Olivier Mondain-Monval

Preparation and characterization of narrow sized (o/w) magnetic emulsion

The preparation of well-defined (o/w) magnetic emulsions from an organic ferrofluid is reported. The ferrofluid synthesis is first described and a complete characterization is achieved by using numerous techniques. The ferrofluid is found to be composed of superparamagnetic maghemite nanoparticles, with a diameter below 10 nm, stabilized in octane by a surrounding oleic acid layer. This magnetic fluid is then emulsified in aqueous media in order to obtain stable ferrofluid droplets. The use of a couette mixer and a size sorting step under magnetic field allowed to produce magnetic emulsion with a narrow size distribution. Morphology and chemical composition of the magnetic emulsion are investigated. Magnetic properties of both ferrofluid and magnetic emulsion are also compared and discussed. In particular, it is showed that the superparamagnetic behavior is still observed after the emulsification process.

Preparation and functionalization of (vinyl)polystyrene polyHIPE®.: Short routes to binding functional groups through a dimethylene spacer

Abstract This paper describes the preparation and post-functionalization of an emulsion-derived polymeric foam, called polyHIPE ® , bearing pendant vinyl functionalities. The large pores and large channels of polyHIPE allow liquids and solvents to be driven through the molded monolith at very low pressure. In the presence of a free-radical initiator, compounds such as HBr and thiols underwent an anti-Markovnikov addition to the residual vinyl groups. Other reactions such as hydroboration with BH 3 followed by hydrolysis with H 2 O 2 have also been investigated. In this paper, a comparison between two ways of functionalization has been made: a ‘batch’ method, corresponding to diffusion of reactants through the pores of small cubes of polymers, and a ‘continuous flow’ method, forcing reactants to flow through the interconnected porous structure by applied low pressure.

Synthesis and functionalisation of polyHIPE® beads

Discrete spheroidal particles of interconnected microcellular foams were synthesised by suspension polymerisation of a high internal phase emulsion. These particles have been surface-grafted with functionalised macromolecular chains by controlled radical polymerisation using a covalently-bond TEMPO initiator.

Preparation and functionalization of (vinyl)polystyrene polyHIPE

Abstract This paper describes the preparation and post-functionalization of an emulsion-derived polymeric foam, called polyHIPE ® , bearing pendant vinyl functionalities. The large pores and large channels of polyHIPE allow liquids and solvents to be driven through the molded monolith at very low pressure. In the presence of a free-radical initiator, compounds such as HBr and thiols underwent an anti-Markovnikov addition to the residual vinyl groups. Other reactions such as hydroboration with BH 3 followed by hydrolysis with H 2 O 2 have also been investigated. In this paper, a comparison between two ways of functionalization has been made: a ‘batch’ method, corresponding to diffusion of reactants through the pores of small cubes of polymers, and a ‘continuous flow’ method, forcing reactants to flow through the interconnected porous structure by applied low pressure.

Soft Acoustic Metamaterials

Soft materials that embed small resonators in a host material can dampen or focus sound. Resonance phenomena occur with all types of vibrations or waves and may play a part in spectacular events, such as the collapse of structures—for example, the fall of the Broughton suspension bridge near Manchester in 1831 (1). Indeed, the oscillations of a structure submitted to harmonic excitation reaches its maximum amplitude at the resonance frequency ω0 of the system. At low driving frequencies (ω < ω0), its response is in phase with the forcing but becomes out of phase just beyond (ω0 < ω). Such an out-of-phase response has been exploited with “locally resonant materials” (2). The proposed strategy is to embed a large enough collection of identical mechanical resonators in a passive structure to control wave propagation. These features are used to reach unusual macroscopic behaviors such as ultradamping of noise or negative refraction for imaging (3).

Formulation and mechanical properties of emulsion-based model polymer foams

We produce cellular material based on the formulation of model emulsions whose drop size and composition may be continuously tuned. The obtained solid foams are characterized by narrow cell and pore size distributions in direct relation with the emulsion structure. The mechanical properties are examined, by varying independently the cell size and the foam density, and compared to theoretical predictions. Surprisingly, at constant density, Young’s modulus depends on the cell size. We believe that this observation results from the heterogeneous nature of the solid material constituting the cell walls and propose a mean-field approach that allows describing the experimental data. We discuss the possible origin of the heterogeneity and suggest that the presence of an excess of surfactant close to the interface results in a softer polymer layer near the surface and a harder layer in the bulk.

Colloidal and Physicochemical Characterization of Highly Magnetic O/W Magnetic Emulsions

Abstract A study of magnetic emulsion droplets is reported using various analytical and physico‐chemical techniques. The chemical composition of the droplets (i.e., the amount of iron oxide, organic phase, and surfactant) is determined using the combination of thermogravimetry, elemental analysis, and gas chromatography. The electrokinetic properties of the magnetic emulsion are investigated as a function of pH and ionic strength in order to estimate the surface potential (ψ0) and the shear‐plane position. The droplet surface potential and charge density are deduced from electrophoretic mobility measurements. The surface potential value obtained is in good agreement with one deduced from independent force measurements performed between emulsion droplets. The emulsion stability as a function of the continuous phase ionic strength is evaluated through the experimental determination of the critical coagulation concentration (CCC). A value for the Hamaker constant (A) that quantifies the van der Waals attraction between two emulsion droplets is deduced. In addition, the colloidal force between magnetic droplets is investigated as a function of pH and salt concentration and the surface potentials obtained are compared to the measured zeta potential and deduced diffuse potentials. The results from such an analysis are consistent with those reported in the literature.

Photonic control of surface anchoring on solid colloids dispersed in liquid crystals.

The anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans-cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (trans-azobenzene) to homogeneous planar (cis-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant.

Stokes-Einstein diffusion of colloids in nematics

We report the experimental observation of anisotropic diffusion of polystyrene particles immersed in a lyotropic liquid crystal with two different anchoring conditions. Diffusion is shown to obey the Stokes-Einstein law for particle diameters ranging from 190 nm up to 2 μm. In the case of prolate micelles, the beads diffuse four times faster along the director than in perpendicular directions, D||/D[Symbol: see text] ≈ 4. In the theory part we present a perturbative approach to the Leslie-Ericksen equations and relate the diffusion coefficients to the Miesovicz viscosity parameters η(i). We provide explicit formulas for the cases of uniform director field and planar anchoring conditions which are then discussed in view of the data. As a general rule, we find that the inequalities η(b) D[Symbol: see text].

Sharp acoustic multipolar-resonances in highly monodisperse emulsions

We report the achievement of highly monodisperse emulsions exhibiting about ten acoustic Mie resonances. Thanks to robotics, the effective acoustic properties of such strongly scattering media can be precisely targeted by means of the production of calibrated (random) liquid-droplets. Ultrasonic experiments are compared, with an excellent quantitative agreement, to theoretical predictions derived within the framework of the independent scattering approximation. The dependence of the sound speed and of the acoustic attenuation on both the size and the volume fraction of droplets is quantitatively examined for dilute and more concentrated emulsions, and is presented in a dimensionless way.