Stripe Domain Phase of a Thin Nematic Film and the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>K</mml:mi></mml:mrow><mml:mrow><mml:mn>13</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>Divergence Term

Lavrentovich, Oleg D.; Pergamenshchik, V. M.

doi:10.1103/physrevlett.73.979

Cited by 121 publications

(125 citation statements)

References 27 publications

(37 reference statements)

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“…[A periodic striped texture has been observed in very thin (<1 m) nematic films due to surface gradient terms in the elastic free energy expression [14,15]. Our samples are thick enough that the surface gradient terms are not significant.]…”

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confidence: 83%

Buckling Instability in Liquid Crystalline Physical Gels

et al. 2006

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In a nematic gel we observe a low-energy buckling deformation arising from soft and semisoft elastic modes. We prepare the self-assembled gel by dissolving a coil -side-group liquid-crystalline polymercoil copolymer in a nematic liquid crystal. The gel has long network strands and a precisely tailored structure, making it ideal for studying nematic rubber elasticity. Under polarized optical microscopy we observe a striped texture that forms when gels uniformly aligned at 35 C are cooled to room temperature. We model the instability using the molecular theory of nematic rubber elasticity, and the theory correctly captures the change in pitch length with sample thickness and polymer concentration. This buckling instability is a clear example of a low-energy deformation that arises in materials where polymer network strains are coupled to the director orientation. DOI: 10.1103/PhysRevLett.96.147802 PACS numbers: 61.30.ÿv, 61.41.+e, 82.70.Gg Liquid-crystalline elastomers and gels combine the orientational order of liquid crystals and the translational order of a polymer network. The coupling of these two effects in liquid-crystalline elastomers and gels gives rise to unique phenomena such as soft elasticity in which certain director rotations and network strains cost no free energy in the ideal case. Soft elastic modes are predicted and well understood theoretically by a molecular theory of nematic rubber elasticity [1][2][3]. However, a significant limitation in testing existing theories is the difficulty in preparing well-characterized networks. Many approaches have been undertaken to prepare liquid crystal elastomers, including free-radical polymerization of bulk liquid crystal acrylates and diacrylates [4], free-radical polymerization of diacrylates in the presence of liquid crystals [5,6], and side-coupling reactions with poly(methylsiloxane) [7]. Unfortunately, these approaches result in nematic elastomers and gels with heterogeneities in the polymer network and, in the case of many gels, phase separation of the polymer network from the liquid crystal host [8,9]. More recently, self-assembly of block copolymers has been used as an approach to create model liquid crystal networks [10 -12]. We have utilized self-assembly of a triblock copolymer that has a narrow molar mass distribution and a very long midblock to produce well-defined nematic gels [10,11]. This gel facilitates comparison to present theories which assume the material is lightly cross-linked and single phase.In this Letter, we present a study of the effects of nematic rubber elasticity in thin layers of uniformly aligned gels contained between rigid parallel plates. Using polarized optical microscopy, we observe a banded texture that arises in response to small temperature changes. We propose that this texture represents a buckling deformation made possible by the presence of soft and semisoft elastic modes. We formulate a description based on the molecular theory of nematic elastomers that incorporates soft and semisoft elasticity as well as l...

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confidence: 83%

Buckling Instability in Liquid Crystalline Physical Gels

et al. 2006

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“…Thick films are expected to show a simple and homogeneous structure in the layer plane (defining a 2D c director sketched in Fig. 1B) but when the thickness h decreases to a fraction of microns, a spatial xy modulation of the director field and periodic domain patterns, due to an elastic instability, are observed (22,23). We checked that the particle-free films deposited on PVA/water behave similarly.…”

Section: Significancementioning

confidence: 97%

“…We checked that the particle-free films deposited on PVA/water behave similarly. The simple uniform texture is observed down to a thickness of ∼ 0.4-0.5 μm and thinner films display also periodical instabilities (22). In the following, we mostly focus on films whose thickness is between 0.4 μm and 3 μm.…”

