Defect configurations and dynamical behavior in a Gay-Berne nematic emulsion

To model a nematic emulsion consisting of a surfactant-coated water droplet dispersed in a nematic host, we performed a molecular dynamics simulation of a droplet immersed in a system of 2048 Gay-Berne ellipsoids in a nematic phase. Strong radial anchoring at the surface of the droplet induced a Saturn ring defect configuration, consistent with theoretical predictions for very small droplets. A surface ring configuration was observed for lower radial anchoring strengths, and a pair of point defects was found near the poles of the droplet for tangential anchoring. We also simulated the falling ball experiment and measured the drag force anisotropy, in the presence of strong radial anchoring as well as zero anchoring strength.Comment: 17 pages, 15 figure

Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order

Particles dispersed in a liquid crystal above the nematic-isotropic phase transition are wetted by a surface-induced corona of paranematic order. Such coronas give rise to pronounced two-particle interactions. In this article, we report details on the analytical and numerical study of these interactions published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially demonstrate how for large particle separations the asymptotic form of a Yukawa potential arises. We show that the Yukawa potential is a surprisingly good description for the two-particle interactions down to distances of the order of the nematic coherence length. Based on this fact, we extend earlier studies on a temperature induced flocculation transition in electrostatically stabilized colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa potential to establish a flocculation diagram for a much larger range of the electrostatic parameters, namely the surface charge density and the Debye screening length. As a new feature, a kinetically stabilized dispersion close to the nematic-isotropic phase transition is found.Comment: Revtex v4.0, 16 pages, 12 Postscript figures. Accepted for publication in Phys. Rev.

Defect structures in nematic liquid crystals around charged particles

We numerically study the orientation deformations in nematic liquid crystals around charged particles. We set up a Ginzburg-Landau theory with inhomogeneous electric field. If the dielectric anisotropy varepsilon_1 is positive, Saturn ring defects are formed around the particles. For varepsilon_1<0, novel "ansa" defects appear, which are disclination lines with their ends on the particle surface. We find unique defect structures around two charged particles. To lower the free energy, oppositely charged particle pairs tend to be aligned in the parallel direction for varepsilon_1>0 and in the perpendicular plane for varepsilon_1<0 with respect to the background director . For identically charged pairs the preferred directions for varepsilon_1>0 and varepsilon_1<0 are exchanged. We also examie competition between the charge-induced anchoring and the short-range anchoring. If the short-range anchoring is sufficiently strong, it can be effective in the vicinity of the surface, while the director orientation is governed by the long-range electrostatic interaction far from the surface.Comment: 10 papes, 12 figures, to appear in European Physical Journal

Symmetries and Elasticity of Nematic Gels

A nematic liquid-crystal gel is a macroscopically homogeneous elastic medium with the rotational symmetry of a nematic liquid crystal. In this paper, we develop a general approach to the study of these gels that incorporates all underlying symmetries. After reviewing traditional elasticity and clarifying the role of broken rotational symmetries in both the reference space of points in the undistorted medium and the target space into which these points are mapped, we explore the unusual properties of nematic gels from a number of perspectives. We show how symmetries of nematic gels formed via spontaneous symmetry breaking from an isotropic gel enforce soft elastic response characterized by the vanishing of a shear modulus and the vanishing of stress up to a critical value of strain along certain directions. We also study the phase transition from isotropic to nematic gels. In addition to being fully consistent with approaches to nematic gels based on rubber elasticity, our description has the important advantages of being independent of a microscopic model, of emphasizing and clarifying the role of broken symmetries in determining elastic response, and of permitting easy incorporation of spatial variations, thermal fluctuations, and gel heterogeneity, thereby allowing a full statistical-mechanical treatment of these novel materials.Comment: 21 pages, 4 eps figure

Orientational order in dipolar fluids consisting of nonspherical hard particles

We investigate fluids of dipolar hard particles by a certain variant of density-functional theory. The proper treatment of the long range of the dipolar interactions yields a contribution to the free energy which favors ferromagnetic order. This corrects previous theoretical analyses. We determine phase diagrams for dipolar ellipsoids and spherocylinders as a function of the aspect ratio of the particles and their dipole moment. In the nonpolar limit the results for the phase boundary between the isotropic and nematic phase agree well with simulation data. Adding a longitudinal dipole moment favors the nematic phase. For oblate or slightly elongated particles we find a ferromagnetic liquid phase, which has also been detected in computer simulations of fluids consisting of spherical dipolar particles. The detailed structure of the phase diagram and its evolution upon changing the aspect ratio are discussed in detail.Comment: 35 pages LaTeX with epsf style, 11 figures in eps format, submitted to Phys. Rev.

Continuum theory of elasticity and piezoelectric effects in smectic A elastomers

The general continuum free energy is derived for a liquid crystalline (smectic) elastomer under small strain and orientational distortion. Using group representation theory we obtain all invariants describing the coupling of translational and orientational degrees of freedom. Possibilities of uniform rotation of the layer system under shear deformation are outlined. It is shown that in centrosymmetric materials (smectic A) the only polarizational response is flexoelectric, that is in the response to non-uniform orientational deformations. Chiral materials (rubbers of smectic A$^\ast$) also have a uniform piezoelectric effect in response to a shear deformation, or an extension at an angle with the layer normal. Possible experimental configurations to observe some of the predicted effects are briefly discussed

Networks of helix-forming polymers

Biological molecules can form hydrogen bonds between nearby residues, leading to helical secondary structures. The associated reduction of configurational entropy leads to a temperature dependence of this effect: the helix-coil transition. Since the formation of helices implies a dramatic shortening of the polymer dimensions, an externally imposed end-to-end distance $R$ affects the equilibrium helical fraction of the polymer and the resulting force-extension curves show anomalous plateau regimes. In this article, we investigate the behaviour of a cross link ed network of such helicogenic molecules, particularly focusing on the coupling of the (average) helical content present in a network to the externally imposed strain. We show that both elongation and compression can lead to an increase in helical domains under appropriate conditions

Networks of helix-forming polymers

Biological molecules can form hydrogen bonds between nearby residues, leading to helical secondary structures. The associated reduction of configurational entropy leads to a temperature dependence of this effect: the "helix-coil transition". Since the formation of helices implies a dramatic shortening of the polymer dimensions, an externally imposed end-to-end distance R affects the equilibrium helical fraction of the polymer and the resulting force- extension curves show anomalous plateau regimes. In this article, we investigate the behaviour of a cross-linked network of such helicogenic molecules, particularly, focusing on the coupling of the (average) helical content present in a network to the externally imposed strain. We show that both an elongation and compression can lead to an increase in helical domains under appropriate conditions.Comment: Latex, 17 pages, 11 figures, final versio

Field-induced winding of chiral polymers

PACS. 61.20.Ja Computer simulation of liquid structure - 36.20.-r Macromolecules and polymer molecules - 87.15.By Structure, bonding, conformation, configuration, and isomerism of biomolecules,

“Soft elasticity” — deformation without resistance in liquid crystal elastomers

Polymeric nematic liquid crystals crosslinked into elastomers (solid liquid crystals) are shown to display novel and complex elasticity. The internal (nematic) direction can experience a barrier to its rotation which couples to standard elasticity. We predict a new phenomena unique to anisotropic rubber – a “soft elastic response”; uniaxial strain is developed without resistance below a critical deformation $\lambda^{\star}$ due to the relaxation of related shear strains and reorientation of the nematic director. We discuss possible experiments to verify this prediction and interpret the existing experimental observation in terms of the concept of “soft elasticity”