Abstract
Coalescence of aqueous droplet on macroscopic oil/water interface was investigated. The interface had intrinsic instability which induced the self-pulsing of the dynamic interfacial tension and the macroscopic interfacial flow due to the Marangoni effect. Di-2-ethylhexyl phosphoric acid was used as a surfactant, and calcium chloride or calcium hydroxide was used as an electrolyte. Coalescence time was compared with the equilibrium interfacial tension and the strength of the spontaneous interfacial flow. The coalescence time decreased with an increase in the aging time during which the oil/water interface kept intact and took a maximum near a certain concentration of the surfactant. An experiment with an artificial interfacial flow due to the Marangoni effect was also performed. Coalescence time was completely independent of the interfacial coverage or composition of the surfactants, and only affected by the strength of the interfacial flow at the interface. We concluded that the spontaneous interfacial flow retarded the coalescence since it interrupted the fusion process with molecular reorganization. One can easily think that the droplet surrounded by the intrinsically unstable surfactant film has a short life time. However, there are cases that the droplet with an unstable surfactant film has a longer life time than that covered by a stable surfactant film.
