Transient Emulsions: A New Paradigm for Dynamic Colloidal Assembly

ConspectusEmulsions formed by dispersing one liquid into another immiscible liquid have been a cornerstone of colloid science for over a century. Conventional emulsions are stabilized by surfactants, which reduce interfacial tension from the range of 30-50 mN/m to 1-10 mN/m, allowing droplets to persist against coalescence. Despite their broad industrial relevance, these systems are fundamentally constrained by their interfacial nature: high-energy input is required to generate small, uniform droplets; surfactants may alter physicochemical properties and introduce toxicity; and droplet morphology is largely restricted to isotropic, spherical shapes. Moreover, Ostwald ripening and droplet coalescence, driven by Laplace pressure differences, inevitably lead to thermodynamic instability. These constraints underscore the need for new emulsification paradigms beyond the classical surfactant-stabilized model.Transient emulsions are based on partially miscible liquid pairs such as water and 1-butanol. In these systems, mutual diffusion at the droplet interface generates a blurred transition miscible layer rather than a sharp phase boundary. As a consequence, interfacial tension approaches zero, fundamentally altering the behavior of emulsified droplets and imparting distinctive features: (i) ultrashort lifetime, (ii) absence of surfactants, and (iii) spontaneous emulsification under weak energy perturbation. Notably, the lack of strong interfacial constraints enables transient emulsions to undergo asymmetric deformations that are inaccessible to conventional emulsions.These unique properties open up a new frontier for dynamic, out-of-equilibrium processes, particularly in the self-assembly of colloidal nanoparticles. Transient emulsions offer a versatile platform for constructing colloidal superstructures that would otherwise be unattainable. Three key advances have been demonstrated: (1) rapid, surfactant-free assembly, enabling plasmonic superstructures to form within seconds; (2) uniform superstructuring across multiple scales, achieved through template-confined emulsification; and (3) asymmetric superstructuring, facilitated by new hollowing mechanisms in transient aerosol emulsions. Together, these advances establish transient emulsions as a unique vehicle for controlling structure, symmetry, and dynamics in colloidal assemblies.Beyond fundamental insights, transient emulsions have enabled the development of superstructures with new functionalities and applications. Gold microsphere arrays fabricated by emulsion-directed assembly combined with nanosecond laser irradiation enable ultrastable anisotropic conductive bonding, offering a compelling alternative to conventional metal-polymer core-shell microspheres for anisotropic conductive films in micro-LED packaging. Silica-based hemispherical superstructures function as detachable microlenses with tunable magnification, enhancing numerical aperture and photon throughput in optical microscopy. Meanwhile, coatings assembled from concave-convex silica superstructures exhibit exceptional optical diffusion, combining low haze with high brightness for next-generation display and photonic technologies.In short, transient emulsions introduce a new paradigm in colloid and interface science. By removing the constraints of interfacial tension, they open up powerful pathways for dynamic colloidal self-assembly and functional material creation. Going forward, diversifying transient emulsion systems, enhancing structural stability, and developing high-resolution emulsion printing methods could further establish this versatile, efficient, and adaptable platform for engineering complex superstructures, connecting fundamental colloid science with advanced photonic and electronic technologies.