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.
An original approach is proposed here to reversibly tune Mie scattering resonances occurring in random media by means of external low induction magnetic fields. This approach is valid for both ...electromagnetic and acoustic waves. The experimental demonstration is supported by ultrasound experiments performed on emulsions made of fluorinated ferrofluid spherical droplets dispersed in a Bingham fluid. We show that the electromagnet-induced change of droplet shape into prolate spheroids, with a moderate aspect ratio of 2.5, drastically affects the effective properties of the disordered medium. Its effective acoustic attenuation coefficient is shown to vary by a factor of 5, by controlling both the flux density and orientation of the applied magnetic field.
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.
A process to synthesize silica mesoporous capsules is reported (see Figure), where mineralization takes place at the oil–water interface of reverse emulsion droplets. The capsules exhibit an ...homothetic morphology. Simple scaling analysis reveals that mineralization takes place above a well‐defined hydrolyzed precursor concentration within the droplets.
This review describes the results obtained in the determination of the nanometric structures of liquid crystal systems using the technique of freeze fracture transmission electron microscopy ...(FF-TEM). I focus the review on two types of major results that were obtained with this technique in respectively lyotropic and thermotropic liquid crystals systems.
The aim of this study is to limit the hurdles generated by the presence of a surfactant, i.e., sodium dodecyl sulfate (SDS), in effervescent detergent tablets containing a chlorine provider. The ...results are highlighted by investigating the tablet’s functional characteristics (mechanical strength, disintegration time). A second objective is to increase the surfactant content of the tablet in order to improve the cleaning properties of the detergent formula without retaining the previous drawbacks. For low tablet porosity, mechanical properties are damaged by the presence of 2% of SDS and while disintegration through an erosion mechanism is slowed down. Experimental evidence indicated that these phenomena are associated with the coexistence of SDS and sodium dichloroisocyanurate (DCCNa). Their separation by locating SDS in the tablet core was encouraging but had limited value due to the slow dissolution of the SDS core. The problem was solved when 2% SDS was concentrated on one face of the tablet; however, a higher concentration induced a delayed disintegration due to the progressive erosion of SDS, which behaved as a massive solid. The coating of the tablet with SDS was beneficial because the dissolution of the film delayed effervescence and consequently disintegration. Neither coating the SDS particles with cellulosic film nor including them in zeolite was an appropriate solution. On the other hand, segregating SDS and DCCNa by placing them in separate layers of the tablet produced very conclusive results when microcrystalline cellulose and an effervescent system were added to the SDS. Furthermore, this bilayer tablet allowed the SDS content to be increased while a satisfactory tensile strength and a low disintegration time were retained.
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 (ψ
0
) 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.
We report on the structures exhibited by two different diblock poly(styrene)-b-poly(acrylic acid) (PS-b-PAA) copolymers in water, a selective solvent. Using a combination of X-ray scattering and ...freeze fracture-transmission electron microscopy (FF-TEM), we show that these structures can be widely swollen while retaining their initial morphology and a high degree of long-range order. The analysis of the FF-TEM pictures also evidences the presence of water crystallites of regular size and shape within the confined water domains. We relate the growth of these crystallites to the high local ionic strength of the water swelling the PAA brushes. Moreover, the confinement of the crystallites growth shows that the swollen phases have a very robust structure, potentially useful for confining colloidal particles.
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