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Mechanical properties of hydrophobically coated gold nanoparticle supercrystals have been studied by small angle X-ray scattering under quasi hydrostatic pressure for various nanoparticle core sizes, ligand length and supercrystal structure.

We report on High Pressure Small Angle X-ray Scattering (HP-SAXS) measurements on 3D face-centered cubic (FCC) supracrystals (SCs) built from spherical gold nanoparticles (NPs). Dodecane-thiol ligands are grafted on the surface and ensure the stability of the gold NPs by forming a protective soft layer. Under a hydrostatic pressure of up to 12 GPa, the SC showed a high structural stability. The bulk elastic modulus of the SC was derived from the HP-SAXS measurements. The compression of the SC undergoes two stages: the first one related to the collapse of the voids between the NPs followed by the second one related to the compression of the soft matrix which gives a major contribution to the mechanical behavior. By comparing the bulk modulus of the SC to that of dodecane, the soft matrix appears to be less compressible than the crystalline dodecane. This effect is attributed to a less optimized chain packing under pressure compared to the free chains, as the chains are constrained by both grafting and confinement within the soft matrix. We conclude that these constraints on chain packing within the soft matrix enhance the stability of SCs under pressure.

International audience; Seeding-growth method for gold nanoparticles with a high level of size predictivity and a very low polydispersity.We report a seed-mediated synthesis strategy to control the size of gold nanoparticles at the atomic scale in the 2-5 nm size range. Starting from 2 nm seeds, a regrowth in organic solvent with a designed amount of precursor can achieve in a predictive fashion a precise mean size with a 0.3 nm resolution. We show that these monodisperse nanoparticles assemble into a 2D hexagonal lattice over a distance that can span tens of micrometers.

International audience; In this article, we show how advanced hierarchical structures of topological defects in the so-called smectic oily streaks can be used to sequentially transfer their geometrical features to gold nanospheres. We use two kinds of topological defects, 1D dislocations and 2D ribbon-like topological defects. The large trapping efficiency of the smectic dislocation cores not only surpasses that of the elastically distorted zones around the cores but also the one of the 2D ribbon-like topological defect. This enables the formation of a large number of aligned NP chains, within the dislocation cores that can be quasi-fully filled without any significant aggregation outside the cores. When the NP concentration is large enough to entirely fill the dislocation cores, the LC confinement varies from 1D to 2D. We demonstrate that the 2D topological defect cores induce a confinement that leads to planar hexagonal networks of NPs. We then draw the phase diagram driven by NP concentration, associated with the sequential confinements induced by these two kinds of topological defects. Owing to the excellent large-scale order of these defect cores, not only the NP chains but also the NP hexagonal networks can be oriented along the desired direction, suggesting a possible new route for the creation of either 1D or 2D highly anisotropic NP networks. In addition, these results open rich perspectives based on the possible creation of coexisting NP assemblies of different kinds, localized in different confining areas of a same smectic film that would thus interact thanks to their proximity but also would interact via the surrounding soft matter matrix.

Grazing Incidence X-ray Diffraction (GIXD) studies of monolayers of biomolecules at the air-water interface give quantitative information of in-plane packing, coherence lengths of the ordered diffracting crystalline domains and the orientation of hydrocarbon chains. Rheo-GIXD measurements revel quantitative changes in the monolayer under shear. Here we report GIXD studies of monolayers of Alamethicin peptide, DPPC lipid and their mixtures at the air-water interface under the application of steady shear stresses. The Alamethicin monolayer and the mixed monolayer show flow jamming transition. On the other hand, pure DPPC monolayer under the constant stress flows steadily with a notable enhancement of area/molecule, coherence length, and the tilt angle with increasing stress, suggesting fusion of nanocrystallites during flow. The DPPC-Alamethicin mixed monolayer shows no significant change in the area/DPPC molecule or in the DPPC chain tilt but the coherence length of both phases (DPPC and Alamethicin) increases suggesting that the crystallites of individual phases are merging to bigger size promoting more separation of phases in the system during flow. Our results show that Rheo-GIXD has the potential to explore in-situ molecular structural changes under rheological conditions for a diverse range of confined biomolecules at the interfaces., 21 pages, 11 figures, 2 tables

International audience; 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 ageing 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 nanoparticles 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 towards 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.

