Simulations level-set d'un amas de bulles cisaillées : écoulement et dynamique des tensioactifs

Abstract : Foams are assemblies of gas bubbles in a liquid matrix that are used in several applications (building construction, ore flotation, cosmetics, food, etc). Foam is an out of equilibrium material (therefore instable) and a complex fluid intermediate between solid and liquid. Relating these macroscopic properties to foam dynamics and to foam scales (bubbles, liquid films, and surfactants) is still an open question, in particular regarding the coupling between the flow and surfactant dynamics. Because they allow to trace surfactants (difficult to do experimentally), and to deal with complex geometries (beyond the theoretical lubrication approximation), numerical simulations are a suitable tool to identify the dissipation mechanisms occurring at bubbles and liquid film scales. We thus implemented for the first time in two-phase flow level-set simulations, the surfactant dynamics –diffusion, adsorption/desorption– and its coupling with fluid mechanics –surfactant advection and stress jump across evolving interfaces. The developed code is used to simulate 2D bubble rearrangements under shear stress. These rearrangements represent the process of flow in foams, and can be the cause of foam instability. We thereby obtain a complete description of these dynamics. We study in particular the mechanical response of the system, varying the Péclet, Capillary and Biot numbers. We show that the presence of surfactants and their dynamics can affect bulk dissipation, thereby modifying the flow nature (extensional or shear flow), by changing the boundary condition at liquid/gas interfaces (mobile or rigid interfaces). The presence of surfactants can also be responsible for an effective surface dissipation, related to the work of surface forces. This dissipation is maximal for Péclet number values close to unity. Finally, we present some preliminary results regarding the effect of surface viscosity.
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Andrea Titta. Simulations level-set d'un amas de bulles cisaillées : écoulement et dynamique des tensioactifs. Mécanique des fluides [physics.class-ph]. Université Claude Bernard Lyon 1, 2017. Français. ⟨tel-01633117⟩

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