Capillary-induced giant elastic dipoles in thin nematic films

Abstract

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...

Phase-field model for elastocapillary flows of liquid crystals

“…We consider two-phase flows of a nematic liquid crystal (LC) and an immiscible isotropic fluid under coupled elastic stresses of the LC and capillary forces of the interface. Understanding such flows is important for a number of fundamental and applied scenarios, such as drop dynamics with a nematic drop or host [e.g., 1-5], selfassembly of soft colloids in a nematic phase [e.g., 6,7], elastocapillary interaction of particles on LC interfaces [e.g., [8][9][10], and dynamics of biological matter and cells [e.g., [11][12][13]. The presence of an interface leads to the interplay among surface tension, the anchoring of LC molecules, and the bulk elasticity, and gives rise to novel dynamics.…”

Colloidal Particles in Thin Nematic Wetting Films

“…This texture was characterized by a preferred radial director field around the particle (promoted by a perpendicular anchoring of the nematic director at the bead surface) and the presence of a topological defect at far distance (typically a hundred times the particle size), mandatory to restore a uniform director field far away. We also showed that simple 2D approximations of the free interfacial and elastic energies were sufficient to capture the evolutions of both this “giant dipole” texture and the distortions with the film thickness …”

“…These elastic effects strongly modify the colloidal behavior with respect to the one observed in simple fluids, mainly under capillary forces. We have, for instance, recently shown that the elastic distortions of a thin nematic film caused by the presence of solid particles trapped at both interfaces were sufficient to prevent their expected capillary aggregation . In a thin nematic film, the orientational elasticity changes the usual picture of a simple fluid film.…”

Elastocapillary Driven Assembly of Particles at Free-Standing Smectic-A Films

“…Studies of the dynamics of rodlike particles at the free surface of thin hybrid nematic films reveal complementary roles for elasticity and capillarity on the assembly of cylindrical particles. For particles trapped in thin nematic films, long-range interactions mediated by elastocapillarity occur via the formation and interactions of novel defect configurations. , While these recent works focus on capillarity at nematic interfaces, little is known about capillary effects at more complex LC phases such as smectic films. An overview of experimental and theoretical progress on understanding colloidal interactions in liquid crystals, such as nematic and smectic phases, can be found in refs and .…”

“…For particles trapped in thin nematic films, long-range interactions mediated by elastocapillarity occur via the formation and interactions of novel defect configurations. 39,40 While these recent works focus on capillarity at nematic interfaces, little is known about capillary effects at more complex LC phases such as smectic films. An overview of experimental and theoretical progress on understanding colloidal interactions in liquid crystals, such as nematic and smectic phases, can be found in refs 41 and 42.…”