The mysteries of droplet birth in microfluidic cross junctions

In microfluidics, flow focusing is widely used to produce water-in-oil droplets in microchannels at high frequency. We here present an experimental study of droplet formation in a microfluidic cross-junction with a minimum number of geometrical parameters. We mostly focus on the dripping regime. The dynamics of droplet formation is composed of two distinct steps : filling and pinching. The duration of each step (and cor- responding volumes of each phase) are analyzed. They vary according to both water and oil flow rates. These variations reveal several insights about the fluid flows in both phases. We propose some scaling laws to relate the droplet volume and frequency to the flow rate of both phases. We also discuss the influence of surfactant on droplet formation.