Optically pumped light emissions in imperfectly aligned dye-doped cholesteric cells with glance and frosted glass substrates of three different cell gap thicknesses are experimentally studied. Alignment imperfections show up in emission spectra by a broadening of the photonic bandgap (PhBG) lasing (allowed) lines at short- and long-wavelength PhBG edges and by an additional (forbidden) emission line inside the PhBG. Forbidden and allowed lines differ distinctively by their stability in the course of pumping. The origin of the forbidden line is discussed.

Hydrophobically coated gold nanoparticles have been inserted in swollen surfactant lamellar phases. Insertion of the gold nanoparticles is observed only above a critical hydrophobic thickness that depends on the particle size. Such a system can thus be used as a size filter for hydrophobically coated gold nanoparticles. Above this critical thickness, large amounts of nanoparticles can be inserted, forming a 2D fluid in each lamella. The interaction between the nanoparticles has been investigated using small-angle X-ray scattering experiments on well-oriented doped lamellar samples. A strong repulsive interaction between the particles when confined in the lamellar phase is observed. This interaction, which depends on the oil swelling rate, is highly competitive with the attractive van der Waals interaction and can thus be used to tailor the interaction between gold nanoparticles.

Slow reduction of platinum salt in soft template made by direct hexagonal mesophases leads to Pt nanoballs with hexagonal cells formed by connected nanowires. The mesophases can be doped by high concentrations of platinum (up to 0.2 M) which leads to relatively large quantities of Pt nanostructures.

International audience; The Frank-Kasper phases were already known in 1982 when quasicrystals have been discovered but their complex architectures are now considered as making a link between simple close-packed periodic structures and some quasiperiodic 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, den-dritric 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 (MgZn 2 type, also labelled C14) in superlattices of monodis-perse hydrophobically-coated gold particles at room temperature obtained from sus-pensions in various solvents. The existence of such a structure in this system is analysed 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 exhibit a Frank and Kasper phase built by one-size objects. This result opens a route towards nanoparticle superlattice with complex structures and thus original physical properties.

Interaction between hydrophobically-coated gold nanoparticles suspended in oil is usually described as the combination of strong attractive van der Waals attraction between the gold cores and interaction between the ligands. The latter interaction is expected to be purely repulsive if the suspending medium is a good solvent for the ligands or partially attractive for a bad solvent. By measuring the structure factor of interacting gold nanoparticles in various solvents, we show that the chemical affinity of the ligand with the solvent is not the only parameter that controls the interaction between the ligands and that the solvent conformation (small rigid or long flexible molecules) also plays a crucial role. Gold nanoparticles covered with hexanethiol or dodecanethiol thus undergo a larger attraction in n-dodecane or n-hexadecane compared to toluene or cyclohexane. As a consequence, self-assembly of these nanoparticles into superlattices appears at a much lower volume fraction than predicted in n-hexadecane or n-dodecane. Analogy with the behavior of polymer grafted colloids in a polymer melt is proposed to explain these unexpected results.

International audience; Ultrathin Au nanowires were prepared following two routes: the reduction of the lamellar phase [OY–AuICl] in an excess of pure oleylamine and the direct reduction of AuIII in a solution of oleylamine in hexane. The superiority of the reduction of chloride gold precursor in isotropic environment is evidenced by small-angle X-ray scattering (SAXS) measurements. A reaction yield of 75% is observed, an order of magnitude higher than the route involving the lamellar phase. On the basis of a detailed SAXS analysis, the soft template role of the [OY–AuICl] intermediate solid phase was discarded for the nanowire growth. In the case of the liquid phase synthesis, density functional theory (DFT) calculations evidence a cooperative adsorption and organization of ions pairs at the surface of the ultrathin Au NWs. We propose that this charged backbone plays a key role in the unique growth mechanism of such anisotropic objects.

