Spontaneous wave driven by chemical autophobing with oscillatory dynamics at glass/oil/water interfaces

The spontaneous wave generation in an oil/water interface was studied focusing on the surface properties of a sample container. The organic phase with iodine and potassium iodide and the aqueous phase containing trimethylstearylammonium chloride C18TAC were used. Dependency of the waveform on pH in the aqueous phase showed that the waveform and the traveling velocity were controlled by the ζ-potential of the container sidewall. We found that hydrophobic wetting of the glass plate was driven by the surfactant adsorption with the desorption by chemical reactions. This caused an accelerated wetting and dewetting, which constituted an oscillatory change in the contact angle. When there is a sufficient width with the sidewall, the oscillatory change lead to the hydrodynamic growth. The change in the level of the interface was necessary for the well-reproducible traveling wave. The spontaneous wave which was generated by the oscillatory wetting and the level change was understood on the basis of the stick-jump motion of surfactant solutions. This understanding well explained the dependency of the waveform on pH, the amplitude of the wave and the effect of the sidewall curvature on the wave-amplitude. At the high pH corresponding to the strong adsorption of cationic head group to the glass surface, the pattern should be a step-wise form. While the glass surface is electrically neutral, the wave shape must be similar to fingering-like pattern. These characteristics were also kept for the present traveling wave. The wave-amplitude itself and the dependency on the sidewall curvature were well estimated by the capillary-gravity balance which can also predict the amplitude of the stick-jump motion. Furthermore, significant differences between the present wave and the stick-jump motions were also pointed out.