Confinement and curvature have dramatic effects in liquid crystals. For instance, when a nematic is confined between two spherical surfaces, complex defect structures arise [1]. These defect structures stem from the unavoidable topological constraints associated to the curvature of the confining space. The total topological charge of the system is fixed by the boundary conditions for the director n at the two confining spheres; however, the number and type of defects depend on energetical considerations. While changing the boundary conditions has been shown to induce fascinating transitions between different defect structures [2], the effect of changing the elastical properties of the liquid crystal has not been explored yet, despite the richness of behavior expected. In this work, we study the structural transitions undergone by a nematic shell when the ratio between the bend (K33) and splay (K11) elastic constants is changed. To produce the shells, we employ a coaxial microfluidic device, where a liquid crystal and two aqueous phases are combined to produce double emulsions [3]. The middle phase is a liquid crystal, 4-n-octyl-4-cyanobiphenyl (8CB), and the aqueous phases contain poly-vynil-alcohol (PVA), which stabilizes the double emulsions and enforces planar boundary conditions at the two interfaces. We use temperature to change the K33/K11 ratio. In the nematic phase, 40°C < T < 33°C, the energy associated to bend and splay distorsions is similar (K33 and K11 have similar values). In this situation, we observe the three defect structures typically observed with pentyl-cyano-biphenyl (5CB) [1]. However, those structures are no longer stable near the nematic-smectic transition temperature, T ~ 33°C, where K33 diverges and bend distorsions become energetically prohibited [4]. At this temperature, the system undergoes a transition in which the defects repel each other and relocate along a great circle of the sphere, corroborating existing predictions [5,6]. The topological charge and nature of the defects play a key role in this transiton. Finally, novel defect structures never reported before were observed in the smectic phase at T < 33°C. References [1] A. Fernandez-Nieves, V. Vitelli, A.S. Utada, D.R. Link, M. Marquez, D.R. Nelson and D.A.Weitz, Phys. Rev. Lett. 99, 4 (2007). [2] T. Lopez-Leon and A. Fernandez-Nieves, Phys. Rev. E 79, 5 (2009). [3] A.S. Utada, E. Lorenceau, D.R. Link, P.D. Kaplan, H.A. Stone and D.A. Weitz, Science 308, 537 (2005). [4] N.V. Madhusudana and R. Pratibha, Mol. Cryst. Liq. Cryst 89, 249 (1982). [5] H. Shin, M. Bowick and X. Xing, Phys. Rev. Lett. 101, 037802 (2008). [6] M. Bates, J. Chem. Phys. 128, 104707 (2008).

Quenched disorder refers to a disorder which is frozen; i.e., it does not change in time. In most experimental situations, this noise has a peculiar statistical signature with correlations decaying on some finite length as opposite to uncorrelated and scale invariant white noise [1]. In liquid crystals the orientational quenched disorder (OQD) profoundly affects the phase order close to a phase transition [2,3]. Unfortunately a direct characterization of these surface random fields necessary for an overall understanding of the measured phenomena is still in its early stages [4]. In this paper we show a complete set of measurement embracing different anchoring layers and liquid crystals. The obtained results support the universality of the measured OQD. We propose a possible physical mechanism which accounts for the measured OQD. In experiments the easy axis distribution was measured via the transmitted intensity trough a hybrid cell made by a hometropic counter-plate and filled with the nematic liquid crystal. Using a polarizing microscope equipped with a high resolution CCD camera we are able to measure the easy axis orientation at the pixel size (0.22 µm). The OQD on solid substrates as rubbed and photo-aligned polymers and SiO, presents the same statistical signature consisting in a compressed spatial auto-correlation function C(R) with unusually large (µm) correlation length (points in the Figure). In the model we consider the adsorption of the director fluctuation modes on the surface. The model main idea is that all fast modes fluctuating with characteristic times lower than a typical adsorption time T are averaged to zero. The model predictions agree with the experimental results by adjusting T ~= 1msec (lines in the Figure). The microscopic origin of the adsorption time will be discussed. References [1] M. A. Rubio, C. A. Edwards, A. Dougherty, and J. P. Gollub, Phys. Rev. Lett. 63, 1685 (1989). [2] T. Bellini, M. Buscaglia, C. Chiccoli, F. Mantegazza, P. Pasini, and C. Zannoni., Phys. Rev. Lett. 85, 1008 (2000). [3] M. Marinelli, F. Mercuri, S. Paoloni, and U. Zammit, Phys. Rev. Lett. 95, 237801 (2005). [4] M. Nespoulous, C. Blanc, and M. Nobili, Phys. Rev. Lett. 104, 9 (2010).