The 2×2 ray tracing matrix (RTM) method is employed for the description of optical aberrations caused by the refractive index mismatch (RIM) in fluorescent confocal polarization microscopy. We predict and experimentally confirm that due to the RIM a liquid crystal layer with highly non-uniform director distribution appears to be imaged as a layer with non-uniform thickness, which shows up in the roughness of the rear surface. For the off-axial focusing of the probing beam in a droplet dispersed in an immiscible liquid, we have developed an extended method still keeping the 2×2 dimensionality of the RTM.

International audience; 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 ligand’s 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.

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.

Surfaces, like minimal surfaces, are commonly used to describe intricate thermotropic liquid crystalline phase structures: cubic phases, TGB phases, quadratic phases, blue phases, and smectic blue phases. Such geometrical models are good tools to visualize the competition between the various molecular interactions generating these phases although they often cannot lead to the prediction of the thermodynamic phase diagrams.

We report the first experimental observation of a commensurate twist-grain-boundary smectic- $A$ phase $({\mathrm{TGB}}_{A})$. The number of blocks per pitch is found to vary with temperature but is independent of the thickness of the well aligned sample. For the first time the variations with temperature of the distance between the screw dislocations ${l}_{d}$ and the grain boundaries ${l}_{b}$ have been determined in the ${\mathrm{TGB}}_{A}$ phase. The ratio ${l}_{b}{/l}_{d}$ is found to be very close to the one predicted by Renn and Lubensky. We also report the first structural study of the phase presumed to be the chiral line liquid phase, the liquid-crystal analog of the vortex liquid phase in type-II superconductors.

We report the first X-ray study on monodomains of the recently discovered blue phases with presumed smectic order. We have succeeded in growing large, well oriented monodomains of BP2. X-ray scattering performed on these monodomains clearly show that smectic order is present in these blue phases but with a smaller correlation length than in the neighbouring TGB phase. Moreover the smectic order possesses the symmetry of the blue phase lattice.

Une nouvelle sequence de phase: phase a joints de grains suivie directement par des phases bleues sans phase cholesterique a recemment ete decouverte dans un compose chiral. Nous avons analyse ce compose en rayons X pour etudier l'ordre smectique a travers la sequence de phases. Le resultat principal que nous avons obtenu est la persistence de l'ordre smectique dans les phases bleues avec une longueur de correlation inferieure au pas TGB.

We demonstrate a unique shear-induced crystallization phenomenon above the equilibrium freezing temperature in weakly swollen isotropic and lamellar mesophases with bilayers formed in a cationic-anionic mixed surfactant system. Synchrotron rheological X-ray diffraction study reveals the crystallization transition to be reversible under shear (i.e., on stopping the shear, the nonequilibrium crystalline phase melts back to the equilibrium mesophase). This is different from the shear-driven crystallization below , which is irreversible. Rheological optical observations show that the growth of the crystalline phase occurs through a preordering of the phase to an phase induced by shear flow, before the nucleation of the phase. Shear diagram of the phase constructed in the parameter space of shear rate vs. temperature exhibits and transitions above the equilibrium crystallization temperature , in addition to the irreversible shear-driven nucleation of in the phase below . In addition to revealing a unique class of nonequilibrium phase transition, the present study urges a unique approach toward understanding shear-induced phenomena in concentrated mesophases of mixed amphiphilic systems.

Une phase hexagonale orientee du systeme lyotrope ternaire, decyl sulfate de sodium/2-octanol/eau, se transforme en une phase lamellaire suite a une lente evaporation de l'eau au cours du temps tandis que le rapport molaire 2-octanol/decyl sulfate de sodium reste constant. La transformation se fait en plusieurs etapes. La sequence suivante est identifiee par diffraction de RX aux petits angles : phase hexagonale → phase hexagonale en coexistence avec une phase rectangulaire → phase cubique → phase cubique en coexistence avec une phase lamellaire → phase lamellaire. Des relations epitaxiales entre les differentes phases sont mises en evidence. Le cliche de diffraction de la phase lamellaire presente en plus des reflexions caracteristiques de cette phase, de l'intensite diffusee en dehors des reflexions de Bragg qui resulte de fluctuations de densite dans les bicouches d'amphiphiles. En accord avec les reflexions de Bragg, la phase cubique est centree et nous proposons un nouveau modele pour cette derniere. La structure rappelle celle de la phase cubique dont le groupe d'espace est Im3m, mais les tiges des deux sous-reseaux ne sont plus des cylindres comme dans la Im3m mais des rubans et dans ce cas le groupe d'espace est Im3.

