Step emulsification for monodisperse droplet creation using a connection-free PDMS microchip

Microfluidic technology for generating monodisperse droplets has significantly advanced various scientific and industrial applications. However, reliance on complex infrastructure, such as external pumps and intricate connections, limits its widespread adoption. In this study, we focused on passive droplet formation through step emulsification (SE) using a connection-free polydimethylsiloxane (PDMS) microchip. Our microchip design enabled droplet formation without the need for external pumps for active flow control, instead utilizing the pressure differential created between inlet reservoirs and a shared sealed-outlet space when degassed PDMS was exposed to atmospheric pressure. We began with a simple design and through progressive refinement of the microchip geometry, we achieved a more advanced design that facilitated detailed analysis of droplet formation dynamics. This design allowed for real-time observation of the droplet formation process, including time-dependent variations in droplet formation rate and the associated gentle changes in droplet size, which were intrinsic characteristics of connection-free PDMS microchips. Experiments demonstrated that a triangular nozzle design significantly improved droplet size uniformity, with a coefficient of variation in droplet diameter below 2 % under optimal conditions. These results highlighted the potential of connection-free SE microchips for generating highly uniform droplets in a simplified and efficient manner.