Most of the potential applications in carbon nanotubes and graphene-based composites require suitable methods for making aligned assemblies on a large scale. Liquid crystal ordering is an opportunity to develop such materials and applications [1]. In this talk, we will present a review of our recent results in the preparation and characterization of lyotropic liquid crystals based on concentrated aqueous suspensions, stabilized by surfactants, of single-walled carbon nanotubes (SWNT) or reduced graphene oxide (RGO). In the first part we will focus on anisotropic conductive films, which are prepared by shearing and drying the LC. In particular, we will show how the electrical conductivity anisotropy increases with the orientational order parameter of the nematic liquid crystal. The order parameter can be tuned by controlling the length and entanglement of the nanotubes [1-2]. In the second part we present recent results on the morphology and anisotropy of thin conductive lines of SWCNT, inkjet-printed. Its Their morphology can be tuned from rail track to quasi-continuous lines by increasing nanotube concentration and drop density. The average order parameter is in the range 0.2–0.4 for all samples. The electrical resistivity is larger for rail tracks with respect to continuous layers, due to large amounts of electrical dead-ends in and between the inner edges of rail tracks [4]. Finally we will present how to prepare water-based Graphene Oxide (GO), and Reduced Graphene Oxide (RGO) liquid crystals stabilized by surfactant molecules. We will discuss their structural and thermodynamic characterizations, which provide indirect but statistical information on the organizations and dimensions of the graphene flakes [1-3].
1. C. Zakri et al, Phil. Trans. R. Soc. A. 371 (2013) 201204995(15)
2. Zamora-Ledezma, C. et al. J. Phys. Chem. Lett., 3 (17), pp 2425–2430 (2012)
3. Yuan J. et al. Nat. Commun. 6:8700 doi: 10.1038/ncomms9700 (2015).
4. F. Torres-Canas et al, Mater. Res. Express. DOI: 10.1088/2053-1591/aa5687 (2017)

N.A.

Nematic Liquid Crystals (CL) dominate the market for high-resolution display devices.Unfortunately, their relatively slow response times (> 1 ms) greatly limit their use in thedevelopment of more efficient screens (3D screens) and components (switches, couplers,filters). In this context, newly discovered Twist-Bend Nematic (TBN) similar to the usualnematic phases, but spontaneously deformed at the level of a few nanometers, due to thecurved shape of the molecules, are very promising [1]. In fact, the more rigid structure ofTBN makes it possible to achieve ultra-short electrical field response times (<1 μs). Thestudy of TBN and their potential in new applications requires to master their anchoring, i.e.their orientation on the solid confinement walls.
In this paper we present an experimental study of a newly synthesized TBN the: N,N0 -Bis(4,40-cyanobiphenymethylidene)-nonane-1,9-diamine (CBI-9-ICB), under well controlledanchoring and confinement conditions.
In the experiment, the compound is inserted by capillarity in a cell made by two parallel glassplates with few microns gap. Different anchoring conditions have been examined. We showthat rubbed polyvinyl alcohol (PVA) and Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) treated substrates give different anchoring and texture fromthat given by usual nematic as 4-Cyano-4'-pentylbiphenyl (5CB). In Figure 1 is represented atypical TBN texture observed under Polarizing Optical Microscope in a cell withhomogeneous planar anchoring obtained with rubbed PVA. Stripes parallel to the anchoringorientation having a wavelength proportional to the cell thickness are measured. The sametexture is also found in rubbed Polyimide substrates. Currently, two models are proposed toexplain this texture. The first is related to the Helfrich-Hurault instability due to the presenceof TBN pseudo-layers [2]. The second is based on the peculiar elasticity of the TBN directorfield [3]. Although it is not clear how these models could reproduce the observed lineardependence of pattern periodicity as a function of the cell gap. In Figure 2 is reported theTBN texture for silane treated glass plates that gives, for ordinary nematic, homeotropic (i.e.perpendicular to the plate) anchoring. The resulting anchoring for TBN is surprisingly planardegenerated as one can infer from the presence of half-strength disclinations. A mosaictexture with homogeneously oriented domains reminiscent of SmC mosaic also appears.Such textures will be discussed in the framework of the elastic theory of TBN.
References:
[1] C. Meyer, G. R. Luckhurst and I. Dozov, Phys. Rev. Lett., 2013,111 (6), 0031-9007.
[2 P. K. Challa,V. Borshch,O. Parri, Phys. Rev. Lett., 2014, 89 (6), 1539-3755.
[3] V. P. Panov, M. Nagaraj, J. K. Vij, Phys. Rev. Lett., 2010, 105 (16), 0031-9007.

