Coalescence of surfactant-stabilized adjacent droplets using surface acoustic waves

A novel, on-demand microfluidic droplet merging mechanism is presented in this paper. We demonstrate that a narrow beam surface acoustic wave, targeted at the oil buffer, causes nearby surfactant-stabilized droplets to coalesce. The lack of direct exposure of the droplet to the excitation stimulus makes this method ideal for sensitive samples as harm will not occur. This powerful technique works on a straight channel with no special design, is not affected by surfactant concentration and droplet volume hence promises seamless integration into existing microfluidic systems. It offers high-throughput, biologically safe, on-demand droplet merging for applications ranging from fast reaction kinetics to microfluidic high throughput screening. We thoroughly characterize the physical mechanism triggering droplet-droplet coalescence and observe a cutoff distance from the center of the acoustic beam to the droplet-droplet interface after which the merging mechanism does not work anymore. We establish that the most likely mechanism for merging is acoustic streaming induced droplet deformation.