Inertial Marangoni Propulsion: Experiments

In this presentation, the experimentally observed Marangoni-driven motion of disk-shaped and spherical particles located at an air-water interface atop a deep layer of water will be presented. The particle self-propulsion is due to the release of a surface active agent (here alcohol or surfactants) from its rear which locally reduces the surface tension. The Marangoni surfer is expected to undergo a pure translation. However, at a high enough Reynold numbers, the left-right symmetry was broken giving rise to rotation of the particle. After the initial rotation, the particle was found to follow a nearly circular path. Particle tracking and PIV measurements were used to understand the origin of the rotational motion. When the strength of the propulsion by changing the concentration of alcohol released, a critical Reynolds number of approximately Re = 200 was discovered beyond which an asymmetric vortex shedding was observed in the wake of the disks and spheres. The asymmetric vortex shedding results in a periodic torque on the particle that can result in the observed particle rotation. In the limit of vanishingly small Reynolds numbers, particle rotation can also be induced through an interaction with interface curvature near the edges of the dish.