Differentiation of Back-and-Forth Motion in Self-Propulsion Emerging via Complex Formation

Self-propulsion of a thymol acetate (TA) droplet on a sodium dodecyl sulfate (SDS) aqueous solution buffered at pH 9, which was driven by thymol (TOH) produced from TA, was investigated as a function of the concentration of SDS (C_SDS) to elucidate its control effect. The TA droplet exhibited three types of back-and-forth motion depending on C_SDS, i.e., partial back-and-forth motion with oscillatory motion and full back-and-forth motion with either oscillatory or continuous motion. For the first one, i.e., the partial mode, a rest stage was observed between two displacements, while the second one showed continuous motion. To evaluate the driving force of self-propulsion, the surface tension of the aqueous phase with and without TOH and the speed of the Marangoni flow around the droplet pinned on the aqueous phase were measured at different values of C_SDS. The changes from oscillatory to continuous motion and from partial to full back-and-forth motion depending on C_SDS are discussed in terms of the driving force of self-propulsion and adsorption-desorption processes of TOH at the air/water interface in the presence of SDS. The present study suggests that characteristic features of self-propulsion and deformation of the droplet can be created in a reactive system involving adsorption-desorption dynamics of the product.