Des solutions colloïdales sont assez facilement obtenues à partir de nanoparticules de carbone (nanotubes ou dérivés du graphène) dispersées dans l’eau [1]. L’obtention de mésophases nécessite cependant un soin expérimental dans le tri des nanoparticules et dans la qualité des dispersions. La persistance de fortes interactions attractives et d’agrégats dans une solution peuvent en effet déstabiliser lentement ces phases ou au mieux modifier fortement leurs propriétés visco-élastiques. Depuis plusieurs années, nous étudions comment obtenir à partir de ces solutions, des films et couches minces de propriétés variables en contrôlant les conditions de préparation et de dépôts (orientation des particules, texturation).
Dans ce travail, nous décrirons comment créer et stabiliser des textures périodiques de grande échelle dans des phases nématiques d’oxyde de graphène (voir fig.1). Ces textures ont été caractérisées en microscopie optique et électronique et nous discuterons de leur origine et de leur stabilité.
Références:
[1] C. Zakri, et al. Phil. Trans. R. Soc. A., , 2013, 371, 20120499.

Le confinement de microparticules dans un film mince de fluide isotrope est généralement accompagné d’une déformation des interfaces qui donne naissance à de fortes interactions capillaires entre colloïdes, de nature attractive dans la plupart des cas [1]. La situation devient différente lorsqu’il s’agit d’un film mince de fluide complexe à l’exemple d’un cristal liquide nématique. Nous avons dans un premier temps étudié et compris le comportement surprenant des particules colloïdales individuelles piégés dans un film nématique mince étalé sur un liquide isotrope [2,3].
Nous avons poursuivi l’étude du couplage capillarité/élasticité qui donne naissance à une grande richesse de phénomènes colloïdaux. A l’aide d’approches numériques (éléments finis) et d’une modélisation simplifiée de l’élasticité dans un film nématique hybride nous avons calculé les déformations et textures attendues pour des particules anisotropes, ainsi que déterminé les interactions de paires en fonction de la distance. Le calcul des positions d’équilibre mécaniques entre deux particules confinées et les observations expérimentales sont en très bon accord, ce qui valide la démarche utilisée.
[1] P. A. Kralchevsky and K. Nagayama, Adv. Colloid Interface Sci. 85, 145 (2000).
[2] H. Jeridi, M. A. Gharbi, T. Othman, and C. Blanc, Proc. Natl. Acad. Sci. 112, 14771 (2015).
[3] H. Jeridi, M. Tasinkevych, T. Othman, and C. Blanc, Langmuir 32, 9097 (2016).

N.A.

This article deals with the fundamental problem of light-matter interaction in the quantum theory. Although it is described through the vector potential in quantum electrodynamics, it is believed by some that a hamiltonian involving only the electric and the magnetic fields is preferable. In the literature this hamiltonian is known as the Power-Zienau-Woolley hamiltonian. We question its validity and show that it is not equivalent to the minimal-coupling hamiltonian. In this article, we show that these two hamiltonians are not connected through a gauge transformation. We find that the gauge is not fixed in the Power-Zienau-Woolley hamiltonian. The interaction term is written in one gauge whereas the rest of the hamiltonian is written in another gauge. The Power-Zienau-Woolley hamiltonian and the minimal-coupling one are related through a unitary transformation that does not fulfill the gauge fixing constraints. Consequently, they predict different physical results. In this letter, we provide the correct quantum theory in the multipolar gauge with a hamiltonian involving only the physical fields.