On-demand, parallel droplet merging method with non-contact droplet pairing in droplet-based microfluidics

We demonstrate a simple approach for merging droplets in an on-demand, parallel manner via noncontact pairing of two droplets. The non-contact pairing can be achieved by exploiting flow-induced deformability of a droplet with a unique structure of merging element. Non-contact paired droplets, which are initially stabilized by surfactant molecules, can be merged simply by destabilizing the droplet interface and bringing two droplets into contact. On-demand, parallel droplet merging is performed with a proper pressure profile achieved by a pneumatic pressure supply system, and merging process is solely dependent on the pressure-driven fluid flow. We achieved an average merging efficiency of 90.0 % (SD = 3.14, n = 450) in performing parallel merging in a non-contact paired droplet array. We also evaluated the on-demand merging performance by measuring the average merging delay time (mean = 3.25 s, SD +/- 1.09 s, n similar to 180). Furthermore, we demonstrated the applicability of our device for the initiation of a chemical reaction through the merging of two droplets with different chemical contents. We believe that the proposed method will be useful for studying various droplet-based reactions.