Selective fusion of anchored droplets via changes in surfactant concentration.

We present a robust method to fuse in parallel an array of anchored droplets in a microchannel. Pairs of anchored droplets are fused by the removal of surfactant from the droplet interface by reducing the surfactant content in the flowing external oil phase. By controlling the flow of multiple oil inlets, the selective fusion of rows of droplets in a larger array is demonstrated. The technique is compatible with cells as shown with a trypan blue exclusion vitality assay. The method is easy to implement, requires no active components and is applicable to oil/water combinations where the surfactant is soluble in the external phase.