  We study numerically the dynamics of jets and drops in a microcapillary
co-flow device. The co-flow stream encounters different flow regimes, including
dripping, jetting. Using a level-set/finite element axysimmetric numerical
simulation, we study the dynamics of breakup of a jet subject to flow rate
perturbations. A scaling law for the width of the unperturbed jet is presented
and compared to existing experimental results as well as numerical
measurements. Then, we show that the introduction of a sinusoidal perturbation
of the inner fluid flow rate can facilitate breakup of the fluid in regimes
where a jet is usually observed. Moreover, the flow rate perturbation leads to
a good control over the size and the frequency of the resulting droplets. Using
theoretical modelling, we provide a criterion to determine the optimal
frequency to break up the jet. We also derive scaling laws to determine the
volume of inner fluid encapsulated in the emulsion droplets as a function of
the frequency and to estimate the distance for the jet to break up as a
function of the amplitude of perturbation. These scaling laws are in good
agreement with results of numerical simulations. Our work suggests a novel
approach and offers guiding principles to break up liquid jets in cases where
dripping is difficult to achieve.
