A surfactant reaction model for the reciprocating motion of a self-propelled droplet.

We report herein experimental observations of the reciprocating motion of a self-propelled droplet floating on the surface of an aqueous surfactant solution and a simple reaction model capable of reproducing the observed behavior of the droplet. The reciprocating motion was observed in a quasi-one-dimensional annular channel, so the reciprocation was not caused by reflections at boundaries. To understand the reciprocation, our model assumes a reaction between the surface active substance emitted from the droplet and surfactants dissolved in the aqueous solution. This reaction invokes an inversion of the surface tension gradient and thus the droplet's reciprocation. We show that the model can reproduce experimental results semi-quantitatively using numerical simulations with realistic parameters.