Title: Multiscale Modeling and Microfluidic Study of Particle-Laden Emulsions and Foams
Description: The aim of this thesis is to gain new insights into the physics underlying the long-term stability and instability of liquid foams and emulsions in the absence and presence of particles. By using Finite element based and mesoscopic Lattice Boltzmann techniques along with the microfluidic tools at our disposal, we tackled this question using two very different, yet complementary, approaches. In the first part, we went down to the smallest scale of foam, by observing a single bubble where the particle would straddle at interfaces of thin films. This brought a novel understanding to the observation that the torque on the particle is independent of film thickness and was mainly contributed by contact line stresses. We then precisely measured the hydrodynamic and dielectrophoretic interactions of a particle armored bubble treating the bubble as a flat surface and showed that its resistance to the motion was much less for hydrophobic particles compared to other wetting particles while the dielectrophoretic forces were more for hydrophobic particles as the latter protruded more in the oil phase.

These findings are of utmost importance when designing particle-stabilized foams and dielectrophoresis-based particle separation techniques because they guide the choice of the particles to use for a particular application. In the second part, we studied the foam at a larger scale, by analyzing the evolution of a large population of identical bubbles produced in microfluidic geometries. This monodisperse foam destabilizes through Ostwald ripening or Coarsening toward a well-known self-similar state. However, we have shown that the transient regime leading to that state is not homogeneous in space. The microfluidic model that we develop predicts how the disorder grows in the foam, which is a valuable asset in applications where an ordered organization of the bubbles is required resisting foam coarsening. Furthermore, multiscale Lattice Boltzmann simulations of emulsion drainage based on frustrated long-range interactions are developed using the images from the microfluidic experiment as the initial phase thus providing a global understanding of emulsion stabilization and drainage dynamics.

The key parameters investigated for particle-induced emulsion stabilization were solid particle concentration, particle size, wettability, heterogeneity and particle shape. The resulting emulsion droplets adopted pronounced non-spherical polyhedral shapes with time, indicating a high elasticity of the interface. The stability and the remarkable non-spherical shape of the emulsion droplets stabilized by the particles were features which bear resemblance with foam stabilization of bubbles using hydrophobic particles in flotation processes.