether such a phase has been positively identified in bent-core liquid crystals. One reason for the discrepancy is that there is currently no way to rule out surface effects or anchoring transitions, both of which may give a false positive identification of a uniaxial-biaxial nematic transition. We have developed a technique that uses a magnetic field to align the uniaxial director, thus widening its application to any bent-core nematic material.
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