Surface acoustic wave based droplet merging on demand

In this study, a microfluidic chip is developed where surface acoustic waves (SAWs) are used to merge multiple droplets on demand. Acoustic radiation forces induced in the liquid medium are shown to trap droplets while they are passing from a microchannel to an expansion chamber. Droplets stay immobilized until successive droplets arrive and merge. As the droplet volume increases beyond a critical value, drag forces exceed the existing acoustic radiation forces and cause the merged droplet to exit the expansion chamber. The number of successive droplets that can be merged depend on the initial droplet size and applied power for actuation.