We explored how to increase liquid crystal sensitivity to external magnetic fields. Suspensions consisting of a mixture of liquid crystal and spindle type monodispersed hematite nano-particles at concentrations lower than 1 wt% were prepared.The Freedericksz transition threshold for the suspensions appeared to be lower than for the pure liquid crystal. It was proved that adding canted antiferromagnetic nano-particles in liquid crystals increased their sensitivity to magnetic field while no change of the basic mesogenic properties of the matrix occurred.

By means of direct imaging, we map the surface heterogeneities of the nematic director orientation on a SiOx anchoring layer. The spatial correlations of surface director orientations are well fitted with a compressed exponential with exponent of 1.5 and typical correlation length of few microns. To discuss these results a formal analogy is established between the equation governing the nematic surface torques and the Langevin equation. Based on this analogy we prove that the disorder is spatially correlated orientational quenched disorder. The measured correlation length is discussed in terms of substrate morphology and molecular adsorption.

(en cours)

Résumé (à complèter)

Dispersing colloidal particles (1nm-1µm) in fluids has been an empirical activity for a long time, either to create new materials (colloidal dispersions, emulsion stabilization) or to modify the properties of existing materials (polymer mechanical reinforcement for example, rheological changes). In the last decade, great efforts have led to a better control of the size, the shape or the functionality of small solid particles and it is possible now to better relate their characteristics to the final materials. We will discuss here the possibilities offered by complex fluids to design and organize nano-objects, but also how to use them to enhance the properties of a complex fluid. In the first part, I will have a short review on the properties of self-organized phases of surfactants, lyotropic and thermotropic liquid crystals and consider the main problems related to the dispersions of nanoparticles. We will discuss for example the attempts to orient Single Wall Carbon Nanotubes in a nematic phase, or to achieve bidimensional layers of nanoparticles in a smectic phase. In a second part I will discuss the enhancement of the properties of a complex fluid. I will particularly focus on the dispersion of magnetic particles in a liquid crystal and the role of the shape in increasing the magneto-optical properties of the nematic phase.

Aqueous suspensions of single-wall carbon nanotubes (SWNT) are mixed with solutions of polyvinylalcohol (PVA) and dried to prepare free standing SWNT/PVA composite films (~100 μm thick). For nanotube concentrations up to 1wt %, the composites are homogeneous and the nanotubes are well dispersed both at mesoscopic scales and down to the individual scale as revealed by microscopic pictures and strong photoluminescence (PL) signals, repectively. We study the alignment of SWNT in the composites using polarized Raman and PL spectroscopies. Alignment can be continuously adjusted by hot-stretching the PVA composites. We propose a simple relation to measure the order parameter, based on the assumption that only one single component of the Raman polarizability tensor is non-zero. This assumption is validated experimentally by the observation of similar profiles for spectra measured in different polarization configurations. We modelize the orientation of the nanotubes under uniaxial stretching by a simple affine model assuming deformation at constant volume. For high nanotube concentrations, we discuss the effect of absorption on the Raman/PL results and show how it can be taken into account to measure the order parameter accurately.

Carbon nanotubes exhibit exceptional physical properties at the individual scale. In order to optimize these properties at the macroscopic scale, a good control of both the dispersion and alignment of the nanotubes is required. In this work we study the dispersion and alignment of single-wall carbon nanotubes in lyotropic nematic suspensions and we use these suspensions for the elaboration of anisotropic thin films. Individual nanotubes are stabilized by denaturated DNA in aqueous suspensions. Lyotropic nematic phases are prepared by concentrating nanotubes/DNA aqueous suspensions and the nematic is shear-aligned in thin cells. At each step of the preparation, the dispersion and alignment are studied by polarized optical microscopy, photoluminescence and Raman measurements. We show that thin homogeneous anisotropic films can be prepared from wet aligned suspensions. In a first series of experiments, the nematic and the corresponding films exhibited a quite weak order parameter (S about 0.2) [1], which was assigned to some entanglements of the nanotube network. In this talk, we will show that the order parameter of the nematic and the corresponding films can be significantly improved by controling the aspect ratio and/or the bundling of the nanotubes, for example using fractionation through controled ultra-centrifugation during the nematic phase formation. [1] C. Zamora-Ledezma, C. Blanc, M. Maugey, C. Zakri, P. Poulin, E. Anglaret. Nanoletters 8 (12), 4103-4107, (2008).