I developed a numerical platform to model droplet production process in flow focusing microchannels, using a front-tracking method to track the interface and a FENE-CR model for viscoelasticity of the continuous fluid. By investigating flow dynamics for various physical conditions, the simulations showed three distinct regimes of droplet formation: squeezing, dripping and jetting regimes. In squeezing and dripping regimes, droplets were generated close to the orifice. However, in the jetting regime, a long liquid threat was formed in the expansion region before the pinch-off of the small droplets. Droplet size decreased by increasing dispersed fluid flow rate, while a sudden raise in droplet size occurred in the transition between dripping and jetting regimes. Enhancing viscoelasticity mainly increased droplet size, with an abrupt increase in the average droplet volume for a fluid with high polymeric viscosity ratio, at high Weissenberg numbers. The variation of droplet size and size distribution as a function of $Wi$ for various polymeric viscosity ratios, showed that the viscoelasticity does not have a significant influence on the droplet size until a critical Weissenberg number (Wicr) is reached, i.e., Wi < Wicr. When the Weissenberg number exceeded the critical value, the droplet size increased abruptly. The critical Weissenberg number weakly depended on the polymeric viscosity ratio and slightly decreases as the polymeric viscosity ratio increases. The polymeric viscosity ratio had a negligible influence on the droplet size when Wi < Wicr but the droplet size increased rapidly with increasing polymeric viscosity ratio after Wi > Wicr due to the well known strain-rate hardening effects of polymeric fluids.
This work showed the importance of rheological behavior of fluids in the process of droplet formation, which was unexplored in previous computational researches. This work was supported by the Scientific and Technical Research Council of Turkey (TUBITAK) through Grant No. 112M181 and Turkish Academy of Sciences (TUBA).