114 research outputs found
Distinctive dielectric properties of nematic liquid crystal dimers
We provide an overview of the effect of the molecular structure on the dielectric properties of dimers exhibiting nematic and twist-bend nematic phases with special focus on how the conformational distribution changes are reflected by the dielectric behaviour. Nematic dimers show distinctive dielectric properties which differ from those of archetypical nematic liquid crystals, as for example, unusual temperature dependence of the static permittivity or dielectric spectra characterised by two low-frequency relaxation processes with correlated strengths. The interpretation of such characteristic behaviour requires that account is taken of the effect of molecular flexibility on the energetically favoured molecular shapes. The anisotropic nematic interactions greatly influence the conformational distribution. Dielectric behaviour can be used to track those conformational changes due to dependence of the averaged molecular dipole moment on the averaged molecular shape. Results for a number of dimers are compared and analysed on the basis of the influence of details of the molecular structure, using a recently developed theory for the dielectric properties of dimers.Postprint (author's final draft
The phase behavior of a binary mixture of rodlike and disclike mesogens: Monte Carlo simulation, theory, and experiment
Published versio
Accessible Light Bullets via synergetic nonlinearities
We introduce a new form of stable spatio-temporal self-trapped optical
packets stemming from the interplay of local and nonlocal nonlinearities.
Pulsed self-trapped light beams in media with both electronic and molecular
nonlinear responses are addressed to prove that spatial and temporal effects
can be decoupled, allowing for independent tuning. We numerically demonstrate
that (3+1)D light bullets and anti-bullets, i. e. bright and dark temporal
solitons embedded in stable (2+1)D nonlocal spatial solitons, can be generated
in reorientational media under experimentally feasible conditions.Comment: 17 pages, 4 figures A scale error in the index perturbation vs
average power dependence has been fixe
Bifurcations in annular electroconvection with an imposed shear
We report an experimental study of the primary bifurcation in
electrically-driven convection in a freely suspended film. A weakly conducting,
submicron thick smectic liquid crystal film was supported by concentric
circular electrodes. It electroconvected when a sufficiently large voltage
was applied between its inner and outer edges. The film could sustain rapid
flows and yet remain strictly two-dimensional. By rotation of the inner
electrode, a circular Couette shear could be independently imposed. The control
parameters were a dimensionless number , analogous to the Rayleigh
number, which is and the Reynolds number of the
azimuthal shear flow. The geometrical and material properties of the film were
characterized by the radius ratio , and a Prandtl-like number . Using measurements of current-voltage characteristics of a large number of
films, we examined the onset of electroconvection over a broad range of
, and . We compared this data quantitatively to
the results of linear stability theory. This could be done with essentially no
adjustable parameters. The current-voltage data above onset were then used to
infer the amplitude of electroconvection in the weakly nonlinear regime by
fitting them to a steady-state amplitude equation of the Landau form. We show
how the primary bifurcation can be tuned between supercritical and subcritical
by changing and .Comment: 17 pages, 12 figures. Submitted to Phys. Rev. E. Minor changes after
refereeing. See also http://mobydick.physics.utoronto.c
Squeezing a drop of nematic liquid crystal with strong elasticity effects
The One Drop Filling (ODF) method is widely used for the industrial manufacture of liquid crystal devices. Motivated by the need for a better fundamental understanding of the reorientation of the molecules due to the flow of the liquid crystal during this manufacturing method, we formulate and analyze a squeeze-film model for the ODF method. Specifically, we consider a nematic squeeze film in the asymptotic regime in which the drop is thin, inertial effects are weak, and elasticity effects are strong for four specific anchoring cases at the top plate and the substrate (namely, planar, homeotropic, hybrid aligned nematic, and Ï-cell infinite anchoring conditions) and for two different scenarios for the motion of the top plate (namely, prescribed speed and prescribed force). Analytical expressions for the leading- and first-order director angles, radial velocity, vertical velocity, and pressure are obtained. Shear and couple stresses at the top plate and the substrate are calculated and are interpreted in terms of the effect that flow may have on the alignment of the molecules at the plates, potentially leading to the formation of spurious optical defects (âmuraâ)
VOLUMETRIC STUDIES OF THE HOMOLOGOUS SERIES OF ALKYL-CYANO-BIPHENYL LIQUID CRYSTALS
Des mesures de densitĂ© en fonction de la tempĂ©rature ont Ă©tĂ© rapportĂ©es pour les sĂ©ries homologues de cristaux liquides d'alkyle-cyano-biphĂ©nyle CnH2n+1ĂĂCN pour n = 5 Ă 9. Des lignes cubiques Bsp ont Ă©tĂ© trouvĂ©es en accord avec les rĂ©sultats expĂ©rimentaux et leurs dĂ©rivĂ©es ont donnĂ© les coefficients d'expansion thermique. Des changements de volume transitionnels ont Ă©tĂ© dĂ©terminĂ©s par extrapolation de la densitĂ© des phases de chaque cĂŽtĂ© d'une transition, et les variations de volume de nĂ©matique Ă isotrope montrent un effet pair-impair marquĂ©. Les densitĂ©s de phases nĂ©matique et isotrope en Ă©quilibre ont Ă©tĂ© utilisĂ©es avec une thĂ©orie de van der Waals des cristaux liquides pour fournir une information sur les paramĂštres d'un potentiel de champ moyen, les paramĂštres d'ordre transitionnel et les rapports longueur sur largeur molĂ©culaire.Measurements are reported of densities as a function of temperature for the homologous series of alkyl-cyano-biphenyl liquid crystals CnH2n+1.Ă.Ă.CN for n = 5 to 9. Cubic B-splines have been fitted to the experimental results and their derivatives have given the coefficients of thermal expansivity. Transitional volume changes have been determined by extrapolation of the densities of phases each side of a transition, and the nematic to isotropic volume changes show a marked odd-even effect. The densities of the nematic and isotropic phases in equilibrium have been used in conjunction with a van der Waals theory of liquid crystals to provide information on mean field potential parameters, transitional order parameters and molecular length to breadth ratios
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