Experimental and numerical investigations on interface coupling of coaxial liquid jets in co-flow focusing

Abstract

The rupture of coaxial liquid jets in co-flow focusing is studied experimentally and numerically, aiming to deepen the understanding of interface coupling and its effect on the morphology of the generated compound droplets. The weak, transitional, and strong coupling regimes of interface instabilities are identified in experiments by varying the flow rate ratio of the inner jet to the coaxial jets (denoted by rQ). The dynamics of coaxial liquid jets are further resolved by direct numerical simulations after being validated against experiments. The flow structures and pressure contours for the coaxial jets can be given under different coupling conditions. Moreover, scaling laws are proposed to correlate the breakup length of coaxial jets and the sizes of compound droplets and encapsulated cores with rQ, and a good agreement is obtained. It can be found that the transition from weak, transitional and strong coupling occurs approximately at rQ = 0.15 and rQ = 0.4, respectively. This work provides underlying insight into the interface coupling of coaxial liquid jets, which would provide theoretical guidance for microcapsule generation in various potential applications.