Self-propelled camphor disk dependent on the depth of the sodium dodecyl sulfate aqueous phase

Abstract

We studied the self-propulsion of a camphor disk placed on a surfactant solution to clarify the relationship between the speed of motion and the depth of the aqueous phase. At lower concentrations of sodium dodecyl sulfate (SDS) in the aqueous phase, the speed of motion for the deeper aqueous phase was higher than that for the shallower aqueous phase. However, this relationship was reversed at higher SDS concentrations. The surface tension of the aqueous phase and diffusion rate around the camphor disk were measured to determine this reverse relationship at higher SDS concentrations. Numerical calculation considering the flow field coupled with the concentration field qualitatively reproduced the experimental results on the diffusion rate depending on the SDS concentration. These results suggest that the amount of camphor dissolved in the aqueous phase plays an important role in reversing the speed of motion depending on the depth of the aqueous phase.