Self-propulsion of symmetric interfacial swimmers

Marangoni effects are involved in a variety of capillary and interfacial phenomena. One instance which made its way into popular science is interfacial boats, which self-propel by releasing surfactant asymmetrically. Remarkably, as demonstrated experimentally with millimetric camphor-loaded disks, self-propulsion is also possible for symmetric objects through a spontaneous symmetry breaking, which, however, remains only partially elucidated. We propose a generic toy model, amenable to analytic treatment, that captures the swimming mechanism and predicts semi-quantitatively the swimming velocity. To go beyond this simple picture, we use numerical methods which can handle the complex couplings at work. We identify the nature of the bifurcation where motion sets in and characterize the swimming far above this threshold, and show that the role of Marangoni flow is very different in those two regimes.