Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale requires both a good dispersion of individual tubes and a control of their orientational order at a large scale. The use of a liquid crystal as a structured solvent for aligning the tubes is attractive and several studies have already examined the dispersion of SWNTs in thermotropic or lyotropic liquid crystals. In this work, we used Disodium Chromoglycate (DSCG), a chromonic liquid crystal (LCLC) to disperse a large SWNT concentration (more than 0.1 %) in an aqueous nematic phase. The doped nematics and their orientation were studied by polarized microscopy, polarized Raman and photoluminescence spectroscopies (from individual semiconducting SWNT only). A quantitative approach [1-3] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1.

In a seminal work, [1] D. R. Nelson proposed that nematic coatings could be used to promote localized functionalization on colloidal particles. Topological defects are indeed necessarily present on genus-0 surfaces and it could be possible to attach ligands to them. Such a realization, however, still remains an experimental challenge. As a first step, the fabrication of thin nematic shells from double emulsions drops was reported a few years ago in Ref. [2]. Since then, the equilibrium structure of the topological defects on a sphere has been studied in details both theoretically and experimentally. It not only depends strongly on the nematic elastic constants [3] but also on the homogeneity of the shell thickness [4-5]. The defects structure on a nematic shell can therefore be finely controlled. As a next step, we have recently examined how a small number of solid particles can be trapped on nematic shells. We have used water/5CB/water double emulsions produced by microfluidics techniques. The anchoring of the nematic at the water-5CB interface is strongly planar due to the presence of polyvinyl alcohol in aqueous solutions. The colloids are silica beads treated with silane to provide a homeotropic alignment. We especially have examined how the particles interact with the topological defects, where they get trapped, and how the equilibrium textures depend on their size. In our experiments we have observed a great diversity of defect structures and patterns, different from the ones observed in simple nematic shells. I will describe their main features and explain how they result from the competition between elasticity and capillarity effects, during the formation of the shells. I will show how this last mechanism can be used to control the position of the ligands on the shell. References: [1] D.R. Nelson, Toward a Tetravalent Chemistry of Colloids, Nanoletters, 2, 1125 (2002). [2] A. Fernandez-Nieves et al., Novel Defect Structures in Nematic Liquid Crystal Shells Phys. Rev. Lett. 99, 157801 (2007). [3] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Nematic-Smectic Transition in Spherical Shells, Phys. Rev. Lett. 106, 247802 (2011). [4] T. Lopez-Leon,V. Koning, K. B. S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Frustrated nematic order in spherical geometries, Nature Physics, 7 391 (2011). [5] D.Sec et al., Defect trajectories in nematic shells: Role of elastic anisotropy and thickness heterogeneity , Phys. Rev. E, 86, 020705(R) (2012).

Liquid crystals shells exhibit fascinating behaviors, for instance, they display topological defects in their ground states. The number and distribution of these defects depend on factors such as shell thickness, anchoring of the liquid crystal at the boundaries, or the type of liquid crystal phase considered. In this work, we study the dramatic consequences of inducing a nematic-smectic phase transition in a liquid crystal shell. Our nematic shells are double emulsions produced in a microfluidic capillary device [1]. The middle fluid is a thermotropic liquid crystal, 4-n-octyl-4-cyanobiphenyl (8CB), while the outer and inner fluids are aqueous solutions containing polyvinyl alcohol (PVA), which stabilizes the double emulsion and forces the nematic molecules to be tangentially anchored at the interface. In the nematic phase, far from the nematic-smectic phase transition temperature (TNS) we observe a configuration with four s = +1/2 defects, as predicted by theory [2]. Since experimental shells are typically heterogeneous in thickness, the four defects appear grouped together at the thinnest part of the shell in an almost square arrangement, Fig. 1(a). By contrast, when approaching TNS, the defects reorganize themselves along a great circle of the sphere, Fig. 1(g), confirming recent theoretical and simulation results [3]. The transition between these two limiting configurations proceeds continuously through a series of equilibrium states in which the defects associate in two pairs to depict the diagonals of a rhombus, Fig. 1(b) and (c). By means of numerical simulations, Fig. 1(d-f), we show that these new defect structures stem from the elastic anisotropy of the system: K3 diverges with respect to K1 when T approaches TNS. Novel types of defects, not observed in shells before, appear when the nematic order is replaced by smectic order [4]. In particular, we observe the formation of primary, secondary, and tertiary curvature walls, which provoke a wavy modulation of the smectic layers. We provide recent results which show that the number of curvature walls and the wavelengths of the associated undulations are correlated to the thickness of the shell. REFERENCES: [1] A.S. Utada, E. Lorenceau, D.R. Link, P.D. Kaplan, H.A. Stone, and D.A. Weitz, Monodisperse Double Emulsions Generated from a Microcapillary Device, Science. 308, 537 (2005). [2] T. Lopez-Leon, V. Koning, K.B.S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Frustrated Nematic Order in Spherical Geometries, Nature Phys. 7, 391 (2011). [3] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Nematic-Smectic Transition in Spherical Shells, Phys. Rev. Lett. 106, 247802 (2011). [4] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Smectic Shells, J. Phys.: Condens. Matter 24, in press (2012).

