Corentin Tregouët (IPR, Rennes) - Instability in capillary flows, from Marangoni to soap films
Capillary flows are imposed by the surface to the bulk, and their motors are surface minimization or surface tension gradients (Marangoni flows). These flows occur especially when a surfactant-laden interface is deformed unevenly, which is the case in a lot of multiphasic processes. However, due to the complexity of the coupling between diffusion and convection, a lot remains to be understood about these phenomena. I will present two examples of capillary flows.
In the first example, I generate Marangoni flows by depositing a concentrated solution of surfactant at the center of an air/water interface. While these flows may occur over finite distances on the surface, subsequent secondary flows can be observed on much larger lengthscales. I will show that if a Marangoni flow of soluble surfactants is confined laterally, the flow forms an inertial surface jet, whose velocity profile is predicted analytically and successfully compared with experimental observations. Moreover, this straight jet eventually destabilizes into meanders. Quantitative comparison between the theory and our experimental observations yields a very good agreement in terms of critical wavelengths.
I will then focus on the dynamics of soap films, for which capillary flows naturally have a critical effect. The stability of soap bubbles and hence of liquid foams is a set by the film thinning, which is known to occur locally: rounds patches of thin films appear at the edges of the film before moving to the top of the film by buoyancy. This phenomenon, the marginal regeneration, has been observed for decades, but its origin hasn’t yet been established. It involves the appearance of a localized pinch between the film and the meniscus, which dynamics has been entirely characterized by assuming its invariance in the direction of the meniscus. We identify a limit in which the bulk drainage and the surface rearrangements are decoupled, the film thus evolving in a sliding-puzzle-like dynamic. In this frame, we study theoretically and numerically the stability of this straight marginal pinch, and show that it is unstable to long wavelengths. We predict a critical wavelength of fastest destabilization and a thickness ratio between the thin and thick parts of the film, both in good agreement with experimental observations.
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- mardi 26 janvier 2021 11:00