  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 100 ns 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 time scale (order of
microseconds) whereas the dominant pressure oscillations are linked to the
bubble pinch-off and propagate at a much faster time scale (order of
nanoseconds).
