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 w_(m) approximately tau(1/3) between the neck width w_(m) and the time tau before breakup indicates that breakup is driven by the inertia of both gas and liquid, not by capillarity.

Lab-on-a-chip devices can handle very tiny amounts of fluids, as small as a picolitre. Samples of gas are conveniently encapsulated and transported within bubbles. The generation of these calibrated bubbles involves specific microfluidic devices that we describe. When the bubble concentration is high enough in a channel, they come into contact, and they flow as a crystalline foam. The flow of the foams depends strongly on the arrangement of bubbles within the channel, which entails original dynamical behaviours such as super-stability of the flow, or on the contrary spontaneous oscillations.