Time-resolved velocity and pressure field quantification in a flow-focusing device for ultrafast microbubble production

Flow-focusing devices have gained great interest in the past decade, due to their capability to produce monodisperse microbubbles for diagnostic and therapeutic medical ultrasound applications. However, up-scaling production to industrial scale requires a paradigm shift from single chip operation to highly parallelized systems. Parallelization gives rise to fluidic interactions between nozzles that, in turn, may lead to a decreased monodispersity. Here we study the velocity and pressure field fluctuations in a single flow-focusing nozzle during bubble production. We experimentally quantify the velocity field inside the nozzle at 100ns time resolution, and a numerical model provides insight into both the oscillatory velocity and pressure fields. Our results demonstrate that, at the length scale of the flow-focusing channel, the velocity oscillations propagate at fluid dynamical timescale (order of μs) whereas the dominant pressure oscillations are linked to the bubble pinch-off and propagate at a much faster timescale (order of ns).