Impingement of droplet train with a immiscible liquid jet: Dynamics and outcomes*

Droplet-based microfluidics can generate compound microdroplets of various morphologies, and promote developments of functional materials synthesis, biochemical analysis, and drug manufacturing, etc. However, droplet microfluidics usually uses liquid as an environmental fluid with limited flow rates. In-air microfluidics (IAMF, Ref: Visser et al., Sci. Adv. 2018;4: eaao1175) generates compound microdroplets in the air by impinging microdroplets with a immiscible liquid jet. This study carries out direct numerical simulations to reveal physical details during generations of compound microdroplets using IAMF. A three-phase volume-of-fluid method combining with an  adaptive-mesh-refinement technology is used to predict impingement dynamics. Processes such as droplet-jet interaction, triple-line motion, and jet pinch-off are investigated to show effects of flow and physical parameters on shapes (spherical, ellipsoidal and band-shaped) and morphologies (Janus and core-shell) of compound droplets. This study provides physic basis and optimization criteria for the development of IAMF.