Droplets containing large solid particle inside formation and breakup dynamics in a flow-focusing microfluidic device

The breakup dynamics of compound droplets containing large particle inside (S/W/O) in a flow-focusing microfluidic device were systematically investigated, and four typical flow regimes, regarding multiple-encapsulation, slug, transition and cobble, are distinguished. At low flow rate of outer fluid, Q(c) < 140 mL/h, the neck breakup can be divided into squeezing stage (W-n/W-c >= 0.25), and rapid pinch-off stage (W-n/W-c <= 0.25) during the whole formation process. However, for Q(c) >= 140 mL/h, the neck rupture behavior appears various, in which only rapid pinch-off stage can be observed. Generally, the neck dimensionless width, W-n/W-c variation with remaining time usually obeys a power law function. Moreover, the liquid film thickness always obtains a critical value as the flow rate of outer fluid further increases. Specially, it suggests that the differences in the breakup dynamics are mainly caused by the existence of solid particles. Finally, the corresponding driving mechanisms were also discussed.