Self-motion of a camphor disk on an aqueous phase depending on the alkyl chain length of sulfate surfactants.

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.