Straight-to-curvilinear motion transition of a swimming droplet driven by the Marangoni effect

The swimming microdroplet is a simple experimental model of active matter. When water microdroplets are introduced into an oil-surfactant solution, the Marangoni effect causes convection on the droplets' surfaces and induces the self-propelled motions of the droplets [1]. The variety of observed motions makes the experimental system interesting as an example of nonequilibrium phenomena of fluid dynamics and simultaneously attractive as a real space model of actively swimming objects such as swimming microorganisms. In this study, we measured swimming microdroplets' motion to identify the straight-to-curvilinear motion transition, one of the fundamental transitions observed in the self-propelled motion. We found that the transition occurs as the droplet size increases. To elucidate the mechanism behind this phenomenon, we developed a three-dimensional theoretical model based on the advection-diffusion and Stokes equations combined by the surface tension distribution. The model explains the results of the experiments and reveals the characteristics of the transition.
[1] Z. Izri, M. N. van der Linden, S. Michelin, and O. Dauchot, Phys. Rev. Lett. 113, 248302 (2014).