Height-dependent oscillatory motion of a plastic cup with a camphor disk floated on water.

We have developed a self-propelled object, which is composed of a plastic cup and a camphor disk, on water to reflect its three-dimensional shape in the nature of motion. The self-propelled object, of which the driving force of motion is the difference in the surface tension, exhibited oscillatory motion between motion and rest. The period and the maximum speed of oscillatory motion increased and decreased depending on the height of the cup, h , respectively. Two types of diffusion coefficients were estimated based on the diffusion of camphor molecules which were indirectly visualized using 7-hydroxycoumarin. The experimental result on the period of oscillatory motion depending on h could be reproduced by the numerical calculation based on the diffusion of camphor molecules around the object and the diffusion coefficients which were experimentally estimated. The experimental results suggest that characteristic features of motion can be created based on the three-dimensional shape of the object.