A simplified two dimensional coaxial flow-focusing geometry model was developed for computation domain, and then a volume of fluid based on continuum surface force model was carried out to study the influence of flow parameters on the droplet formation in a coaxial flow-focusing microfluidic device. The effects of flow rates, viscosities and the surface tensions of the three phases which are called the outer fluid, middle fluid and inner fluid on the size and morphology of the droplets were investigated. The results reveal that if the velocity and viscosity of the outer fluid are increased, the sizes of outer and inner droplets become smaller. By increasing the velocity of the middle fluid, the outer droplets become bigger, while the inner droplet size decreases. As the velocity of inner fluid increases, more inner fluid is injected, which leads to an increment with both outer and inner droplet size. Both of the outer and inner droplet sizes become bigger as the outer surface tension coefficients increase, and for the same reason, the increment with the outer surface tension result in an increase with the outer droplet size, but has no effect on the inner droplet size. Similarly, the droplets morphology almost does not vary with the viscosity of both middle and inner fluid. In fact, the principles revealed above are related with the interaction between the viscosity shear stress and the surface tension.

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