Oil droplets can “swim” autonomously while dissolving in a water-surfactant solution: They move about their environment in a manner similar to that of microorganisms. As viscosity is increased, the droplets lose their sense of direction, turn frequently, and finally switch to a chaotic “stop-and-go” motion. Yet, naively, one would expect oil droplet swimmers to move steadily in a viscous medium. To explain this observation, we experimentally visualize the spread of dissolved oil around a droplet in a water-glycerine mixture and characterize the droplet’s movement at different mixture viscosities.

As the droplet dissolves, tension builds up on its surface if there is enough asymmetry in the distribution of dissolved oil. This tension drives a flow that moves the droplet and reinforces that initial asymmetry, resulting in continuous, self-powered motion.

Using fluorescence microscopy, we find that at low viscosities the droplet moves steadily, showing a twofold flow pattern. At higher viscosities, the droplet changes the surrounding flow into a fourfold pattern, trapping it in place. The unsteady motion is caused by a constant alternation between the two flow patterns, driven by interactions with the dissolved oil. This kind of gait switching is similar to how some organisms explore their environment somewhat randomly but still efficiently because they have left traces of where they have been. We can make droplets explore a very similar “self-avoiding walk” by turning up the viscosity.

Our findings show that gait-switching motion is not necessarily a “conscious decision,” but can be triggered by external conditions. Similar environment-driven flow switching could be used in micropumps that redistribute their surroundings depending on their conditions. This result also shows that persistent memory effects can significantly change the dynamics of self-propelling microagents, such as those used for drug delivery, and should be exploited in their design.