Microspheres (MS), such as emulsion droplets, multiple emulsions, microparticles, microcapsules, and liposomes, have been utilized in various industries. However, size control of MS is not so easy. Recently, we proposed a novel method for preparing monodispersed emulsion droplets with a coefficient of variation less than 5% from a microfabricated channel (MC) array. In this study, we analyzed a droplet-formation mechanism from a MC using a microscope high-speed camera system. During droplet formation, the dispersed phase passed through the channel inflated on the terrace in a disklike shape, and the droplets were formed in 0.01 s. A droplet-formation mechanism was suggested in which the distorted dispersed phase on the terrace is cut off spontaneously into spherical droplets by interfacial tension. The mechanism is shown to be an adequate model from the viewpoint of interfacial free energy. This emulsification technique exploits the interfacial tension, which is the dominating force on a micrometer scale. It is a promising technique for producing MS requiring extreme monodispersity because of its simplicity.

Recently, we proposed a microchannel (MC) emulsification technique, which is a novel method for making a monodispersed emulsion from a microfabricated channel array. The droplet formation mechanism for MC emulsification is a unique one, in which the distorted dispersed phase is spontaneously transformed into spherical droplets by interfacial tension. The objective of this study was to characterize the flow in MC emulsification. We investigated the emulsification behavior at different flow velocities of the dispersed phase using MCs with different sizes and analogous shapes and found a critical flow velocity over which the character of flow changed drastically. The formed droplet diameters were almost constant below the critical velocity, and monodispersed emulsions were formed. The droplet diameters increased drastically above the critical velocity, and polydispersed emulsions were formed. The critical velocities were independent of MC size. Analysis using a dimensionless number revealed that the critical point, at which the character of the flow of MC emulsification changes, is determined by the Capillary number (Ca), which is the ratio of viscous force to interfacial tension force. Below the critical Ca, the interfacial tension, which is the driving force for droplet formation, is dominant, and the flow in MC emulsification was based on spontaneous transformation. Above the critical Ca, viscous force is dominant, and the flow is similar to laminar flow. We experimentally confirmed this idea using the dispersed and continuous phases with different viscosities, MCs with different sizes, and different interfacial tensions. These results suggested a law of similarity in MC emulsification, and the transition of the flow was determined by Ca. The information obtained in this study includes the essential physics underlying spontaneous transformation flow in MC emulsification and is useful for practical applications.