Formation of satellite droplets in flow-focusing junctions: volume and neck rupture

This work focuses on the rupture of the neck of a main droplet, leading to the formation of satellite droplets in a flow-focusing junction. The size of these satellites is determined by image analysis. Emphasis is given to the influence of viscosity and rheological behaviour of the fluids, using both Newtonian and non-Newtonian fluids as continuous phases. The scaling of the size of the satellite droplet with the capillary number of the continuous phase shows two different areas separated by a critical capillary number of 10−2. Below this critical capillary number, the size of satellites hardly changes. Above this critical value, the size of the droplet increases almost linearly with the capillary number. This critical value appears also as a difference in shape of the rupture of the neck of the droplet for Newtonian continuous fluids: symmetrical for low capillary numbers and asymmetrical rupture of the neck of the droplet for higher capillary numbers. The higher the viscosity ratio (viscosity of dispersed phase/viscosity of continuous phase), the bigger is the main satellite droplet. For more viscous dispersed phases, numerous satellite droplets can be formed. When the continuous phase is non-Newtonian (polyacrylamide solutions) the number of satellites droplets is even higher and their sizes are found interdependent according to a cascade scenario.