Role of the channel geometry on the bubble pinch-off in flow-focusing devices

The role of the orifice geometry in the production of bubbles by flow focusing of a gas and a liquid in an orifice of rectangular cross-section is investigated. It is experimentally shown that the aspect ratio of the orifice dramatically influences the duration of bubble breakup, characterized by a slow linear 2D collapse, followed by a final fast 3D pinch-off. A stability analysis predicts that the 2D collapse is always stable, whereas the 3D pinch-off is always unstable. The ultimate stage of the pinch- off is recorded by high-speed imaging, yielding a scaling $w_m \sim\tau^{1/3}$ between the neck width $w_m$ and the time $\tau$ before breakup, which indicates that breakup is driven solely by the inertia of both gas and liquid, and that it is not a capillary process. The presented study of the bubble breakup shows that elongated rectangular orifices favors high monodispersity, whereas the highest frequency of bubble production is achieved in square orifices.