International audienceThe mechanical properties of particle laden interfaces are investigated by studying capillary wave propagation along the interface. Interfaces are coated with monodisperse silica hydrophobic particles with diameters ranging from 35 mm to 159 mm. The surfaces are prepared with a particle density just above that required for random close packing, so that no macroscopic wrinkles can be observed. Measurements of wave celerity versus wavelength and a comparison of the results with theory allow us to define the stretching modulus and the bending stiffness of such an interface. These quantities are for the first time measured independently and under a large range of frequencies (100 Hz-900 Hz). Particle diameters and contact angles have been varied systematically. We observe that the stretching modulus does not depend on the particle size, while the bending stiffness exhibits variations with the square of particle diameter. These results are confronted with previous results on quasistatic assays and with theoretical predictions on the capillary origin of bending stiffness, showing a good agreement. Similarities and differences are discussed
International audienceBinary collisions of drops of immiscible liquids are investigated experimentally at well-defined conditions of impact. In the experiments we vary all relevant properties of an aqueous and an oil phase, the impact parameter, the drop size and the relative velocity. The drops observed after the collisions exhibit three main phenomena: full encapsulation, head-on fragmentation, and off-centre fragmentation. The regimes characterized by these phenomena replace the ones observed in binary collisions of drops of the same liquid: coalescence, reflexive separation, and stretching separation. Our aim is a universal description of the two fragmentation thresholds of such collisions. Based on the capillary instability and an energy balance, we establish for head-on collisions a scaling law for the evolution of the threshold impact velocity with the properties of the liquids and the droplet size. The fragmentation threshold for off-centre collisions is compared to established models from the literature, which appear unsatisfactory. Introducing an effective impact parameter, which accounts empirically for the deformation and rotation of the drops upon impact, we describe this fragmentation threshold in a universal way. For both fragmentation thresholds, the agreement between experimental data and their theoretical representation is very good. Our work yields new insight into binary collisions of drops and proposes a perspective to develop a more general description with implications for binary collisions of drops of a single liquid as well
International audienceArmored interfaces refer to fluid interfaces on which a compact monolayer of particles is adsorbed. In this paper, we probe their robustness under impact. For such an investigation, the impact of a drop (covered or not by particles) on a flat armored interface is considered. Two regimes are observed: small drops impacting at low velocities do not coalesce, while bigger drops falling at higher velocities lead to coalescence. The coalescence which occurs when the impacting drop has just reached its maximum extension directly results from the formation of bare regions within the armor. We therefore propose a geometric criterion to describe this transition. This simple modeling is able to capture the dependence of the measured velocity threshold with particle size and drop diameter. The additional robustness experienced by double armors (both drop and puddle covered) results in an increase of the measured velocity threshold, which is quantitatively predicted. © 2013 AIP Publishing LLC
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