L'application d'un champ electrique suffisamment intense sur des monocristaux de phases bleues BP1 et BP2 provoque l'apparition de nouvelles structures cristallines. Notamment, une meme phase BPX a symetrie tetragonale a ete observee quand le champ est dirige suivant un axe d'ordre 2 de la BP1 ou un axe d'ordre 4 de la BP2, ceci quand l'anisotropie dielectrique est positive. A partir de modeles geometriques des phases bleues bases sur l'analogie avec les phases cubiques lyotropes, nous proposons un modele geometrique de la phase BPX et un mecanisme de transformation des phases bleues BP1 et BP2 en BPX

We describe a compact point laser source based on dye-doped cholesteric encapsulated in a flexible polymer film. Optically pumped emission spectra...

International audience; We study charged nanoparticles adsorbed onto surfactant bilayers using small-angle scattering of synchrotron radiation. The in-plane interaction of the particles is well described by a DLVO component (measured independently in solution) and a repulsive dipolar interaction due to the presence of the interface, with an amplitude close to the theoretical prediction. We prove that charged nanoparticles at soft interfaces are well described by the classical model of Poisson-Boltzmann and van der Waals terms; as a corollary, they do not experience the like-charge attraction reported in the literature for some systems of micron-sized spheres at interfaces.

Smectic B phases are 3D ordered phases, but they are not perfect crystals. In these phases there exist strings of molecules, the order of which is easily disturbed by the existence of vacancies. We have shown in previous papers that a significant signature of these defects on X-ray diffraction pattems is the presence of a white diffuse line. The hexagonal symmetry of the medium generates an anisotropic deformation field around each defect. We relate the shape of the white diffuse zone to the elastic constants of the material.

International audience; A novel one-pot synthesis method to prepare gold nanorods of different aspect ratios using radiolysis is presented. The gold nanorods are synthesized by radiolytic reduction of Au-III complexes in a micellar solution formed by a mixture of cationic surfactants (cetyltrimethylammonium bromide and tetraoctylammonium bromide) and in the presence of acetone and silver ions. The mechanisms of reduction of gold complexes and formation of the nanorods are discussed. The nanorods can be trapped in a poly(vinyl alcohol) gel formed under irradiation. The UV-visible absorption spectra of nanorod solutions are compared with calculations performed by using the discrete dipole approximation method, underlying the effect of the rod aspect ratio distribution.

We study the influence of nanoparticle doping on the lyotropic liquid crystalline phase of the industrial surfactant Brij®30 (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.

Using x-ray photon correlation spectroscopy, we studied the dynamics in the nematic phase of a nanorod suspension. The collective diffusion coefficient in the plane perpendicular to the director varies sharply with the wave vector. Combining the structure factor and the diffusion coefficient, we find that the hydrodynamic function of the phase decreases by more than a factor of 10 when going from length scales comparable to the interparticle distance toward larger values. Thus, the collective dynamics of the nematic phase experiences strong and scale-dependent slowing down, in contrast with isotropic suspensions of slender rods or of spherical particles.

International audience; Using pulsed gradient spin−echo NMR, we studied molecular self-diffusion in aligned samples of a hybrid lyotropic lamellar Lα phase. This composite organic−inorganic material was obtained by doping the lamellar phase of the nonionic surfactant Brij-30 with the [PW12O40]3− polyoxometalate (POM). Both water and POM self-diffusion display a large anisotropy, as diffusion is severely restricted along the normal to the bilayers. Water diffusion in planes parallel to the bilayers does not depend on the POM concentration but depends on the lamellar period, which is due to a variable fraction of “bound” water molecules. POM diffusion in the hybrid La phase is almost 2 orders of magnitude slower than in aqueous solution. Moreover, it is not at all affected by the thickness of the aqueous medium separating the bilayers. This proves that the POM nanoparticles do not freely diffuse in the interbilayer aqueous space but adsorb onto the PEG brushes that cover both sides of the surfactant bilayers.

