The formation of bubbles by flow focusing of a gas and a liquid in a rectangular channel is shown to depend strongly on the channel aspect ratio. Bubble breakup consists in a slow linear 2D collapse of the gas thread, ending in a fast 3D pinch-off. The 2D collapse is predicted to be stable against perturbations of the gas-liquid interface, whereas the 3D pinch-off is unstable, causing bubble polydispersity. During 3D pinch-off, a scaling wm~tau1/3 between the neck width wm and the time tau before breakup indicates that breakup is driven by the inertia of both gas and liquid, not by capillarity.
Dollet, B., van Hoeve, W., Raven, J-P., Marmottant, P. G. M., & Versluis, M. (2008). Role of the Channel Geometry on the Bubble Pinch-Off in Flow-Focusing Devices. Physical review letters, 100(3), 034504-1-034504-4. https://doi.org/10.1103/PhysRevLett.100.034504