Title: Pickering-like emulsion stabilized via fine bubbles

Abstract

Micro-nanobubbles exhibit unique properties, such as mass transfer, electroflotation, disinfection, decontamination, bioactivity and cleansing. Among them, we note the cleansing property, in which oil on skin, hair, and cloth can be effectively washed off using water with micro-nanobubbles. We propose that this cleansing property is due to the Pickering-like emulsification of oil induced by fine bubbles. When air, water, and oil exhibit a three-phase contact angle, bubbles can attach to the interface between the water and oil. In this case, the oil is expected to form an oil-in-water emulsion stabilized via bubbles even without surfactants, similar to a Pickering emulsion stabilized via solid particles. Although the lifetime of the bubble-induced Pickering-like emulsion is limited, this emulsion appears transiently and is expected to enhance the cleansing property of fine bubbles. To demonstrate this, we theoretically analyzed the interfacial energy of a system comprising a water–oil interface and a single bubble or multiple bubbles. The results indicate that the interfacial energy decreases when a bubble residing in water attaches to the water–oil interface. This predicts that an oil-in-water emulsion, which includes fine bubbles in water, is stabilized by the attachment of bubbles and forms a Pickering-like emulsion. Furthermore, self-emulsification occurs when this energy decrease exceeds the energy required to generate a new water–oil interface. The conditions required for self-emulsification were depicted as a phase diagram of γ_GW⁄γ_WO vs. γ_GO⁄γ_WO , where γ_GW, γ_GO, and γ_WO represent the air–water, air–oil, and water–oil interfacial tensions, respectively (Figure 1). Hopefully, these theoretical predictions will enliven experimental studies on the interaction between an emulsion and fine bubbles, including direct observation of emulsions incorporating fine bubbles via the scanning electron microscope and high-speed video camera.