The previously produced droplets are reinjected into the microfluidic acoustoinjection device by a pneumatic flow controller (MFCSTM-EZ, Fluigent, Germany). The droplet inlet pressures are varied between 100 and 500 mbar, while the distance between the droplets is adjusted by adding the corresponding continuous phase into a separate inlet channel under certain pressures. For better visualization, black ink is used as droplet injection fluid. The pressure in the injection fluid channel is adjusted to the pressures of the droplet channel to ensure contact between the passing droplets and the injection fluid. For the determination of the injection with different input amplitudes and surfactant concentrations (Fig. ), the inlet pressure is 300 mbar at the droplet inlet channel, 295 mbar at the separation fluid inlet channel and 120 mbar at the injection fluid inlet channel. The exact pressure rates for the injection efficiency experiments under certain droplet passing speeds can be found in Table . Droplet injection is observed with an inverted microscope (Olympus IX71, Olympus, Japan) and high-speed camera videos (Phantom v2511, Vision Research, USA) are taken for further analysis of the injection process.