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.

Le comportement collectif de particules colloïdales piégées à l'interface de fluides immiscibles dérive fréquemment d'une compétition subtile entre interactions directes et interactions capillaires [1]. Si la physique des colloïdes micrométriques confinés dans des cellules nématiques a été largement abordée ces dernières années, seules quelques études expérimentales ont porté sur le comportement de particules piégées aux interfaces des nématiques [2-5]. Ces études ont donné lieu à des interprétations parfois contradictoires que nous avons voulu réexaminer. Nous avons en particulier déterminé les diagrammes de phases de particules sphériques piégées à l'interface libre d'un nématique. Selon la densité surfacique des billes, l'épaisseur des couches nématique et l'ancrage, plusieurs structures sont observées (figure 1). Les potentiels de paires ont été déterminés à l'aide de pièges optiques et permettent d'expliquer la formation de telles structures. Nous discuterons finalement des mécanismes possibles à l'origine d'un tel potentiel.

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.

Liquid crystal ordering is an opportunity to develop novel materials and applications with carbon nanotubes spontaneously aligned on macroscopic scales. Nevertheless, achievement of large orientational order parameter and extended monodomains remains challenging. In this work, we show that shortening nanotubes allows the formation of liquid crystals that can easily be oriented under the form of large macroscopic monodomains. The orientational order parameter of single-wall nanotube liquid crystals measured by polarized Raman spectroscopy at the isotropic-nematic transition exceeds by far the value reported in previous experiments. The presently measured order parameter approaches the value theoretically expected for liquid crystals made of rigid rods in solution. This finding suggests that the production of highly ordered nanotubebased liquid crystals was presumably limited in earlier contributions by the length and waviness of long nanotubes. Both factors increase the material viscosity, can yield some elasticity, and stabilize topological defects.

We report the synthesis of novel chiral catanionic liquid crystals bearing camphorsulfonamide substructures. The phase behaviour of these long-chain substituted imidazolium sulphates and sulfonates was investigated using X-ray diffraction (XRD), polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). We observed that the phase behaviour clearly depends on the substitution of both cation and anion. The chiral camphorsulfonamide substructures have an unfavourable influence on the formation of liquid crystalline (LC-) phases. Contrary to N,N0-di-alkyl-imidazolium salts, the formation of LC phases was only observed when both cation and anion are substituted with long alkyl chains (C12 or C16). Furthermore, the phase transition temperatures depend on the chain length of the alkyl groups, as higher phase transition temperatures were observed for compounds bearing longer alkyl chains. However, no macroscopic evidence for the formation of chiral mesophases was obtained.

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).