20 Novembre – Thesis defense - Raphaël Saiseau

14 h Room 2015 - LOMA (Talence)

Thermo-hydrodynamics in critical systems : instabilities, relaxation and evaporation.

To form a drop at the tip of a liquid column, a pinching process has to occur until it reaches the atomic scale at the final break-up, covering all length scales. Some recent experimental and theoretical results show that this common phenomenon is still poorly understood when the pinching reaches the thermal fluctuations length scales. Here, we try to deepen our understanding by using phase separated near-critical binary liquids as model of fluctuating liquids and interfaces and by looking at different relaxation dynamics of out of equilibrium situations: instability of a liquid column, interface relaxation and droplet evaporation. Hence, the study of these phenomena is performed using ultra-soft liquid interfaces and continuously varying hydrodynamic, thermodynamic and stochastic properties with the shift to the critical temperature. In a first step, the interface of these near-critical binary liquids is initially driven out of equilibrium using the radiation pressure of a laser wave in order to create in situ liquid columns and droplets. Dedicated tools for image analysis of near-critical fluctuating fluids were also developed. Then, we show that, contrary to the classical idea, liquid ligaments break-up triggered by Rayleigh-Plateau instability comes from modes superposition. This enables us, using Fourier analysis, to build the full dispersion relation for spontaneous break-up. Secondly, a preliminary work on drop spreading on solid surface established the existence of two dynamical regimes: one nonlinear relaxation mechanism to a spherical cap followed by an auto-similarity behavior of this spherical cap characteristic of Tanner’s spreading. A significant amount of evaporation was also observed in some spreading dynamics, calling for a work extension considering  adapted models. A last study was performed on single droplet evaporation. It constitutes the first experimental work on conserved order parameter evaporation, furthermore for near-critical binary liquids. Against all odds, the measured evaporation and droplet rising dynamics seem completely unfit when using diffusion and gravity coupling descriptions. In particular, their behaviors are independent to the proximity to critical point. All these behaviors are verified over a large variation of distances to the critical point. As such, they seem to be universal within the criticality meaning. Eventually, the hydrodynamic behavior are verified when the thermodynamic one stay misunderstood. This raises questions on their coupling by means of thermal fluctuations. Nonetheless, thanks to the developed tools, we are now able to simultaneously get the macroscopic scale of the dynamics and the microscopic scale of interface fluctuations opening the way to more complete, multi-scale, analyses, in the fluctutations dominated case of the already observed phenomena.

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