Self-Propelled Motion of a Coumarin Disk Characteristically Changed in Couple with Hydrolysis on an Aqueous Phase

In this study, a coumarin disk was examined as a simple self-propelled object under a chemical reaction. A coumarin disk placed on an aqueous phase containing Na₃PO₄ as a base exhibited continuous and oscillatory motion at lower and higher initial concentrations of Na₃PO₄, [Na₃PO₄]₀, respectively. In addition, the period of the oscillation between rest and motion increased with increasing [Na₃PO₄]₀. The mechanism of mode bifurcation between continuous and oscillatory motion and a change in the period of oscillation were discussed in terms of hydrolysis of coumarin and the surface tension of the aqueous solution as a driving force. A reduced mathematical model based on the reaction kinetics of coumarin around the air/aqueous interface, which adequately reproduced the experimental observation, was constructed. These results suggest that the characteristics of the self-propelled motion were determined by the kinetics of hydrolysis.