We demonstrated mode-switching of self-motion coupled with diffusion of molecules at a solid/liquid interface. A camphor boat moved spontaneously on water and the mode of self-motion depended on the setup of the boat. When a camphor disk was connected to the center of a larger plastic plate, intermittent motion (alternating between rest and rapid motion) was observed. When the position of the camphor disk was changed from the center to one of its edges, the period of intermittent motion decreased, and intermittent motion changed to continuous motion. The features of this self-motion and mode-switching were qualitatively reproduced by a numerical calculation using a mathematical model that incorporates the distribution of camphor molecules at the solid/liquid interface.

A change in the mode of self-motion was investigated for a camphor disk on water upon the addition of sulfate surfactants with alkyl chains of different lengths as a simple autonomous system. With an increase in the concentration of surfactant with a longer alkyl chain (number of hydrocarbons: 14 or 16), two mode changes (continuous --> intermittent (alternating between motion and rest) --> no motion) were observed. With an increase in the concentration of surfactant with a shorter alkyl chain (number of hydrocarbon: 10 or 12), four mode changes (continuous --> intermittent --> continuous --> intermittent --> no motion) were observed. These two types of mode changes are discussed in relation to the solubility of the surfactant and camphor in the water phase and the surface tension of the surfactant, camphor, and a mixture of surfactant and camphor as the driving force of motion.

As a simple example of an autonomous motor, the characteristic features of self-motion coupled with the acid-base reaction were numerically and experimentally investigated at the air/aqueous interface. Oscillatory and uniform motion were categorized as a function of the reaction order by numerical computations using a mathematical model that incorporates both the distribution of the surface active layer developed from a material particle as the driving force and the kinetics of the acid-base reaction. The nature of the self-motion was experimentally observed for a boat adhered to a camphor derivative with a mono-or di-carboxylic acid on a phosphate aqueous phase as the base.2

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