Graphene oxide (GO) flakes dissolved in water can spontaneously form liquid crystals. Liquid crystallinity presents an opportunity to process graphene materials into macroscopic assemblies with long-range ordering, but most graphene electronic functionalities are lost in oxidation treatments. Reduction of GO allows recovering functionalities and makes reduced graphene oxide (RGO) of greater interest. Unfortunately, chemical reduction of GO generally results in the aggregation of the flakes, with no liquid crystallinity observed. We report in the present work liquid crystals made of RGO. The addition of surfactants in appropriate conditions is used to stabilize the RGO flakes against aggregation maintaining their ability to form water-based liquid crystals. Structural and thermodynamical studies allow the dimensions of the flakes to be deduced. It is found that the thickness and diameter of RGO flakes are close to that of neat GO flakes.

We introduce the idea of transformation trajectories to describe the evolution of nematic shells in terms of defect locations and director field when the elastic anisotropy and the shell thickness heterogeneity vary. Experiments are compared to numerical results to clarify the exact role played by these two parameters. We demonstrate that heterogeneity in thickness is a result of a symmetry breaking initiated by buoyancy and enhanced by liquid crystal elasticity, and is irrespective of the elastic anisotropy. In contrast, elastic anisotropy--in particular, disfavored bend distortion--drives an asymmetric defect reorganization. These shell states can be both stable or metastable.

Les outils microuidiques permettent, aujourd'hui, de réaliser facilement des émulsions doubles d'épaisseur nement contrôlable. Lorsque le uide intermédiaire est d'épaisseur très ne, on peut obtenir de véritables coques uides proches de sphères bidimensionnelles modèles. Ces dernière années, nous avons exploré l'emploi de cristal liquide comme uide intermédiaire et avons étudié les singularités et instabilités qui apparaissent dans ces coques en raison du connement bidimensionel et des contraintes imposées en surface. Lorsque le cristal liquide est en phase smectique (caractérisée par un empilement de couches uides), de nombreuses instabilités macroscopiques se développent à la surface des coques [1][2]. En réponse aux contraintes géométriques, les couches forment en eet des pliures caractéristiques, qui s'organisent hiérarchiquement. Nous discuterons des propriétés géométriques des gures observées et de leur origine élastique. [1] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Nematic-Smectic Transition in Spherical Shells, Phys. Rev. Lett. 106, 247802 (2011). [2] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Smectic Shells, J. Phys. : Condens. Matter 24, in press (2012)

In a seminal paper, [1] D. R. Nelson proposed that a nematic coating could be used to promote localized functionalization on spheres by attaching ligands to the defects that necessarily arise in spherical nematics. However, using liquid crystals defects to fabricate novel colloids and novel forms of self-organization at the micron scale is still, to date, an experimental challenge. In this work, we have studied the properties of a simple experimental system which consists of micron-sized particles trapped on nematic shells. We especially have examined how the beads interact with the topological defects, where they get trapped, and how they are at the origin of new textures. We used water/5CB/water double emulsions produced by microfluidics technics. Nematic shells with a planar arrangement are obtained when PVA is added in the aqueous solutions [2]. The colloids are silica beads treated to provide a homeotropic alignment. Figure 1. Examples of textures observed in nematic shells in the presence of colloidal particles. In these shells, we observe various defect structures and patterns, different from the ones observed in simple nematic shells. We will describe their main features and explain how they result from the competition between elasticity and capillarity effects, during the formation of the shells. We will show how this last mechanism, among others [3], can be used to control the position of the ligands on the shell. References: [1] D.R. Nelson, Toward a Tetravalent Chemistry of Colloids, Nanoletters, 2, 1125 (2002). [2] T. Lopez-Leon,V. Koning, K. B. S. Devaiah, V. Vitelli and A. Fernandez-Nieves, Frustrated nematic order in spherical geometries, Nature Physics, 7 391 (2011). [3] T. Lopez-Leon, A. Fernandez-Nieves, M. Nobili and C. Blanc, Nematic-Smectic Transition in Spherical Shells, Phys. Rev. Lett. 106, 247802 (2011). _____________________________________________ * presenting author; E-mail: christophe.blanc02@univ-montp2.fr