Selective droplet coalescence using microfluidic systems

We report a microfluidic approach, which allows selective and controlled 1 : 1, 2 : 1 or 3 : 1 droplet fusion. A surfactant-stabilized droplet with an interfacial surfactant coverage, Gamma, of > 98% will fuse spontaneously with a second droplet when Gamma of the latter droplet is < 16%. However, when Gamma of the second droplet is similar to 66%, the two droplets will not fuse, unless they have previously been brought into contact for critical time tau. Therefore, controlling the number of droplets in contact for time tau allows precise control over the number of fused droplets. We have demonstrated efficient (proportion of droplets coalesced p(c) 1.0, n > 1000) and selective 1 : 1, 2 : 1 or 3 : 1 droplet fusion (proportion of correctly fused droplets p(s) > 0.99, n > 1000). Coalescence in this regime is induced by hydrodynamic flow causing interface separation and is efficient at different Ca numbers and using different dispersed phases, continuous phases and surfactants. However, when Gamma of the second droplet is similar to 96% coalescence is no longer observed. Droplet-based microfluidic systems, in which each droplet functions as an independent microreactor, are proving a promising tool for a wide range of ultrahigh-throughput applications in biology and chemistry. The addition of new reagents to pre-formed droplets is critical to many of these applications and we believe the system described here is a simple and flexible method to do so, as well as a new tool to study interfacial stability phenomena.