We study nematic shells with four s=+1/2 defects and vary the elastic constant anisotropy of the liquid crystal by approaching the nematic-to-smectic phase transition temperature. We find the defects ultimately arrange themselves along a great circle, consistent with recent expectations. Changing the elastic constant anisotropy provides an alternative route to changing the shell thickness inhomogeneity for controlling the defects location.

We present in this Rapid Communication experimental evidence of an acceleration of the zenithal easy-axis dynamics of a nematic liquid crystal (NLC) with the age of a NLC-polymer layer. The comparison with other hard alignment layers strongly indicates that the polymer softness and its ability to reorient in the nematic ordering field is at the origin of the measured dynamics. The unusual acceleration of the dynamics with the polymer age is discussed in terms of this unique coupling with the NLC order. The NLC behaves like a physical plasticizer as a result of the coupling between the NLC and the polymer orders.

Le confinement et la courbure ont généralement des effets dramatiques sur les systèmes ordonnés tels que les cristaux liquides. Une de leurs principales conséquences est l’apparition de défauts topologiques qui altèrent localement l’ordre moléculaire du système. En particulier, quand un nématique est confiné dans une coque sphérique, une configuration avec quatre défauts organisés de manière tétraédrique est prédite [1]. Ces défauts pourraient être fonctionnalisés afin de créer des colloïdes tétravalents, ce qui permettrait de reproduire à l’échelle micrométrique la structure type diamant du carbone, un des défis actuels de la photonique. Malgré l’intérêt suscité par cette prédiction théorique, l’ordre nématique dans les coques sphériques n’a que peu été exploré expérimentalement. Ici, nous étudions les différentes structures des défauts observés expérimentalement dans des coques nématiques. Les coques étudiées sont des émulsions doubles générées par des techniques de microfluidique [2, 3]. La phase du milieu est un cristal liquide thermotropique, 4-n-octyl-4-cyanobiphenyl (8CB), alors que les phases intérieure et extérieure sont des solutions aqueuses contenant de l’alcool polyvinylique (PVA) qui stabilise l’émulsion double et force un ancrage planaire. En plus d’observer la structure tétraédrique originalement prédite par la théorie, nous montrons que cette structure n’est qu’une des nombreuses configurations résultant de la variation de l’épaisseur des coques [4]. Cette richesse dans la palette des configurations observées est multipliée lorsqu’on introduit, par l’intermédiaire de la température, une anisotropie entre les constantes élastiques K3 (flexion) et K1 (ébrasement) du cristal liquide. Une divergence du ratio K3/K1 empêche énergétiquement les distorsions de type flexion. En conséquence, les défauts se réorganisent autour d’un grand cercle de la sphère, confirmant ainsi les résultats récents des simulations [5]. Lorsque 1

The fascinating idea of mimicking molecular chemistry with colloids (nano-objects) has motivated the research of scientists in many fields. However, colloids typically lack the ability to bond along specific directions as atoms and molecules do, which limits the complexity of the structures that they can spontaneously form. In a seminal paper, it was suggested that liquid crystal properties could be exploited to tackle this problem [1]. The idea is to coat spherical colloidal particles with a thin layer of nematic liquid crystal and add ligands to the unavoidable defects or bald spots that arise when the nematic order is established on the surface of a sphere. Such ligands would act as linkers between particles and direct their interactions along specific directions, which are determined by the defect positions. Despite the big impact that these nano-structured systems could have in photonics or material science, this topic has been barely explored experimentally, mainly because of the experimental difficulties associated to the fabrication of a nematic shell. In the FP7-PEOPLE-2008-IEF-236091 project, we have overcome this problem by taking advantage of a novel microfluidic technique that enable a controlled fabrication of multiple emulsions, i. e., droplets containing smaller droplets inside [2,3]. Specifically, our nematic shells are double emulsions that consist of droplets of nematic liquid crystals containing a single aqueous droplet inside. When the size of the inner droplet is comparable to the size of the outer one, the aqueous core is coated by a thin nematic shell. With these experimental shells, we have not only corroborated theoretical predictions, but we have been able to systematically control the number and position of the defects formed. For thin shells, we have shown that these defects can be engineered to emulate the linear, trigonal and tetrahedral directionalities of sp, sp2 and sp3 carbon bonds [4,5]. References: [1] D. R. Nelson, Nano Lett., 2 (2002) 1125. [2] A. S. Utada, E. Lorenceau, D. R. Link, P. D. Kaplan, H. A. Stone and D. A. Weitz, Science, 308 (2005) 537. [3] A. Fernandez-Nieves, V. Vitelli, A. S. Utada, D. R. Link, M. Marquez, D. R. Nelson and D. A. Weitz, Phys. Rev. Lett., 99 (2007) 157801. [4] T. Lopez-Leon, V. Koning, K. B. S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Nature Phys., 7 (2011) 391. [5] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Phys. Rev. Lett. 106 (2011) 247802.

We study the nematic-smectic phase transition of a thermotropic liquid crystal confined to a spherical shell. Far from the nematic-smectic phase transition temperature, TNS, we observe a configuration with four þ1=2 defects, as predicted by theory. Since in this case K1 ~~ K3, the four defects are confined at the thinnest part of the shell to minimize the energy associated with the defect cores. By contrast, near TNS, where K3 >> K1, bend distortions become prohibited and the defects organize themselves along a great circle of the sphere, confirming recent theoretical and simulation results. During this structural change, the defects associate in two pairs that behave independently. In the smectic phase, we observe a new configuration displaying curvature walls.

We examine the behavior of spherical silica particles trapped at an air-nematic liquid crystal interface. When a strong normal anchoring is imposed, the beads spontaneously form various structures depending on their area density and the nematic thickness. Using optical tweezers, we determine the pair potential and explain the formation of these patterns. The energy profile is discussed in terms of capillary and elastic interactions. Finally, we detail the mechanisms that control the formation of a hexagonal lattice and analyze the role of gravity for curved interfaces.

Colloidal particles trapped at the interface between two immiscible fluids usually display various bidimensional phases depending on the competition between direct and capillary interactions [1]. On the other hand, the physics of colloids confined in a nematic cell has been thoroughly investigated in the recent years. The structures formed by embedded solid spheres strongly depend on the nematic elasticity, the nematic topological defects and the anchoring at interfaces. Only few studies are devoted to colloids at nematic interfaces [2-5] where these interactions combine. Here, we examine the behavior of spherical solid particles at nematic liquid crystal (NLC) interfaces with different geometries. First, we will detail the phase behavior of beads with a strong homeotropic anchoring at an air-NLC interface [6]. Depending on their area density, the nematic thickness and the anchoring, colloids spontanously form various structures (Fig.1). Using optical tweezers, we determine the pair potential and we probe the different roles of capillary and elastic forces. Subsequently, we report the behavior of silica beads at more complex NLC interfaces. By controlling the geometry of the interfaces and the colloids density with microfluidic techniques, several structures have been observed. The competition between anchoring conditions and liquid crystal elasticity, but also the topology of the curved surface is responsible of long-range interactions and the formation of new ordered structures. In particular we examine how the behavior of colloids trapped at the interface of a curved nematic interface modifies the overall director field (Fig.2).