25 Novembre – Thesis defense - Goce Koleski

14 h Amphi - Centre de Recherche Paul Pascal (Pessac)

Flower-like azimuthal instability of a divergent flow at the water/air interface.

Axisymmetric flows on a water-air interface prove to be azimuthally unstable. In this thesis work, we design two setups to explore this fact : (1) a small subaquatic fountain propelling a jet against the waterair interface where it creates a centrifugal radial flow, (2) a laser – heated microbead in partial wetting at the surface of water that induces a divergent thermocapillary flow. At sufficiently high jet speeds or laser powers appears a symmetry – breaking of the toroidal base flow in the form of counter – rotating vortex pairs surrounding the source. Morphological traits of the torus and the dipole are uncovered through a wealth of laser tomography and dye injection experiments. In the water jet experiment, we show that the torus size is primarily fixed by the distance between the injector and the surface. In both experiments, the tracking of tracer particles evidences a ‘locked’ interface in the toroidal regime, whereas it ‘unlocks’ when a dipole sets in. Such a phenomenon is conditioned by surface elasticity. Cogent evidence is brought by the elastic response to laser shutdown of a surfactant layer adsorbed at the water surface. Unveiling the key role of surface elasticity in the scenario of the instability is the main achievement of this work.
On a theoretical level, we focus on thermocapillary convection induced by a fixed point source of heat sitting across the water-air interface. We solve the incompressible Stokes equation within the water – filled half – space and derive an exact solution to the advective nonlinear regime in the far – field axisymmetric limit. We then lay the groundwork on which to build a model of the instability. This thesis work paves the way for understanding how a hot microsphere found on the water surface triggers such an instability, thereby becoming an ‘active particle’ able to achieve self – propulsion at large speeds.

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