Microchannel Emulsification Devices for Generating Highly Uniform Droplets

Monodisperse emulsions consisting of uniform droplets have received a great deal of attentions over the past decade due to their high-tech applications, e.g., monodisperse microparticles as spacers for electronic devices and monodisperse micro-carriers for drug delivery systems (DDS). Our group proposed microchannel (MC) emulsification, which enables generating highly uniform droplets with the smallest coefficient of variation of below 5% using MC arrays of unique geometry. The resultant droplet size can be precisely controlled by MC geometry. Droplet generation for MC emulsification is very mild and does not require any external shear stress; a dispersed phase that passed through MCs is transformed spontaneously into uniform droplets inside a continuous-phase domain. The aim of this paper is to present recent developments in MC emulsification devices, particularly focusing on straight-through MC arrays consisting of uniform straight-through holes for large-scale production of monodisperse emulsions. A straight-through MC array device of a standard 24 × 24-mm size was made of single-crystal silicon, and a straight-through MC array consisting of numerous MCs was positioned within a 10 × 10-mm central region of the device. We initially designed symmetric straight-through MCs with circular and oblong sections. Highly uniform droplets with average sizes of 4 to 100 μm were generated using oblong straight-through MCs. The simulation results using CFD (computational fluid dynamics) agreed well with the experimental results and provided useful information, such as the movement of the oil-water interface around the MC outlet during droplet generation. Below the critical value of the dispersed phase flux, monodisperse emulsions were produced via suitable oblong straight-through MCs, with droplet size and size distribution independent of the flux value. The development of asymmetric straight-through MC arrays consisting of numerous pairs of microslots and circular MCs improved the productivity of highly uniform droplets and stability during droplet generation.