11/28/2011
A method of achieving homeotropic alignment of liquid crystals (LCs) by ultraviolet light induced phase separation of LC from its mixture with photocurable prepolymer is reported. Vertical polymer columns and microfibrils developed during the phase separation promote homeotropic alignment of the LC director (i.e., alignment perpendicular to the LCsubstrate interface), suitable for devices based on LCs possessing negative dielectric anisotropy. These vertical structures extend between two substrates and permit the fabrication of highly flexible electrooptical devices with high contrast coupled with fast response times. This simple singlestep technique eliminates the need for the traditional polymer alignment layer predeposited on substrates. (C) 2011 American Institute of Physics. [doi:10.1063/1.3663966]
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07/15/2008
Recent experiments report that the longlookedfor thermotropic biaxial nematic phase has been finally detected in some thermotropic liquid crystalline systems. Inspired by these experimental observations, we concentrate on some elementary theoretical issues concerned with the classical sixthorder Landaude Gennes free energy expansion in terms of the symmetric and traceless tensor order parameter Q(alpha beta). In particular, we fully explore the stability of the biaxial nematic phase giving analytical solutions for all distinct classes of the phase diagrams that theory allows. This includes diagrams with triple, critical, and tricritical points and with multiple (reentrant) biaxial and uniaxial phase transitions. A brief comparison with predictions of existing molecular theories is also given.
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08/15/1991
The directorfield configuration of a nematic liquid crystal confined to cylindrical cavities of polycarbonate Nuclepore membranes ranging from 0.3 to 0.05mum in radius is determined using deuterium nuclear magnetic resonance (H2 NMR). Spectral patterns from cavities of radius 0.3mum reveal the escapedradial configuration with singular point defects, but as the cylinder size is decreased, the elastic energy imposed by the curvature of the confining walls competes with the anchoring energy to tilt the directors away from their preferred perpendicular anchoring direction, preventing the expected transition to the planarradial configuration. A surface fitting parameter is directly determined by simulating H2NMR line shapes, and by studying a series of samples with different radii, the molecularanchoring strength W0 and surface elastic constant K24 are extracted.
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04/15/1995
Two compounds of the 1(4’alkoxyphenylamino3[5’’(2’’menthylpirydil)]propen1one3 (PIRn) series that exhibit the hexaticB (HexB) to smecticA (SmA) and the crystalB (CryB) to HexB phase transitions were studied by high resolution ac calorimetry. The HexB–SmA phase transition was found to be of first order and continuous for PIR7 and PIR9, respectively. The tricritical point was located at the concentration for which TCryB–HexB/THexB–SmA =0.980±0.004, i.e., a hexatic range of 7 K. At both sides of the tricritical point, nonclassical and nonthreedimensionalXY specific heat exponents α were observed. The CryB–HexB phase transition was of first order and was associated with a small or immeasurable Cp anomaly.
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03/01/2000
We study the Landau model of the class of incommensurate systems with a scalar order parameter where the modulated phase is driven by a gradientsquared term with negative coefficient. For example, theoretical studies of cholesteric liquid crystals in a field (electric or magnetic) suggest that such an modulated phase should exist at high chirality. The bulk phase diagram in the presence of a bulk external field which couples linearly to the order parameter exhibits a modulated phase inside a loop in the temperaturefield plane, and a homogeneous phase outside. On analyzing the same model for a semiinfinite system, we find a surprising result; the system exhibits surface states in a region where the bulk phase is homogeneous (but close to the modulated region). These states are very different from the wellknown surface states induced either by a surface field or by enhanced interactions at the surface, for they exist and are energetically favored even when the sole effect of the surface is to terminate the bulk, as expressed by free boundary conditions taken at the surface. Near the surface, the surfacestate order parameter is very different from the bulk value (in fact, it has the opposite sign). When the temperature or the bulk field are varied to move away from the modulated state, we find a surface phase transition at which the surface states become energetically unfavorable, though they continue to exist as metastable states. We then study how a surface field changes the surface phase diagram.
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10/01/2009
The dye sunset yellow (SY) forms columnar aggregates via ππ stacking in aqueous solutions. These aggregates develop orientational and translational order at elevated concentrations to exhibit the nematic (N) and columnar (C) mesophases. Positionalorder correlation lengths of the aggregates in the directions parallel and perpendicular to the stacking direction were measured as functions of temperature, concentration, and ionic content of solutions with synchrotron xray scattering in magnetically aligned samples. Average length of aggregates (i.e., the number of SY molecules in an aggregate) grows monotonically with concentration while their effective transverse separation decreases. The scission energy, E, determined from the Arrhenius thermal evolution of the longitudinal correlation length, is found to be 4.3±0.3 kBT and 3.5±0.2 kBT, in the N and C phases, respectively. Temperature and concentration dependence of E suggests that chromonic aggregation is not an isodesmic process. The aggregate length decreases with decreasing pH when HCl is added to the solution.
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04/01/2001
A novel method for liquid crystal alignment using insitu exposure to linearly polarized ultraviolet (LPUV) light during imidization of polyimide has been devised. The alignment layers prepared by this method exhibit higher thermal stability than the conventional method that employs LPUV exposure after imidization. Multidomain cells can be easily fabricated with the use of a photo mask and multistep insitu LPUV exposure during hard bake. With this method, it is also possible to generate pretilt angle using twostep LPUV exposure during imidization.
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02/01/2000
The most frequently observed focal conic domains (FCD’s) in lamellar phases are those based on confocal paris of ellipse and hyperbola. Experimentally, the eccentricity of the ellipse takes a broad range of values 0<~eeisolated FCD reaches a minimum only at e⃗1 (under the constraint of a fixed major semiaxis of the ellipse); exceptions include situations with large saddlesplay elastic constant and small domains where the applicability of the elastic theory is limited. In realistic cases, a value of eccentricity smaller than 1 is stabilized by factors other than the curvature energy: by dislocations emerging from the FCD’s with e≠0, compression of layers and surface anchoring.
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02/16/1998
We explore the SaffmanTaylor instability of a gas bubble expanding into a shear thinning liquid in a radial HeleShaw cell. Using Darcy's law generalized for nonNewtonian fluids, we perform simulations of the full dynamical problem. The simulations show that shear thinning significantly influences the developing interfacial patterns. Shear thinning can suppress tip splitting, and produce fingers which oscillate during growth and shed side branches. Emergent length scales show reasonable agreement with a general linear stability analysis.
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07/15/1991
The results of a highresolution xraydiffraction study of the smectic phases in mixtures of DB7OCN and 8OCB show that the incommensurate smecticAi2 phase, previously reported in this mixture, is in fact a coexistence of the partially bilayer smecticAd and the bilayer smecticA2phases at a firstorder phase transition between them.
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