Controllable Droplet Coalescence in the T-Junction Microchannel with a Funnel-Typed Expansion Chamber

Droplet coalescence in a novel funnel-typed expansion chamber in the T-junction microchannel was visually investigated to improve and precisely control droplet coalescence. Three types of coalescence regimes were observed: noncoalescence, two-droplet coalescence, and multiple coalescence. For two-droplet coalescence, three subtypes were observed: slipping coalescence, colliding coalescence, and squeezing coalescence. The different trajectories of the two droplets in the three subtypes and the influences of the two-phase flow rate and the length-to-width ratio of the expansion chamber on the coalescence percentage for two-droplet coalescence were studied systematically. The results indicate that the coalescence percentage increases first and then declines with the increase of a continuous phase flow rate or the decrease of the dispersed phase flow rate. Under a lower continuous phase flow rate, the coalescence percentage rises with an increasing length-to-width ratio of the chamber, while an inverse trend was found in the higher continuous phase flow rate.