Mechanisms and Control of Viscoelastic Double Emulsion Droplet Formation in Microchannels via Pulsatile Flow

In droplet microfluidics, active velocity pulsing is emerging as an effective method for regulating the droplet formation. However, the role of velocity pulsation in preparing viscoelastic double emulsion droplets is still unclear. In this work, direct numerical simulations are utilized to study the droplet formation mechanism, integrating the volume of fluid method to handle the two-phase interface. The viscoelastic properties of the polymeric fluid are modeled using the Phan-Thien–Tanner constitutive equation. The results demonstrate that pulsatile inlet velocities significantly influence the flow behavior. The higher pulsation frequency promotes the transition from the inner multiple and outer synchronized mode to the inner and outer synchronized mode, while the increased amplitude drives the system to irregular modes. A model is proposed for predicting mode transitions of viscoelastic double emulsion droplet pinch-off in microchannels. In addition, elasticity has a negligible effect on double emulsion droplet formation at low Weissenberg number (Wi), but it inhibits the pinch-off process once Wi exceeds a certain threshold. Finally, a dimensionless empirical model is developed to correlate the active control parameters with the droplet particle size, with a regulation range of 2.5 times and a droplet radius prediction error of less than 5%, which is conducive to the controlled production of viscoelastic polymer double emulsion droplets.