Thèses

Jeudi 24 Octobre 2024 à 14h00.

Marangoni swimmer in external flows

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Directeur de thèse / thesis director :
Christophe Ybert, Cécile Cottin-Bizonne, François Detcheverry

Membres du jury / jury members :
M. Christophe Ybert CNRS Directeur de thèse
Mme. Cécile Cottin Bizonne CNRS Co-Directrice de thèse
M. François Detcheverry CNRS Co-Directeur e thèse
Mme. Isabelle Cantat Université de Rennes Examinatrice
M. Jean Phillipe mattas Université Lyon 1 Examinateur
M. Etienne Reyssat CNRS Examinateur
Mme. Emanuelle Riot Université Paris Saclay Rapportrice
M. Mathieu Roché CNRS Rapporteur

Résumé / Abstract :

Interfacial swimmers are objects that self-propel at an interface by generating surface tension gradients, typically through the continuous release of surfactants. If the propulsion mechanism of symmetric swimmer is now well understood, the experimental characterization remains for now limited, as it focuses primarily on the swimming velocity. This global measure, however, conceals several underlying mechanisms that are intricately interconnected. A more detailed experimental characterization is needed which would provide deeper insights into the key physical mechanisms driving the complex problem of Marangoni swimmer. Moreover, the interactions of these interfacial swimmers with external potential remain unexplored. Accordingly, the goals of this thesis are twofold.

First, to provide the first comprehensive characterization of a Marangoni swimmer in a stationary state, assessing the forces acting on the swimmer, the flow field, and the surfactant distribution. Second, to study the interaction of Marangoni swimmers with external couplings, such as a flow field or a harmonic potential. We design a novel experimental setup that allows simultaneous measurement of force and flow in a stationary state, offering a fresh perspective by examining a fixed swimmer in a controlled flow. Our approach reveals several key findings. First, the force exerted by the fluid on the swimmer changes sign when the flow velocity increases and cannot be simply reduced to the sum of capillary force and drag experienced by a passive disc. Second, we could measure the stationary interfacial flow fields under different advection conditions. Third, we develop a new approach that combines experimental data with numerical modeling to reconstruct the surface pressure field from the observed surface flow. Using an equation of state, we then derive the surface concentration field of surfactant, overcoming the curse of "hidden variables". Finally, we explore the swimmer behavior under external potentials, examining two scenarios: coupling with a spring and interaction with a water flow. In the spring scenario, the swimmer exhibits an ellipsoidal trajectory
with amplitude decreasing with advection, a behavior that was qualitatively reproduced by a simple model, suggesting that the primary factors are the swimmer self-propulsion and the changes in surfactant distribution induced by advection. In the second scenario, we analyze the trajectories of the swimmer in simple shear flow and vortex conditions. Despite the system complexity, a toy model—disregarding the details of surfactant distribution and propulsion mechanisms and focusing only on spontaneous velocity and rotation by the flow—was able to capture the qualitative behavior of the swimmers.

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