Nanoparticle-stabilized water-in-water emulsions: A critical review of formation, stabilization, and applications.

Water-in-water (W/W) emulsions are unique colloidal systems composed of two immiscible aqueous phases, featuring ultra-low interfacial tension, mild preparation conditions, and excellent biocompatibility. Owing to their distinctive interfacial properties and non-toxic aqueous environment, W/W emulsions have attracted extensive attention across colloid and interface science, food science, biomedicine, and pharmaceutical engineering, and have gradually become a hot research topic in interdisciplinary fields. In particular, recent advances have highlighted the importance of nanoparticle-stabilized Pickering W/W emulsions, which provide enhanced stability despite the intrinsically ultralow interfacial tension of such systems. This paper reviews the formation and phase separation mechanisms of W/W emulsions, summarizes the characteristics and stabilization strategies of Pickering stabilizers, discusses the regulatory rules of colloidal particle intrinsic properties, interfacial characteristics, and external environmental factors on emulsion stability, and explores their applications in the environment, medicine, food, and other fields. Finally, current challenges and future perspectives for the development of stable and functional W/W Pickering emulsions are discussed.