Droplet dynamics in asymmetric microfluidic junctions

Abstract

Droplet-based microfluidics has received increasing interest over the last two decades, due to its extensive applications in drug delivery, cell encapsulation, protein crystallization, biochemical synthesis and high-throughput screening. As one of the key units in the microfluidic channels, asymmetric microfluidic junctions provide more opportunities to control and organize the droplets with different sizes, however, few studies have been reported to explore the effect of the junction angle on droplet behaviour in the asymmetric junctions. This work presents a numerical investigation of the droplet behaviour in asymmetric microfluidic junctions. The effect of the junction angle on the droplet breakup is explored in detail. Droplet breakup and non-breakup are found in asymmetric junctions. In the regime of the asymmetric breakup, the volume of the daughter droplet in the uphill branching channel is always larger than that in the downhill branching channel. Interestingly, in the regime of non-breakup, droplets are sorted into the uphill or downhill branching channels, suggesting that this asymmetric junction could be used for passive droplet sorting. The physical mechanisms behind droplet breakup and sorting are explored. Both the junction angle and the initial tip-to-tip length of the droplet play essential roles in the droplet behaviour in the asymmetric junctions. A phase diagram of the droplet behaviour is presented in this work. A scaling model is proposed to predict the transition between the breakup and non-breakup of the droplets in the asymmetric junctions.