Hypothesis: Predicting formation mode of double emulsion drops in microfluidic emulsification is crucial for controlling the drop size and morphology.
Experiments and modelling: A three-phase Volume of Fluid-Continuum Surface Force (VOF-CSF) model was developed, validated with analytical solutions, and used to investigate drop formation in different regimes. Experimental investigations were done using a glue-free demountable glass capillary device with a true axisymmetric geometry, capable of readjusting the distance between the two inner capillaries during operation.
Findings: A non-dimensional parameter (ζ) for prediction of double emulsion formation mode as a function of the capillary numbers of all fluids and device geometry was developed and its critical values were determined using simulation and experimental data. At logζ>5.7, drops were formed in dripping mode; the widening jetting occurred at 5<logζ<5.7; while the narrowing jetting was observed at logζ<5. The ζ criterion was correlated with the ratio of the break-up length to drop diameter. The transition from widening to narrowing jetting was achieved by increasing the outer fluid flow rate at the high capillary number of the inner fluid. The drop size was reduced by reducing the distance between the two inner capillaries and the minimum drop size was achieved when the distance between the capillaries was zero.
Keywords: Core-shell droplets; Double emulsions; Dripping regime; Dripping-to-jetting transition; Droplet microfluidics; Glass capillary device; Narrowing jetting; Velocity profile; Volume of fluid–continuum surface force model; Widening jetting.
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