Active drops driven by surface and polymorphic phase transitions: Current understanding and emerging perspectives

Small emulsion droplets typically adopt spherical shapes under positive interfacial tension, minimizing unfavorable oil-water contact. This shape, along with the initial drop size, are generally preserved upon drop freezing or melting. However, in a series of studies, we demonstrated that simple temperature fluctuations near the melting point of the dispersed oil phase can spontaneously induce a wide range of dynamic behaviors in droplets. These activities include morphogenesis into various non-spherical shapes such as hexagonal, triangular, and tetragonal platelets, rods and fibers; the formation of complex composite micrometer-sized structures in the presence of adsorbed latex particles on initially spherical droplets; spontaneous desorption of the initially adsorbed particles; the generation of synthetic microswimmers capable of self-propulsion through the continuous phase, driven by the rapidly growing elastic filaments; spontaneous drop fragmentation and bursting into smaller particles (with sizes down to 20 nm) without any mechanical energy input; and the engulfment of the surrounding media spontaneously producing double water-in-oil-in-water droplets. All these phenomena were found to be intricately related to surface and polymorphic phase transitions proceeding within the droplets. The underlying mechanisms and control parameters were systematically investigated and published in a series of papers. The present review aims to summarize the key discoveries, present them within a unified conceptual framework, and compare them with other processes reported in the literature to lead to similar outcomes. Furthermore, the practical implications of these phenomena are discussed, and potential future research directions in this emerging area at the intersection of emulsion science and phase transition phenomena are outlined.