B. Pansu

Frustration and related topology of blue phases

Abstract Blue Phases remained up to very recent years quite mysterious. First observations were done at the end of the last century by Reinitzer in 1888 and by Lehmann in 1906. The precise nature of these phases was a puzzling problem. At present it is firmly established that they are real phases and not textures of other phases. These Blue Phases are observed in a very narrow range of temperature in between the isotropic and the cholesteric phases. Up to now, three different Blue Phases have been discovered. They appear in the following order by increasing the temperature:

Photochromic hybrid organic-inorganic liquid- crystalline materials built from nonionic surfactants and polyoxometalates: elaboration and structural study

This work reports the elaboration and structural study of new hybrid organic-inorganic materials constructed via the coupling of liquid-crystalline nonionic surfactants and polyoxometalates (POMs). X-ray scattering and polarized light microscopy demonstrate that these hybrid materials, highly loaded with POMs (up to 18 wt %), are nanocomposites of liquid-crystalline lamellar structure (Lalpha), with viscoelastic properties close to those of gels. The interpretation of X-ray scattering data strongly suggests that the POMs are located close to the terminal -OH groups of the nonionic surfactants, within the aqueous sublayers. Moreover, these materials exhibit a reversible photochromism associated to the photoreduction of the polyanion. The photoinduced mixed-valence behavior has been characterized through ESR and UV-visible-near-IR spectroscopies that demonstrate the presence of W(V) metal cations and of the characteristic intervalence charge transfer band in the near-IR region, respectively. These hybrid nanocomposites exhibit optical properties that may be useful for applications involving UV-light-sensitive coatings or liquid-crystal-based photochromic switches. From a more fundamental point of view, these hybrid materials should be very helpful models for the study of both the static and dynamic properties of nano-objects confined within soft lamellar structures.

Growth and Self-Assembly of Ultrathin Au Nanowires into Expanded Hexagonal Superlattice Studied by in Situ SAXS

We report the self-assembly of gold nanowires into hexagonal superlattices in liquid phase followed by in situ small-angle X-ray scattering and give new insights into their growth mechanism. The unprecedented large interwire distance of 8 nm strongly suggests the stabilization of the ultrathin gold nanowires by a ligands double layer composed of oleylamine and oleylammonium chloride. The one-dimensional growth is discussed, opening perspectives toward the control growth and self-assemblies of metallic nanowires.

Repulsion Between Inorganic Particles Inserted Within Surfactant Bilayers

We study by synchrotron small-angle x-ray scattering highly aligned lamellar phases of a zwitterionic surfactant, doped with monodisperse and spherical hydrophobic inorganic particles as a function of particle concentration. Analysis of the structure factor of the two-dimensional fluid formed by the particles in the plane of the bilayer gives access to their membrane-mediated interaction, which is repulsive, with a contact value of about 4kBT and a range of 14 angstroms. Systematic application of this technique should lead to a better understanding of the interaction between membrane inclusions.

Evidence for a C14 Frank–Kasper Phase in One-Size Gold Nanoparticle Superlattices

The Frank-Kasper phases were already known in 1982 when quasi-crystals were discovered, but their complex architectures are now considered as making a link between simple close-packed periodic structures and some quasi-periodic ones. These tetrahedrally close packed structures are observed in many materials from elements to intermetallics as well as self-assembled soft materials like micellar systems, dendritric liquid crystals, star polymers, and more recently block copolymers or heated gold nanocrystal superlattices. We report here the existence of a Frank-Kasper phase with hexagonal symmetry (MgZn2 type, also labeled C14) in superlattices of monodisperse hydrophobically coated gold particles at room temperature obtained from suspensions in various solvents. The existence of such a structure in this system is analyzed in terms of geometrical parameters including gold core diameter, ligand length, and grafting density and an energetic approach based on van der Waals attraction. Hydrophobically coated gold nanoparticles is a new system that exhibits a Frank and Kasper phase built by one-size objects. This result opens a route toward a nanoparticle superlattice with complex structures and thus original physical properties.

