The peculiar linear temperature-dependent swelling of core-shell microgels has been conjectured to be linked to the core-shell architecture combining materials of different transition temperatures. Here the structure of pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of temperature using small-angle neutron scattering with selective deuteration. Photon correlation spectroscopy is used to scrutinize the swelling behaviour of the colloidal particles and reveals linear swelling. Moreover, these experiments are also employed to check the influence of deuteration on swelling. Using a form free multi-shell reverse Monte Carlo approach, the small-angle scattering data are converted into radial monomer density profiles. The comparison of ‘core-only’ particles consisting of identical cores to fully hydrogenated core-shell microgels, and finally to H core/D shell architectures unambiguously shows that core and shell monomers display gradient profiles with strong interpenetration, leading to cores embedded in shells which are bigger than their isolated ‘core only’ precursor particles. This surprising result is further generalized to different core cross linker contents, for temperature ranges encompassing both transitions. Our analysis demonstrates that the internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous and strongly interpenetrated, presumably allowing only progressive core swelling at temperatures intermediate to both transition temperatures, thus promoting linear swelling behaviour.

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.

We present a class of dark matter models, in which the dark matter particle is afeebly interacting massive particle (FIMP) produced via the decay of anelectrically charged and/or colored parent particle. Given the feebleinteraction, dark matter is produced via the freeze-in mechanism and the parentparticle is long-lived. The latter leads to interesting collider signatures. Westudy current LHC constrains on our models arsing from searches for heavycharged particles, disappearing tracks, displaced leptons and displacedvertices. We demonstrate not only that collider searches can be a powerful probeof the freeze-in dark matter models under consideration, but that an observationcan lead as well to interesting insights on the reheating temperature and thuson the validity of certain baryogenesis models.

Microfluidics is one of the most fascinating fields that researchers have been trying to apply in a large number of scientific disciplines over the past two decades. Among them, the discipline of food and pharmaceutical formulation encountered several obstacles when combining microfluidics with aqueous media. Indeed, the physical properties of liquids at micrometric volumes being particular, the droplet generation within microfluidic devices is a big challenge to be met. This focus review is intended to be an initiation for those who would like to generate microdroplets in microfluidic systems involving aqueous continuous phases. It provides a state-of-the-art look at such systems while focusing on the microfluidic devices used, their applications to form a wide variety of emulsions and particles, and the key role held by the interface between the device channels and the emulsion. This review also leads to reflections on new materials that can be used in microfluidic systems with aqueous continuous phases.