Spontaneous periodic pulsation of contact line in oil/water system--frequency control with divalent cations and applied voltage

Periodic oscillatory change of hydrophilicity (or hydrophobicity) of a glass surface was studied. A glass capillary was immersed normally at an oil/water interface. The water phase contained the cationic surfactant trimethyloctadecylammoniumchloride, and the oil phase contained bis(2ethylhexyl) phosphate. Adsorption of the surfactant molecules and their desorption via anionic chemicals dissolved in the oil generated a gradual wetting by the water, followed by a rapid wetting by oil. The three phase contact line exhibited a pulse-like motion that continued, at least for a few minutes. The frequency depended on the cation species dissolved in water and the applied voltage across the oil/water interface. Four kinds of cations, Mg(2+), Ca(2+), Sr(2+) and Ba(2+) were used. While the frequency order was Ba(2+)>Sr(2+)>Mg(2+), the Ca(2+)-containing interface did not show any motion irrespective of the applied voltage. There was a threshold voltage and concentration of anionic chemical that was necessary for the onset of this motion. The pulsation mechanism and its ion selectivity are also discussed. This interfacial motion was a typical nonlinear oscillation with an ion-selective nature. In this regard, this interfacial motion had biomimetic characteristics.