In this article, we show how advanced hierarchical structures of topological defects in the so-called smectic oily streaks can be used to sequentially transfer their geometrical features to gold nanospheres. We use two kinds of topological defects, 1D dislocations and 2D ribbon-like topological defects. The large trapping efficiency of the smectic dislocation cores not only surpasses that of the elastically distorted zones around the cores but also the one of the 2D ribbon-like topological defect. This enables the formation of a large number of aligned NP chains, within the dislocation cores that can be quasi-fully filled without any significant aggregation outside the cores. When the NP concentration is large enough to entirely fill the dislocation cores, the LC confinement varies from 1D to 2D. We demonstrate that the 2D topological defect cores induce a confinement that leads to planar hexagonal networks of NPs. We then draw the phase diagram driven by NP concentration, associated with the sequential confinements induced by these two kinds of topological defects. Owing to the excellent large-scale order of these defect cores, not only the NP chains but also the NP hexagonal networks can be oriented along the desired direction, suggesting a possible new route for the creation of either 1D or 2D highly anisotropic NP networks. In addition, these results open rich perspectives based on the possible creation of coexisting NP assemblies of different kinds, localized in different confining areas of a same smectic film that would thus interact thanks to their proximity but also would interact via the surrounding soft matter matrix.
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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