Ion-selective Marangoni instability--chemical sensing of specific cation for macroscopic movement

Spontaneous motion and tension oscillation of an oil/water interface responding to specific cation Ca(2+) or Fe(3+) were observed when the oil phase containing the anionic surfactant bis(2-ethylhexyl) phosphate came in contact with the cation-containing water. Both the dynamics were the results of Marangoni instability. Complex formation between the anionic surfactant and cation caused the instability. The results showing the level of cation extraction and degree of interfacial tension revealed that the surfactant-cation combination forms an oil-soluble complex with reduced surface activity. Brewster angle microscopy indicated that molecules of the complex tend to aggregate at the interface. This aggregation affected the desorption rate of the complex. We were able to generate ion-selective instability by imposing mechanical and electrochemical perturbations to the interface at equilibrium. The results from these efforts suggested that the aggregation is a type of thermodynamic transition and is required for the onset of instability: Desorption probably occurs as an exfoliation of the aggregated complex, which generates the gradient of interfacial tension. For the standard experiment of biphasic contact, two neighboring interfacial flows compress the local interface between them. We considered that this compression provides mechanical work to the local interface, resulting in desorption of the aggregates and occurrence of instability. Both complex formation and aggregation are possible in the presence of the specific cation. The interface detects the cation via the chemical and thermodynamic processes in order to develop the macroscopic movement, a form of biomimetic motion of the oil/water interface.