Ion-Selective Marangoni Instability of Oil/Water Interface under a DC Voltage

Ion-selective Marangoni instability (MI) was studied by electrochemical measurements in order to determine the non-hydrodynamic nonlinear dynamics inherent to the process. An oil-soluble surfactant and calcium ion reacted at the interface, where the reaction products form aggregates. We generated a macroscopic interface where interfacial processes such as reaction and aggregation were synchronized. In this case, a non-hydrodynamic regime was observed before convection: Immediately after applying voltage, we observed an unsynchronized interface and an irregular interfacial flow. However, when the amount of ion transport across the interface exceeded a threshold value, the interface became synchronized, and successive occurrence of MI appeared as a periodical oscillation of interfacial tension. An aggregate of reaction products was considered to proceed homogeneously over the whole synchronized interface. We used a hemispherical droplet shape, and thus an increase in interfacial tension decreased the interfacial area. The interface was then compressed, providing a mechanical work for the desorption of aggregated products. This triggered a hydrodynamic instability.