Single Wall Carbon Nanotubes (SWNTs) display remarkable anisotropic features (mechanical, optical and conductivity properties). Exploiting them at a macroscopic scale in composite materials 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. Dispersing the nanotubes at an individual scale is however not straightforward in organic solvents (and in thermotropic liquid crystals) where bundles and a reaggregation are usually observed. The use of lyotropic liquid crystals is then favored since excellent SWNTs dispersions are usually achieved in water with surfactants and strong sonication. The large amount of amphiphilic molecules however may destabilize the SWNTs, severely limiting the amount of doping. An accurate study of the orientation of individual nanotubes in a LC is therefore still an open challenge. 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. We characterized the quality of the dispersions using the strong photoluminescence, characteristic of individual tubes only. The doped nematics and their orientation were studied by polarizing microscopy, polarized Raman and photoluminescence spectroscopies. In aligned cells, both photoluminescence and Raman signals were shown to be extremely polarized with typical polarization ratios of a few tens. A quantitative approach [1,2] allowed us to determine accurately the order parameter of the tubes, which was found to be in the range 0.9-1. We will discuss the origin of such an high value. Acknowledgment: This work was partially supported by the French ANR Grant Natali. [1] C. Z. Ledezma, C. Blanc, M. Maugey, C. Zakri, P. Poulin, E. Anglaret, Nano. Lett. 2008, 8, 4103. [2] C. Z. Ledezma, C. Blanc, E. Anglaret, Phys. Rev. B 2009, 80, 113407.

Using particle tracking by optical microscopy, diffusion coefficient of polystyrene and silica particles were measured at the air/water interface. Diffusion is faster at interface than in the bulk water (D2D > D3D) because a significant fraction of the surface area of the particle is submitted to the friction of a low viscosity fluid. To quantitatively account for these experimental results it is crucial to precisely determine the immersion of the particle or equivalently its contact angle. To this purpose we coupled particle tracking and phase shift interferometry, so that the contact angle, the radius and diffusion coefficient are measured on the same particle. Small but significant and reproducible discrepancy between experimental results and theoretical models suggest a role of particle roughness and possible coupling with rotational diffusion.

Liquid crystal defects are used as probes to study the local reorientation dynamics of the nematic surface director on SiOx alignment layers. The tracking of the defect's motion reveals the presence of solid friction forces, unexpected in this complex viscous fluid. We identify the director pinning due to a surface quenched disorder as a possible mechanism that gives rise to the measured solid friction.

Nematic liquid crystals are typically composed by anisotropic molecules that orientate along a preferred direction n called the director. Due to the existence of internal order, these systems are dramatically affected by confinement and curvature. One of the most outstanding consequences is the presence of topological defects in the ground state of the system. For a spherical nematic shell, the equilibrium defect structure depends on several factors, such as the thickness of the shell, the molecular anchoring at the two confining surfaces, or the elastic constants of the liquid crystal [1, 2]. Here, we show the fascinating transitions that take place when we change: i) the geometry of the shell, and ii) the elastic constants of the nematic phase. To make spherical shells of liquid crystal we generate double emulsions using an axi-symmetric microfluidic device [3]. Both the inner and outer fluids are aqueous solutions that contain poly-vynil-alcohol (PVA), which stabilizes the double emulsion and enforces planar boundary conditions for the nematic. The middle fluid is pentyl-cyano-biphenyl, 5CB. In order to modify the original thickness of the shell, we induce a difference in osmotic pressure between the inner and outer phases, which allows us to swell or de-swell the inner droplet in a controlled way. For thin shells, we observe a continuous transition in which the defects progressively increase their separation distance as the shell becomes thinner. Conversely, we observe a discontinuous transition in thicker shells, where only two equilibrium defect structures exist. More complex transitions arise when using 4-n-octyl-4-cyanobiphenyl (8CB) as liquid crystal. In that case, the decreasing of temperature towards the nematic-to-smectic phase transition temperature provokes a dramatic change in the elastic constants of the system. References: [1] A. Fernandez-Nieves, V. Vitelli, A.S. Utada, D.R. Link, M. Marquez, D.R. Nelson and D.A.Weitz, Phys. Rev. Lett. 99, 4 (2007) [2] T. Lopez-Leon and A. Fernandez-Nieves, Phys. Rev. E 79, 5 (2009) [3] A.S. Utada, E. Lorenceau, D.R. Link, P.D. Kaplan, H.A. Stone, D.A. Weitz, Science 308, 537 (2005)