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We investigate experimentally the freezing of a rivulet, obtained by flowing water on a cold solid substrate (0.C to -35.C). Surprinsingly, we show that a peculiar 3D structure is formed, composed of an ice pyramidal base-layer on which a vertical wall develops.
This structure grows until reaching a stationary shape after tens of seconds. This stationary state is shown to result from the competition between the water cooling and its convection as the growing ice layer lowers the cooling ability of the substrate.
In these studies, we were interested in the acoustical response of microbubbles covered with particles. The bubbles are around 100 um diameter and the particles between 1-10 um.
We first looked at the shape oscillations of the bubbles and identified a synchronization mechanism between different modes of deformation. This effect leads to greater local deformation of the interface and ultimately can trigger the desorption of particles in specific locations.
In a second study, we tried to look precisely at the dynamic organization of the particles at the interface for gentle oscillations. We discovered the formation of a network of strings at the surface for sufficiently low coverage. It results from a subtle combination of capillary interactions (statics but also due to inertial deformation) and hydrodynamics interactions (streaming around the particles). We were able to reproduce the experimental results with numerical simulations.
In these studies, we were interested in the compressive behaviour of particle-laden interfaces. Using X-rays, we probed the adsorption and rearrangements occuring at an oil-water interface with ligand-grafted nanoparticles adsorbed. We showed a two-stage adsorption mechanism. Under compression at different speeds, the interface behaved differently. For slow compression, the ligands have time to rearrange and allo for smaller interparticle distances before buckling of the monolayer.
We also looked at the temperature-induced dissolution of bubbles covered with clay particles. Those bubbles were shown to be extremely stable (tens of hours) and resistant to complex temperature variations.
A question as simple as predicting the droplet velocity while pushed by an external fluid at fixed velocity is still not answered. Understanding and thus modelizing it requires the identification of dissipation mechanisms in the droplet, in the dynamical meniscus and in the flat film.
We studied the dynamical properties of lubrication films using an interferometric method (RICM) that has been adapted to microfluidics. We first showed that, in a static case, we are able to measure nanometric film thicknesses with very accurate precision and that it is set by the disjoining pressure, especially the electrostatic part. Then the film was studied when the droplets flow. At low speeds, the film thickness is set by the disjoining pressure, while at higher capilarry numbers we identified a viscous model in agreement with our experimental results. For a micellar solution, we observed spinodal decomposition allowing us to recover interfacial properties (velocity, Marangoni stress).
We were able to develop a lab on a chip allowing droplets manipulations taking advantage of micro-heaters integration. The deformation of the PDMS induced by the heat allowed for propulsion, orientation, sorting of droplets.
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