Inhibition of oscillatory motion of a camphor float due to dissolution

To elucidate significance of nonequilibrium at an air/aqueous interface during self-propulsion of a camphor object, oscillatory motion of a self-propelled object composed of a smaller camphor disk and larger plastic disk floating on an aqueous Na₂SO₄ phase was investigated. Both the frequency and maximum speed of the oscillatory motion decreased with an increase in the concentration of Na₂SO₄ in the aqueous phase, C_i. The decrease in the frequency of the oscillatory motion is attributed to the dissolution rate of camphor in the aqueous phase as a function of C_i. The frequency of the oscillatory motion was numerically calculated as a function of the dissolution rate constant, k, and the saturated concentration of camphor in the aqueous salt solution, C₀. The decrease in the maximum speed of the oscillatory motion as a function of C_i was reproduced by the difference in the surface tension (Δγ) as the driving force of motion and the friction coefficient of motion (μ) which corresponds to the viscosity of the aqueous phase. This study proposes that the nature of self-propulsion can be controlled by the dissolution into the bulk water phase as the 3D space and the nonequilibrium adsorption of energy source molecules on the interface as the 2D space.