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Swimming droplets under confinement
Artificial micro-swimmers have recently become a central field of research in soft-matter. A very promising and original type of swimmer developed in our team, consists in pure water droplet swimming in an oil phase containing micelles of surfactant. The droplet's activity comes from the formation of swollen micelles at its interface which induces Marangoni stresses and then motion of the droplets.
We investigate experimentally the behavior of such self-propelled water-in-oil droplets, confined in capillaries of different square and circular cross-sections. Stretched circular capillaries have been used to explore even stronger confinement. Within the most constricted regions, droplets elongate very strongly. These extremely long droplets reveal unexpected behaviors during their motion, in particular regarding their stability.
We build up on the Bretherthon formalism to rationalize this new kind of self-motion of confined droplets in a tube, not anymore driven by pressure or flow rate but rather by locally-induced interfacial stresses.