When a microparticle is trapped at a fluid interface, particle's electrical charge and weight combine to deform the interface. Such deformation is expected to affect the particle diffusion via hydrodynamics boundary conditions. Using available models of particle-induced electrostatic deformation of the interface and particle dynamics at the interface, we are able to analytically predict particle diffusion coefficient values in a large range of particle's contact angle and size. This might offer a solid background of numerical values to compare with for future experimental studies in the field of particle diffusion at a fluid interface.

Nematic twist-bend phases (N-TB) are new types of nematic liquid crystalline phases with attractive properties for future electro-optic applications. However, most of these states are monotropic or are stable only in a narrow high temperature range. They are often destabilized under moderate cooling, and only a few single compounds have shown to give room temperature N-TB phases. Mixtures of twist-bend nematic liquid crystals with simple nematogens have shown to strongly lower the nematic to N-TB phase transition temperature. Here, we examined the behaviour of new types of mixtures with the dimeric liquid crystal [4 ',4 '-(heptane-1,7-diyl)bis(([1 ',1 ''-biphenyl]4 ''-carbo-nitrile))] (CB7CB). This now well-known twist-bend nematic liquid crystal presents a nematic twist-bend phase below T approximate to 104 degrees C. Mixtures with other monomeric alkyl or alkoxy -biphenylcarbonitriles liquid crystals that display a smectic A (SmA) phase also strongly reduce this temperature. The most interesting smectogen is 4 '-Octyl-4-biphenylcarbonitrile (8CB), for which a long-term metastable N-TB phase is found at room and lower temperatures. This paper presents the complete phase diagram of the corresponding binary system and a detailed investigation of its thermal, optical, dielectric, and elastic properties.

Although the existence of the twist-bend (NTB) and splay-bend (NSB) nematic phases was predicted long ago, only the former has as yet been observed experimentally, whereas the latter remains elusive. This is especially disappointing because the NSB nematic is promising for applications in electro-optic devices. By applying an electric field to a planar cell filled with the compound CB7CB, we have found an NTB-NSB phase transition using birefringence measurements. This field-induced transition to the biaxial NSB occurred, although the field was applied along the symmetry axis of the macroscopically uniaxial NTB. Therefore, this transition is a counterintuitive example of breaking of the macroscopic uniaxial symmetry. We show by theoretical modeling that the transition cannot be explained without considering explicitly the biaxiality of both phases at the microscopic scale. This strongly suggests that molecular biaxiality should be a key factor favoring the stability of the NSB phase.

In this course we will discuss the synthesis and the characterizations of nanomaterials ina liquid environment. Nanoparticles and nanostructured materials are indeed oftensynthesized in solutions via bottom-up routes. In these approaches, chemical and physicalforces operating at the nanoscale are used to assemble basic units into larger structures ofcontrolled morphology.In a first step we will review the most important colloidal interactions and the mechanismsleading to the broad diversity of self-organized structures found in complex fluids. In asecond step we will examine in detail a few examples where monodisperse nanoparticlesor well defined nanostructured solids are obtained via reduction of salts, sol-gelapproaches etc… and how they can be stabilized in solutions.Finally the last sequence of the course will be dedicated to the description of classic tools(such as light scattering techniques, spectroscopies …) that can be used to characterizenanomaterials and follow their formation in solutions. We will present some techniquesand their underlying mechanisms through a few basic examples where the size, thestructure or the properties of the nanomaterials are monitored during their synthesis.

We report the measurement of the interaction energy between a charged Brownian polystyrene particle and an air–water interface. The interaction potential is obtained from the Boltzmann equation by tracking particle interface distance with a specifically designed Dual-Wave Reflection Interference Microscopy (DW-RIM) setup. The particle has two equilibrium positions located at few hundreds of nanometers from the interface. The farthest position is well accounted by a DLVO model complemented by gravity. The closest one, not predicted by current models, more frequently appears in water solutions at relatively high ions concentrations, when electrostatic interaction is screened out. It is accompanied by a frozen rotational diffusion dynamics that suggests an interacting potential dependent on particle orientation and stresses the decisive role played by particle surface heterogeneities. Building up on both such experimental results, the important role of air nanobubbles pinned on the particle interface is discussed.

We have investigated the wetting and surface diffusion of mesoporous colloidal silica particles at the water surface and the adsorption of cationic cetyltrimethylammonium (CTA+) surfactant on these particles. Porous silica colloids diffuse at the surface of water and in the volume, interacting with cationic surfactants that can adsorb inside the pores of the particles. We observed that surfactant adsorption on mesoporous silica depends dramatically not only on the particle pore size but also on specific counterion effects. We measured striking differences both on a macroscopic property of the interface, i.e., surface tension, and also at a single particle level by evaluating the translational diffusion of partially wetted particles at the fluid interface. We varied the pore size from 2 to 7 nm and explored the effects of ions possessing different hydration number and kosmotropic/chaotropic character. At concentrations lower than the critical micellar concentration, we evidence that cationic surfactants adsorb on silica as surface micelles and surfactant adsorption inside the pores occurs only if the pore diameter is larger than the size of surface micelles. With a view to understand the surprising different adsorption behavior of CTA+OH– and CTA+Br– on porous silica particles, we investigated the effect of counterions on the surfactant adsorption on porous silica colloids by tuning the pH and the counterion properties.

Les techniques de microrhéologie basées sur le suivi optique de particules microniques sont très intéressantes lorsqu’il s’agit de mesurer les propriétés viscoélastiques de fluides complexes de petit volume. Elles peuvent également paraître très intéressantes pour sonder localement des milieux hétérogènes. Dans le cas de milieux anisotropes comme les cristaux liquides nématiques, ces techniques doivent cependant être utilisées avec précaution. Nous l’illustrerons à travers le cas d’un cristal liquide lyotrope, le cromoglycate de sodium (DSCG) pour lequel nous avons examiné la pertinence des données rhéologiques extraites à partir de la dynamique de microparticules.Le mouvement Brownien, inhabituel, de microsphères dans la phase nématique de ce système avait en effet attiré récemment l’attention [1,2] suggérant un couplage complexe entre les fluctuations du directeur et les fluctuations de positions des billes. Bien qu’il ait été récemment proposé que des mesures par suivi de particules puissent être utilisées pour obtenir les différentes viscosités de la phase nématique [3], ce phénomène peut en effet conduire à des mesures rhéologiques incohérentes. Afin d’explorer ces différents problèmes, nous avons étudié le mouvement Brownien de particules dans des échantillons alignés en cellules minces (épaisseur 50µm) tout en variant à la fois la concentration de DSCG et la taille des particules (0.5, 1, 3, and 6µm de diamètre).Des propriétés viscoélastiques effectives ont été extraites de nos mesures de microrhéologie passive ainsi que de mesures actives basées sur l’utilisation de pinces optiques et le mouvement forcé des particules[4]. Nos résultats expliquent pourquoi différents comportements viscoélastiques ont été obtenus précédemment sur ce même système et l’origine des modules élastiques mesurés.