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T J Sluckin

Publications and source records attributed to T J Sluckin.

11 recordsLinked to original sources
PubMed2005

Waves at the nematic-isotropic interface: thermotropic nematogen-non-nematogen mixtures.

We develop a theory for surface modes at the nematic-isotropic interface in thermotropic nematogen-non-nematogen mixtures. We employ the dynamical generalization of the Landau-de Gennes model for the orientational (nonconserved) order parameter, coupled with the Cahn-Hilliard equation for concentration (conserved parameter), and include hydrodynamic degrees of freedom. The theory uses a generalized form of the Landau-de Gennes free-energy density to include the coupling between the concentration of the non-nematogen fluid and the orientational order parameter. Two representative phase diagrams are shown. The method of matched asymptotic expansions is used to obtain a generalized dispersion relation. Further analysis is made in particular cases. Orientational order parameter relaxation dominates in the short-wavelength limit, while in the long-wavelength limit viscous damping processes become important. There is an intermediate region (depending on the temperature) in which the interaction between conserved parameter dynamics and hydrodynamics is important.

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PubMed2005

Twist of cholesteric liquid crystal cells: stability of helical structures and anchoring energy effects.

We consider helical configurations of a cholesteric liquid crystal (CLC) sandwiched between two substrates with homogeneous director orientation favored at both confining plates. We study the CLC twist wave number q characterizing the helical structures in relation to the free twisting number q(0) which determines the equilibrium value of CLC pitch P(0) = 2 pi/ q(0) . We investigate the instability mechanism underlying transitions between helical structures with different spiral half-turn numbers. Stability analysis shows that for equal finite anchoring strengths this mechanism can be dominated by in-plane director fluctuations. In this case the metastable helical configurations are separated by the energy barriers and the transitions can be described as the director slippage through these barriers. We extend our analysis to the case of an asymmetric CLC cell in which the anchoring strengths at the two substrates are different. The asymmetry introduces two qualitatively different effects: (a) the intervals of twist wave numbers representing locally stable configurations with adjacent helix half-turn numbers are now separated by instability gaps; and (b) sufficiently large asymmetry, when the difference between azimuthal anchoring extrapolation lengths exceeds the thickness of the cell, will suppress the jumplike behavior of the twist wave number.

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PubMed2004

Annihilation of edge dislocations in smectic-A liquid crystals.

This paper presents a theoretical study of the annihilation of edge dislocations in the same smectic plane in a bulk smectic-A phase. We use a time-dependent Landau-Ginzburg approach where the smectic ordering is described by the complex order parameter psi( r--> ,t) =eta e(iphi) . This quantity allows both the degree of layering and the position of the layers to be monitored. We are able to follow both precollision and postcollision regimes, and distinguish different early and late behaviors within these regimes. The early precollision regime is driven by changes in the phi ( r--> ) configuration. The relative velocity of the defects is approximately inversely proportional to the interdefect separation distance. In the late precollision regime the symmetry changes within the cores of defects also become influential. Following the defect collision, in the early postcollision stage, bulk layer order is approached exponentially in time. At very late times, however, there seems to be a long-time power-law tail in the order parameter fluctuation relaxation.

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PubMed2002

Surface modes at the nematic-isotropic interface.

We examine surface modes at the nematic-isotropic interface using the generalized dynamical Landau-de Gennes theory. We assume an isothermal, infinite, unbounded nematic-isotropic system characterized by a scalar order parameter, both phases having the same density and viscosity, respectively. The generalized dispersion relation is obtained and analyzed in particular cases. Order parameter relaxation dominates in the short wavelength limit, while in the long wavelength limit viscous damping becomes important. We study the crossover between the two regimes and estimate the extent of this region for the liquid crystal 8CB.

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PubMed2002

Light scattering by optically anisotropic scatterers: T-matrix theory for radial and uniform anisotropies.

We extend the T-matrix approach to light scattering by spherical particles to some simple cases in which the scatterers are optically anisotropic. Specifically, we consider cases in which the spherical particles include radially and uniformly anisotropic layers. We find that in both cases the T-matrix theory can be formulated using a modified T-matrix ansatz with suitably defined modes. In a uniformly anisotropic medium we derive these modes by relating the wave packet representation and expansions of electromagnetic field over spherical harmonics. The resulting wave functions are deformed spherical harmonics that represent solutions of the Maxwell equations. We present preliminary results of numerical calculations of the scattering by spherical droplets. We concentrate on cases in which the scattering is due only to the local optical anisotropy within the scatterer. For radial anisotropy we find that nonmonotonic dependence of the scattering cross section on the degree of anisotropy can occur in a regime to which both the Rayleigh and semiclassical theories are inapplicable. For uniform anisotropy the cross section is strongly dependent on the angle between the incident light and the optical axis, and for larger droplets this dependence is nonmonotonic.

