Gelling of Oil-in-Water Emulsions Comprising Crystallized Droplets

We fabricate oil-in-water emulsions above the melting temperature of the oil phase (hexadecane and/or paraffin). Upon cooling, the oil droplets crystallize and the initially fluid emulsions turn into hard gels. The systems evolve by following two distinct regimes that depend on the average droplet size and on the oil nature. In some cases gelling involves partial coalescence of the droplets, i.e., film rupturing with no further shape relaxation because of the solid nature of the droplets. In some other cases, gelling occurs without film rupturing and is reminiscent of a jamming transition induced by surface roughness. We prepare blends of oils having different melting temperatures, and we show that it is possible to reinforce the gel stiffness by applying a temperature cycle that produces partial melting of the crystal mass, followed by recrystallization

Dislocation-loop-mediated smectic melting

We report the direct observation, using freeze-fracture transmission electron microscopy, of topological melting in a lyotropic system in the vicinity of a smectic-cholesteric (N*) phase transition. The proliferation of dislocations leads to at least one, and possibly two intermediate phases, characterised by orientational ordering of the dislocation loops and the subsequent unbinding of the screw-dislocation lines. PAC

Probing the Density Variation of Confined Polymer Thin Films via Simple Model-Independent Nanoparticle Adsorption

International audienceAfter more than 2 decades of intense research, the density variation in confined polymer films still remains a puzzling problem subject to controversy as the methods utilized to determine the density are often model dependent. Here, we propose a direct and model independent method to detect the density/refractive index variations in polymer thin films through the adsorption of ceria nanoparticles (NPs) onto their surface. The amount of adsorbed NP scales with the polymer film refractive index; hence, any increase/ decrease in the NP surface coverage directly indicates an increase/decrease in the film refractive index and density. Experimenting our proposed novel approach on two well-studied polymers, we found that the density of polystyrene (PS) thin films deposited on oxide-free Si substrate increases with a reduction of the film thickness. On the contrary, poly(methyl methacrylate) (PMMA) films deposited on wafers with native silicon oxide show a decrease of their density when the film thickness is reduced

Surfactant-Triggered Disassembly of Electrostatic Complexes Probed at Optical and Quartz Crystal Microbalance Length Scales

A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics-based interactions via competitive binding with ionic surfactants. To this aim, free-standing micrometer-size wires were synthesized using oppositely charged poly­(diallyldimethylammonium chloride) and poly­(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the quartz crystal microbalance technique and that of the wires (>1 μm) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process

Thermally Induced Gelling of Oil-in-Water Emulsions Comprising Partially Crystallized Droplets: The Impact of Interfacial Crystals

We produced triglyceride-in-water emulsions comprising partially crystallized droplets, stabilized by a mixture of protein and low molecular weight surfactant. The emulsions were emulsified in the melted state of the oil phase and stored at low temperature (4 °C) right after fabrication to induce oil crystallization. The systems were then warmed to room temperature for a short period of time and cooled again to 4 °C. Owing to this treatment referred to as temperature cycling or “tempering”, the initially fluid emulsions turned into hard gels.Wefollowed the bulk rheological properties of the materials during and after tempering. The storage modulus, G′, exhibited a dramatic increase when tempering was applied. We showed that the systems evolved following two distinct regimes that depend on the average droplet size and on the surfactant-to-protein molar ratio. Gelling may involve partial coalescence of the droplets, i.e., film rupturing with no further shape relaxation because of the solid nature of the droplets. Alternatively, gelling may occur without film rupturing, and is reminiscent of a jamming transition induced by surface roughness. We discussed the origin of these two mechanisms in terms of the properties (size and protuberance) of the interfacial oil crystals

Surfactant-Triggered Disassembly of Electrostatic Complexes Probed at Optical and Quartz Crystal Microbalance Length Scales

A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics-based interactions via competitive binding with ionic surfactants. To this aim, free-standing micrometer-size wires were synthesized using oppositely charged poly- (diallyldimethylammonium chloride) and poly(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the quartz crystal microbalance technique and that of the wires (>1 μm) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process