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By conducting both a bottle test and isolate drop–drop experiments, we determine the coalescence rates of water droplets within water-in-oil emulsions stabilized by a large amount of Span 80 in the presence of Tween 20, a surfactant that acts as a demulsifier. Using a microscopic model based on a theory of hole nucleation, we establish an analytical formula that quantitatively predicts the coalescence frequency per unit area of droplets whose interfaces are fully covered by surfactant molecules. Despite its simplicity and the strong assumptions made for its derivation, this formula captures our experimental findings on Span 80-stabilized emulsions as well as other results, found in the literature, remarkably well on a wide range of water-in-crude oil systems.

In this work, a viscosimeter implemented on a microfluidic chip is presented. The physical principle of this system is to use laminar parallel flows in a microfluidic channel. The fluid to be studied flows side by side with a reference fluid of known viscosity. By using optical microscopy, the shape of the interface between both fluids can be determined. Knowing the flow rates of the two liquids and the geometrical features of the channel, the mean shear rate sustained by the fluid and its viscosity can thus be computed. Accurate and precise measurements of the viscosity as a function of the shear rate can be made using less than 300 L of fluid. Several complex fluids are tested with viscosities ranging from 10-3to 70 Pa·s.

Control over the synthesis of monodisperse silica particles up to mesoscopic scale is generally made difficult due to intrinsic limitation to submicrometric dimensions and secondary nucleation in seeded experiments. To investigate this issue and overcome these difficulties, we have implemented single step processing by quantifying the effects of the progressive addition of a diluted tetraethyl orthosilicate solution in ethanol on the size and monodispersity of silica particles. Contrary to particles grown in seeded polymerization, monodisperse particles with size up to 2 μm were synthesized. Moreover, the particles exhibit a final diameter (df), which varies with V^-1/3 over more than 2 orders of magnitude in rate of addition (V). On the basis of a kinetic study in the presence of addition showing that particle growth is limited by the diffusion of monomer species, we developed a diffusion-limited growth model to theoretically explain the observed df(V) behavior and quantitatively retrieve the measured amplitude and exponent. Using a single parameter procedure, we can therefore predict and generate in the room temperature range, monodisperse particles of a targeted size by simply adjusting the rate of addition.

Dissymmetric nano/microsized spheres are very appealing because controlled dissymmetry brings an additional degree of freedom for the synthesis of a new generation of materials with spatially separated chemical properties. We explore this aspect by extending to spherical surfaces the application field of lithographic techniques that was up to now essentially limited to planar and cylindrical substrates. The method proposed uses a strongly focused laser beam to generate dissymmetric coatings on microparticles by micro-photochemical deposition in a reactive solution. This is experimentally illustrated by considering the photochemical reduction of chromate ions induced by a continuous Ar⁺ laser wave to “nucleate” and grow a dissymmetry on the surface of silica beads dispersed in a chromate solution. When properly rescaled, the coating growth laws measured at different laser excitations are reduced to a single master behavior that implies a simple strategy to control and predict the desired dissymmetry from its dynamics. The versatility of the technique is then demonstrated by scanning the beam (i) to tailor microscale patterning on one hemisphere and (ii) to assemble beads into ordered structures. Owing to its flexibility, the method can easily be extended to the coating of different types of particles and various photochemical reactions.