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PubMed2002

Shear-induced structural changes of a smectic-A phase: a computer simulation study.

We have carried out a Monte Carlo simulation of a thin sample of the smectic-A phase of the Gay-Berne mesogen GB(4.4,20.0,1,1) sandwiched between two plates and subject to shear. The smectic layers are perpendicular to the confining plates and are pinned at the boundaries. The thickness of the samples studied ranges from about three to twenty molecules. The layers tilt progressively with increasing shear, but rearrange themselves at a critical shear. At this critical shear the layers melt near the center of the sample and reform with a reduced tilt consistent with the layer pinning at the walls. The pseudodynamics of this process as the smectic layers melt and are reformed have been followed during the simulation. The critical layer tilt at which slippage takes place tends to a constant value for thick samples, but for very thin samples the critical shear tends toward half a smectic layer, with a significantly reduced translational order near the sample center just before the critical shear. The simulation results are consistent with the predictions of the mean field theory of this phenomenon developed by Mottram et al.

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PubMed2001

Surface depinning of smectic-A edge dislocations.

Using a Landau-de Gennes approach, we model the formation of an edge dislocation in a smectic-A cell initially in the bookshelf structure. The driving force is the mismatch between the layer thickness in a bulk smectic-A liquid crystal and that imposed by confining plates. The core structure of the dislocation is calculated taking into account spatial variations of the smectic translational order parameter. We numerically determine the critical condition for the surface-driven formation and depinning of the dislocation. By exploiting this phenomenon, we show how the value of the positional anchoring strength at the surface can be determined.

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PubMed2000

Nematic director slippage: role of the angular momentum of light.

We propose a theoretical model of the light-induced director slippage effect. In this effect the bulk director reorientation contributes to the surface director reorientation. It is found that the director and ellipticity profiles, obtained in the geometric optics approximation, are dependent on the ellipticity of the incident light wave. The director distribution is spatially modulated in linearly polarized light but grows monotonically in circularly polarized light. The surface director deviation has been examined, and comparison made with existing experimental data, which then permits the magnitude of the orientational nonlinearity coefficient to be calculated.

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PubMed1999

Dynamics of chevron structure formation. II. Permeation-dominated phenomena.

This paper continues a study of the dynamics of chevron formation in smectic-A liquid crystals in samples with boundary conditions apparently favoring the bookshelf structure, with uniform layers perpendicular to the sample cell plane. The chevron structure that arises when the sample is cooled results from the mismatch between preferred bulk and surface layer thicknesses. In a previous paper we considered relaxation driven by the strong coupling between layer deformation and fluid flow. In this paper we discuss the alternative scenario in which boundary conditions suppress this coupling. Layer deformation now occurs by layer relaxation in the absence of fluid flow. This process is extremely slow and is governed by the nonlinear Fisher-Kolmogorov equation. Chevrons do form under some circumstances, but the process is irregular, and quasimetastable jagged multi-edged multi-tip-like structures can occur on intermediate time scales for suitable layer strains. In the absence of surface layer pinning, layer slippage occurs at the surfaces. We also examine the possibility that deformation may occur through a wave of invasion destroying the bookshelf region.

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PubMed1999

Traveling-wave relaxation in elongated liquid crystal cells.

We have made a theoretical study of Freedericksz relaxation in a long thin nematic liquid crystal cell subject to strong anchoring on the short ends and weak anchoring on the long sides. On removing an imposed magnetic field, three different types of relaxation behavior may be observed. Two of these are simple generalizations of one-dimensional relaxation channels, and are dominated by either the ends or the sides. The third is a traveling wave, nucleated by the strong anchoring ends of the cell but driven by the weak anchoring sides and is the result of a subtle balance between the two classical mechanisms. A phase diagram is derived, identifying the relaxation regimes as a function of the nondimensional initial field and the anchoring strength in the long cell limit. A comparison is made between numerical results and a simple one-dimensional theory derived from an asymptotic analysis. Surprisingly, the traveling wave behavior occurs for a large region of parameter space.

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