The X-ray diffraction patterns of mesomorphic side chain polymers in the SA Phase present diffuse streaks in shape of « butterfly wings ». We show that this diffuse scattering may be due to the presence of edge dislocations. On the basis of a previous description of edge dislocations within the framework of the elastic continuum theory of the SA fihase given by De Gennes, we have calculated the Fourier transform of the deformation field. Optical diffraction experiments on sketches of defects have also been made to reproduce the X-ray scattering patterns. Both methods show that this diffuse scattering may indeed be due to the presence of edge dislocations, Their density may be roughly estimated to some 108/cm~. The size of their cores should be only a few Angstr6ms, From the decay of their elastic deformation field, a typical length A = (K/B)~'~m1.5A can be obtained which shows that the elastic constant B of compression of the layers should be about twoorders of magnitude larger in the polymeric SA Phase than in the conventional one.

International audience; 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 Å. Systematic application of this technique should lead to a better understanding of the interaction between membrane inclusions.

International audience; 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 (Lα), 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 WV 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.

Single-crystalline platinum nanorods, monodisperse in diameter, are synthesized through a simple process at room temperature, in cetyltrimethyl ammonium bromide (CTAB) solution. The complexation of the CTA+ surfactant ion with tetrachloroplatinate in the presence of hexanol leads to the formation of a precipitate with a lamellar crystalline structure. The reduction of Pt(II) metal ions to Pt(0) is carried out using gamma radiolysis. Transmission electron microscopy (TEM) observations of the nanoparticles extracted from the solution, three weeks after radiolysis, revealed single-crystalline Pt nanorods, monodisperse in diameter (3-4 nm) and 20-60 nm long. By following the shape of the nanorods at various stages of the growth, it was found that the single-crystalline nanorods grow by coalescence of spherical seeds 3-4 nm in diameter. This suggests an aggregative mechanism similar to that recently observed for silver particles in solution.

International audience; We propose a high-resolution X-ray study of the twisted liquid crystalline phase appearing between the Twist Grain Boundary smectic A phase (TGBA) and the usual cholesteric liquid crystal N *. The structure of this new phase has been proposed as a liquid of screw dislocations in a smectic A matrix, analogous to the liquid of magnetic vortices found in type-II superconductors in between the ordered Abrikosov lattice and the normal conductor phase. Our data show that the smectic correlation length is consistent with the existence of dislocations, hence supporting the theory of Kamien and Lubensky.

Liquid crystals are phases of condensed matter in which the molecules possess long-range positional or orientational order along some directions of space even though the molecules undergo diffusion as in a simple isotropic liquid along at least one of the other directions @1#. For chiral mesogens, a spontaneous twist of the molecular orientation appears. But this local orientational order can induce frustration, sometimes resulting in complex structures. These include the blue phases ~BP! located between the cholesteric phase and the isotropic phase @2#. Two of these blue phases, BP1 and BP2, exhibit an unusual cubic symmetry in which the orientational ~but not the positional! order is periodic and long range in three dimensions. The blue phase structure involves a twist of the director ~average molecular orientation! extending not only in one direction, as in the cholesteric phase, but radially in two directions of space. This is sometimes called a double twisted structure. This double twisted structure cannot extend perfectly into three-dimensional space. Geometrical models of the BP1 and BP2 phases consist of cubic networks of double twist cylinders separated by defect lines. Thus blue phases can also be seen as a periodic array of disclination

