Self-synchronization of reinjected droplets for high-efficiency droplet pairing and merging

Abstract

Abstract Droplet merging serves as a powerful tool to add reagents to moving droplets for biological and chemical reactions. However, unsynchronized droplet pairing impedes high-efficiency merging. Here, we develop a microfluidic design for self-synchronization of reinjected droplets. Periodical increase of hydrodynamic resistance caused by the droplet blocking in the T-junction enables automatic pairing of droplets. Through further spacing, the paired droplets are merged downstream under electric field. The blockage-based design can achieve a 100% synchronization efficiency even when the mismatch rate of droplet frequencies reaches 10%. Over 98% of the droplets can still be synchronized at non-uniform droplet sizes and fluctuated reinjection flow rates. Moreover, the droplet pairing ratio can be adjusted flexibly for on-demand sample addition. Using this system, we merge two groups of droplets encapsulating enzyme/substrate, demonstrating its capacity to conduct multi-step reactions. We also combine droplet sorting and merging to co-encapsulate single cells and single beads, providing basis for high-efficiency single-cell sequencing. We expect that this system can be integrated with other droplet manipulation systems for broad ranges of chemical and biological applications.