Section: Significancementioning

confidence: 99%

Capillary-induced giant elastic dipoles in thin nematic films

Jeridi

Gharbi

Othman

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Directed and true self-assembly mechanisms in nematic liquid crystal colloids rely on specific interactions between microparticles and the topological defects of the matrix. Most ordered structures formed in thin nematic cells are thus based on elastic multipoles consisting of a particle and nearby defects. Here, we report, for the first time to our knowledge, the existence of giant elastic dipoles arising from particles dispersed in free nematic liquid crystal films. We discuss the role of capillarity and film thickness on the dimensions of the dipoles and explain their main features with a simple 2D model. Coupling of capillarity with nematic elasticity could offer ways to tune finely the spatial organization of complex colloidal systems.A mong the various systems proposed heretofore for bottomup assemblies of solid particles, nematic liquid crystal (NLC) dispersions have attracted a lot of attention. Such systems indeed promote complex anisotropic 2D patterns that can easily resist thermal fluctuations and external perturbations (1-3). The mechanisms responsible for self-assembly in nematics are now well understood. The nematic phase is a fluid with an orientational order, in which locally the molecules spontaneously align in a common direction, the director n. The imposed orientation of the molecules at the surface of solid particles (the so-called anchoring phenomenon) creates a large-scale distortion of the field nðrÞ. Orientational elasticity, far-field alignment, and topological constraints then yield the formation of topological defects associated to the particles (4, 5). A typical system that has been extensively studied is a microsphere with a perpendicular (homeotropic) anchoring immersed in a thin planar-aligned nematic cell. Topologically equivalent to a hedgehog defect for the director field nðrÞ, the spherical particle necessarily creates other topological defects in the uniform far-field director, such as a hyperbolic hedgehog point defect or a Saturn-ring defect (a closed disclination line). The particle-defect pair forms a neutral unit and is stable contrary to a couple of true topological defects that would annihilate. The self-assembly of particles is then explained by the residual distortion of the matrix far from the pair, which yields long-range elastic interactions between particles (5, 6). Whereas the far-distance distortion can be always treated asymptotically, the local director field around the particle depends on several parameters, such as the size and shape of the inclusion or the finite strength of the anchoring, and is accessible only via numerical approaches (7). In all cases, one observes a defect located at a close distance from the particle (of order of its size). Such a scheme is found with particles of different shapes and sizes (8, 9): When introduced in a NLC, a microparticle generates elastic distortions and topological defects in its close neighborhood, i.e., at a distance comparable to its size, and we call this particle-defect pair a common short dipole.In this paper...

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“…This assumption is reasonable: for a metal-liquid crystal boundary, a suitable surface treatment can modify the boundary condition to the tangential boundary condition (director parallel to the surface) [1,11]. Also if the coating is thin, we can assume a tangential boundary condition at the liquid crystal-air interface [12]. Since the anchoring force at the nanoparticle surface can be modified by suitable surface preparation, the director may sometimes satisfy a more general boundary condition, which will not, however, be considered in this Letter.…”

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confidence: 99%

Surface-Enhanced Plasmon Splitting in a Liquid-Crystal-Coated Gold Nanoparticle

Park

Stroud

2005

Phys. Rev. Lett.

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We show that, when a gold nanoparticle is coated by a thin layer of nematic liquid crystal, the deformation produced by the nanoparticle surface can enhance the splitting of the nanoparticle surface plasmon. We consider three plausible liquid crystal director configurations in zero electric field: boojum pair (north-south pole configuration), baseball (tetrahedral), and homogeneous. From a calculation using the Discrete Dipole Approximation, we find that the surface plasmon splitting is largest for the boojum pair, intermediate for the homogeneous, and smallest for the baseball configuration. The boojum pair results are in good agreement with experiment. We conclude that the nanoparticle surface has a strong effect on the director orientation, but, surprisingly, that this deformation can actually enhance the surface plasmon splitting.Colloidal suspensions and emulsions in anisotropic host media, or coated with anisotropic media such as liquid crystals have attracted considerable recent interest, since these systems exhibit a novel class of colloidal interactions [1] or the possibility of forming tetravalent binding sites of colloids [2]. The alignment of the principal axis of anisotropic media (known as the director) around a colloidal surface is a important feature; this new colloidal interaction comes from homeotropic alignment, perpendicular to the surface, while tetravalent binding sites from tangential alignment to the surface.These systems have also been studied for optical applications [3,4,5,6]. For example, related to the emerging nanotechnology, a recent experiment has demonstrated nanoscale control of optical properties on a length scale much less than a light wavelength [5]. The authors observed the surface plasmon splitting of a gold nanoparticle coated with a thin layer of nematic liquid crystal: the surface plasmon resonance frequency in the scattering cross section was shown to depend on the angle between the director and the polarization of incident light. They also showed that, by rotating the director using a static electric field, they could control the change in surface plasmon frequency.A recent calculation [6] showed that this method could indeed produce a measurable change in the surface plasmon frequency, assuming that the director was oriented in the same direction everywhere within the coating or the host media. But in reality, the director field should be influenced near the metal surface [7], and thus the alignment of the liquid crystal director field near the nanoparticle surface may reduce the surface plasmon splitting to hinder some possible applications of this system which involve electrical control of its optical properties. Thus, it is important to investigate quantitatively how an inhomogeneous director field may influence the optical properties of these coated metal particles.In this Letter, we report on novel optical phenomena arising from the surface deformation (or alignment) of the liquid crystal director field. Our main finding is that this deformation can actually enha...

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Stripe Domain Phase of a Thin Nematic Film and theK13Divergence Term

Cited by 121 publications

References 27 publications

Buckling Instability in Liquid Crystalline Physical Gels

Buckling Instability in Liquid Crystalline Physical Gels

Capillary-induced giant elastic dipoles in thin nematic films

Surface-Enhanced Plasmon Splitting in a Liquid-Crystal-Coated Gold Nanoparticle

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