Rheological behaviour of polyoxometalate-doped lyotropic lamellar phases

Abstract.We study the influence of nanoparticle doping on the lyotropic liquid crystalline phase of the industrial surfactant Brij30 ( C12E4 and water, doped with spherical polyoxometalate nanoparticles smaller than the characteristic dimensions of the host lamellar phase. We present viscometry and in situ rheology coupled with small-angle X-ray scattering data that show that, with increasing doping concentration, the nanoparticles act to decrease the shear viscosity of the lamellar phase, and that a shear-induced transition to a multilamellar vesicle “onion” phase is pushed to higher shear rates, and in some cases completely suppressed. X-ray data reveal that the nanoparticles remain encapsulated within the membranes of the vesicles, thus indicating a viable method for the fabrication of nanoparticle incorporating organic vesicles.

Formation of Superlattices of Gold Nanoparticles Using Ostwald Ripening in Emulsions: Transition from fcc to bcc Structure

An efficient method to form 3D superlattices of gold nanoparticles inside oil emulsion droplets is presented. We demonstrate that this method relies on Ostwald ripening, a well-known phenomenon occurring during the aging of emulsions. The key point is that the nanoparticle concentration inside the smaller droplets is increasing very slowly with time, thus inducing the crystallization of the nanoparticles into superlattices. Using oil-in-water emulsions doped with hydrophobic gold nanoparticles, we demonstrate that this method is efficient for different types of oils (toluene, cyclohexane, dodecane, and hexadecane). 3D superlattices of the nanoparticles are obtained, with dimensions reaching a hundred nanometers. The kinetics of the crystallization depends on the solubility of the oil in water but also on the initial concentration of the gold nanoparticles in oil. This method also provides an innovative way to obtain the complete phase diagram of nanoparticle suspensions with concentration. Indeed, during this slow crystallization process, a transition from a disordered suspension to a fcc structure is observed, followed by a transition toward a bcc structure. This evolution with time provides key results to understand the role played by the ligands located at the surface of the nanoparticles in order to control the type of superlattices which are formed.

Gold nanoparticles confined in lamellar mesophases

A lamellar surfactant mesophase is used as a soft confinement medium for gold nanoparticles that are directly synthesized inside the lamellar mesophase by the radiolytic reduction of a gold salt incorporated into the water medium. By increasing the water layer thickness of the mesophase, spherical gold nanoparticles of increasing size are obtained. The same soft confinement medium is used to synthesize directly in situ by radiolysis gold nanorods (aspect ratio about 10). It is also shown that gold nanorods can be inserted by simple mixing into a sufficiently swollen lamellar phase. In all cases, the structure of the lamellar phase is preserved in the presence of the nanoparticles.

A new series with smectic blue phases and SmC*-BPSm2 direct transition

A series of trifluoro-substituted benzoate derivatives: (R)-1-methylheptyl 4-[4-(4-alkyloxy3-fluorobenzoyloxy)-3-fluorobenzoyloxy]-3-fluorobenzoates is reported. The short chain members (n = 8 to 12) display the phase sequence Cr-SmC*-TGBC-TGBA-BPSm-I (except for n = 8, 9 where there is no TGBC phase), whereas for the longer ones (n = 13, 14, 16) a direct SmC*-BPSm2 transition is observed for the first time. We observe for n = 8 to 10, BPSm1 and BPSm3; for n = 11, the full set of three BPSm phases, and for n > 11, only BPSm2 and BPSm3 phases. The mesomorphic properties were studied by optical microscopy, DSC, and electrooptical, optical rotatory power and X-ray scattering measurements. The effect of the positions of fluorine atoms and their influence over mesomorphic behaviour are discussed.

Blue Phases and Periodic Minimal Surfaces

In the geometrical models of blue phases, the axes of double-twist cylinders are directed along the skeleton of some infinite periodic minimal surfaces which can therefore be seen as separating the two networks of cylinders. We show that the director field which minimizes the double-twist energy on these minimal surfaces follows the asymptotic directions (with no curvature).