We have measured lasing spectra from dye-doped droplets of an isotropic liquid located in a cholesteric matrix ('inverted' suspension) and an 'inverse' system, dye-doped cholesteric droplets in an isotropic liquid ('conventional' suspension), with the both systems involving the same cholesteric and isotropic liquid components. For the conventional suspension, we have collected the lasing spectra for about 200 droplets and found that the lasing lines are clearly separated into three groups located at the short-wavelength and long-wavelength edges of a photonic band gap (PhBG) and inside the PhBG. The latter ('forbidden') line is observed for all of the droplets under examination. Occasionally, it can coexist with one or both of the lines located at the short- or/and long-wavelength PhBG edges. We have shown for the first time that, for the inverted isotropic droplets in the cholesteric matrix, a single lasing line is located at the short-wavelength PhBG edge. This reveals a PhBG-nature of that line and corresponds to a zero scalar orientational order parameter for the dye molecules placed in the isotropic liquid.

Smectic blue phases $({\mathrm{BP}}_{\mathrm{sm}})$ are original physical systems of thermotropic liquid crystals, displaying a double geometrical frustration: the extension of chirality in the three spatial dimensions, such as classical blue phases, and the competition between smectic order and helical twist, such as twist grain boundary (TGB) phases. We report experimental evidence of the influence of the underlying $\mathrm{TGB}{(}_{A} or{ }_{C})$ phase on the ${\mathrm{BP}}_{\mathrm{sm}}$ structure.

Smectic blue phases ( ${\mathrm{BP}}_{\mathrm{Sm}}$) are thermotropic liquid crystalline phases which exhibit both three-dimensional orientational order, such as classical blue phases, and smectic positional order. ${\mathrm{BP}}_{\mathrm{Sm}}$ appear as the three-dimensional counterpart of twist grain boundary phases. X-ray scattering and optical polarizing microscopy provide information on the hexagonal and cubic symmetries of these new phases.

Centre de Recherche Paul Pascal, Avenue A. Schweitzer, F-33600 Pessac, France~Received 6 October 1999; revised manuscript received 23 December 1999!Smectic blue phases are liquid crystalline phases which exhibit both three-dimensional-orientational orderand smectic positional order. X-ray scattering experiments reveal that at least one of these phases is not cubic,as classical blue phases, but offers a hexagonal symmetry. A comparison of the experimental patterns with thescattering patterns given by smectic double twist tubes sketched by Kamien is proposed.PACS number~s!: 61.30.Eb, 61.10.2iI. INTRODUCTION

The amplitudes and the polarizations of the different resonant reflections characterizing the modulation of the orientational order in smectic liquid crystals are derived from the molecular tensorial structure factor. In the case of a commensurate helicoidal modulation, our conclusions are consistent with the previous predictions of Dimitrienko. We have extended Dimitrienko's prediction to incommensurate helicoidal structures and to commensurate but nonhelicoidal modulations. We have compared the estimated values for different models of modulations with the same period, with the experimental data obtained on different smectic-C variants. These comparisons enable us to discriminate between the different models.

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.

We discuss the application of in-situ rheological small angle X-ray scattering experiments to the study of complex fluids under shear, implemented using custom Couette cylinder rheometers mounted on the SWING beamline of the SOLEIL Synchrotron. We discuss several applications of this technique to the study of phase transitions in nanoparticle doped liquid crystals and shear alignment of clay suspensions. The concurrent capture of rheological and scattering data provides vital information that relates macroscopic properties such as viscosity to the microstructure of the fluid.

Blue phases are liquid crystalline phases which appear in chiral compounds between the isotropic and the cholesteric phase in a narrow range of temperature1,2. In the cholesteric phase, the periodicity which typically lies in the range of visible light wavelength is only due to orientational ordering. The mean orientation of the molecules or director n is constant inside planes perpendicular to one axis but twists when moving along this axis. This induces periodic properties of physical parameters such as the dielectric tensor in this direction with a period given by π/q where q is the cholesteric pitch. When this pitch is small enough, new phases may appear when temperature increases. These are the blue phases. There exist three blue phases : BPI, BPII and the blue fog. The two first ones are cubic phases with space group symmetry O 8 I4132 et O 2 = P432 as determined by optical scattering experiments. The third one is still mysterious3.