Cation-Dependent Emergence of Regular Motion of a Float.

We report a unique ion-dependent motion of a float at an oil/water interface. The type of motion depended on the cation species that was dissolved in the water. Irregular vibrations occurred when the water contained Ca(2+), back-and-forth motion occurred when the water contained Fe(2+), a type of motion intermediate between these occurred when the water contained Mn(2+), and intermittent long-distance travel occurred when the water contained Fe(3+). This is one of the simplest systems that can be used to show how macroscopic regular motion emerges depending on specific chemicals, which is one of the central issues in the study of biological and biomimetic motions.