06/01/2007
The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ VieillardBaron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression for the distance of closest approach of the centers of two arbitrary hard ellipses as a function of their orientation relative to the line joining their centers. We describe our method for solving this problem, illustrate our result, and discuss its usefulness in modeling and simulating systems of anisometric particles such as liquid crystals.
 Author:

 Format:


06/01/2007
The distance of closest approach of hard particles is a key parameter of their interaction and plays an important role in the resulting phase behavior. For nonspherical particles, the distance of closest approach depends on orientation, and its calculation is surprisingly difficult. Although overlap criteria have been developed for use in computer simulations [ VieillardBaron J. Chem. Phys. 56 4729 (1972); Perram and Wertheim J. Comput. Phys. 58 409 (1985)], no analytic solutions have been obtained for the distance of closest approach of ellipsoids in three dimensions, or, until now, for ellipses in two dimensions. We have derived an analytic expression for the distance of closest approach of the centers of two arbitrary hard ellipses as a function of their orientation relative to the line joining their centers. We describe our method for solving this problem, illustrate our result, and discuss its usefulness in modeling and simulating systems of anisometric particles such as liquid crystals.
 Author:

 Format:


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.
 Author:

 Format:


01/21/2002
The critical divergence of the smecticorder correlation lengths parallel and perpendicular to the director, ξ∥,⊥, and the susceptibility, σ0, near the nematictosmecticA phase transition has been measured in a 5 T magnetic field. The high magnetic field reduced the mosaicity and improved the effective transverse resolution by almost 2 orders of magnitude and permitted measurements nearly over four decades of reduced temperature. The results provide important new insights into mosaicity correction. The values of the critical exponents of ξ∥,⊥ and σ0 were ν∥ = 0.79±0.02, ν⊥ = 0.68±0.02, γ = 1.45±0.04, respectively. They become smaller for a narrower nematic range.
 Author:

 Format:


04/01/1998
The nematic (N) to lamellar (Lα) phase transition in binary mixtures of cesiumperfluorooctanoate (CsPFO) and water has been studied by highresolution synchrotron xray scattering at 46.6 weight % CsPFO. The longitudinal correlation length ξ∥ and the susceptibility σ associated with the lamellar phase fluctuations in the N phase, measured over three decades of reduced temperature, diverge with critical exponents ν∥=0.86±0.04 and γ=1.37±0.11, respectively. These results show that the N to Lα phase transition is quantitatively similar to the N to smecticA phase transition of thermotropic liquid crystals with a wider nematic range.
 Author:

 Format:


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.
 Author:

 Format:


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.
 Author:

 Format:


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.
 Author:

 Format:


01/01/2003
The first examples of achiral bentcore molecules consisting of an azo linkage and five aromatic rings exhibiting bentcore mesophases are reported. They exhibit B1 and B2 phases as identified by optical microscopy, differential scanning calorimetry, Xray diffraction, and electrooptical techniques. The B2 phase of these materials is identified to be the anticlinic–antiferroelectric, SmCAPA phase. The mesophases of these compounds have relatively low transition temperatures and wide temperature ranges. The observation of bentcore phases in azo compounds assumes significance from the fact that the introduction of the –NN– linkage adds a new dimension, namely photochromism, to this field.
 Author:

 Format:


01/01/2006
A homologous series of shapepersistent Vshaped molecules has been designed to form the biaxial nematic phase. Phenyleneethynylene moieties are attached to a bent fluorenone unit to create an apex angle of about 90°, which is determined from the single crystal structure. Two mesogens, one symmetric and another unsymmetric, have been synthesized by attaching a cyano group to one or both of the peripheral phenyl units, respectively. These groups introduce local dipoles essential for the formation of the nematic phases. The tendency to form a crystalline phase is reduced by laterally substituted hexyloxy chains which allow the nematic phase to be supercooled to a glassy state. Two of the three fluorenone derivatives exhibit a transition from the uniaxial nematic to the biaxial nematic phase. This transition has an undetectably small transition enthalpy, but the Xray diffraction, polarizing optical microscopy, and conoscopy reveal the presence of the biaxial order in the low temperature nematic phase.
 Author:

 Format:

