Self-propulsion on liquid surfaces

Surface tension gradients are at the origin of the self-motion and deformation of millimeter-sized floating objects. For (quasi-)non-deformable systems, like solids and gels, the motion-mode is mainly controlled by the shape of the object and by the way the surface active propellant is released on the surrounding surface. Two situations are reviewed. In the first one, the propellant container is the propelled object itself, while in the second case the propellant is placed in a reservoir embarked on a manufactured float. The properties and efficiency of these solid systems are examined and compared for different geometries. They are also compared with the intriguing properties of self-motile liquid lenses/drops which present several additional abilities (spontaneous deformation to adapt their shape to the selected motion-mode, presence of complex fluid flows outside and inside the drops, partial break-ups…). Three mechanisms leading to spontaneous motility have been identified in the literature. Among them two are more largely exemplified in the following as they involve a contribution of the “Marangoni driven spreading” effect, leading to velocities on the cm/s scale. The main theoretical tools usually used for describing the motion and deformation of such self-propelled systems are also reviewed.