Evolution of Self-Propelled Objects: From the Viewpoint of Nonlinear Science.

A variety of moving objects driven by chemical energy have been reported. In this Minireview, we focus on self-propelled objects driven by interfacial tension and explain three types of basic mechanisms for such self-propelled motion, that is, driven by a) surface tension difference, b) contact angle difference, and c) axisymmetric swirling flow in a droplet. Simple behavior induced from the basic mechanisms is then extended by coupling to a chemical reaction or increasing the number of moving objects. Even though the chemicals used here are still simple, the extended systems could show characteristic nonlinear behavior, such as reciprocating motion, oscillatory motion, and spatiotemporal pattern formation. Combining the dynamical information about these characteristic motions with the knowledge of molecular structures will lead to the development of more advanced self-propelled objects. We believe this Minireview can help chemists in investigating self-propelled objects displaying various functional motions observed in a biological system.