11 Septembre – Thesis defense - Hugo Chesneau

14 h LOMA Building 4N - University of Bordeaux

Modelling of a two-phase systems under acoustic or optical stress.

We numerically studied the deformation of two-phase fluid systems under acoustical or electromagnetic stress. We developed and used a numerical method based on the boundary element method to solve simultaneously the scalar Helmholtz equation and Stokes equation to consider at the same time the wave’s propagation and the hydrodynamic of the system. On the first hand, we considered the deformation of initially flat interfaces actuated by two kinds of waves’ induced stresses. First, we studied the scattering force, a bulk force induced by the propagation and the scattering of an incident electromagnetic waves into a turbid liquid phase. This force leads to the formation of eddies which in turn induce viscous stresses on the interface. Under high laser power, the viscous stresses can destabilize the interface into liquid microjets. We characterized numerically the shapes and the fluid flow of these microjets. Then, we focused on the acoustic and electromagnetic radiation pressure at the fluid/fluid interface. We showed that the induced deformations can be described by a universal model and these deformations act as liquid waveguides who are matching perfectly the incident wave which has induced them. On the other hand, we studied the deformation of micrometric droplets and liposomes with optical tweezers and optical stretcher. For droplets, we retrieved similar behaviour than for flat interfaces i.e., a coupling between the deformation of the droplets and the propagation of the wave into the latter. Additionally, we numerically characterize for the first time the liposome deformation in an optical stretcher and compared our result with recent experiments carried out in the laboratory. These initial studies are promising for the microrheology of liquid drops or biological systems.

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