Real-time size modulation and synchronization of a microfluidic dropmaker with pulsed surface acoustic waves (SAW).

We show that a microfluidic flow focusing drop maker can be synchronized to a surface acoustic waves (SAW) triggered by an external electric signal. In this way droplet rate and volume can be controlled over a wide range of values in real time. Using SAW, the drop formation rate of a regularly operating water in oil drop maker without SAW can be increased by acoustically enforcing the drop pinch-off and thereby reducing the volume. Drop makers of square cross-sections (w = h = 30 µm, with width w and height h) that produce large drops of length l = 10 w can be triggered to produce drops as short as l ~ 2w, approaching the geometical limit l = w without changing the flow rates. Unlike devices that adjust drop size by changing the flow rates the acoustic dropmaker has very short transients allowing to adjust the size of every single drop. This allows us to produce custom made emulsions with a defined size distribution as demonstrated here not only for a monodisperse emulsion but also for binary emulsions with drops of alternating size. Moreover, we show that the robustness and monodispersity of our devices is enhanced compared to purely flow driven drop makers in the absence of acoustic synchronization.