Flow field around the encapsulated droplets on impact-driven liquid-liquid encapsulation: Effect of interfacial layer and impact Weber number

This study visualized the complex flow dynamics around the encapsulated droplets generated by impact-driven liquid-liquid encapsulation. In our experiment, a core drop is dispensed from a needle tip and penetrates the interfacial layer formed on a host liquid, resulting in the formation of a drop encapsulated by the interfacial layer. While the hydrodynamics of the encapsulation process significantly influence the morphology and stability of the droplet, detailed knowledge of the surrounding flow field remains scarce. To fill this gap, we conducted comprehensive flow measurements using two-dimensional particle image velocimetry, capturing the flow field during the encapsulation process—impact, interfacial penetration and necking, and cargo separation. Our findings reveal that both velocity and vorticity distributions exhibit similar trends, regardless of the presence of the interfacial layer. Furthermore, we explored the effect of the impact Weber number, revealing that the flow field in encapsulation cases can be categorized into three distinct types based on time series vorticity data obtained from the drop's wake. These insights advance the understanding of the encapsulation process and offer a new framework for analyzing flow behaviors in liquid-liquid systems.